The deposits on the western border of the Andes are excellently exposed

in the Majes Valley, one of the most famous in Peru, though its fame

rests rather upon the excellence and abundance of its vineyards and

wines than its splendid geologic sections. Its head lies near the base

of the snow-capped peaks of Coropuna; its mouth is at Camaná on the

Pacific, a hundred miles north of Mollendo. It is both narrow and deep;

one may ride across its floor anywhere in a half hour. In places it is a

narrow canyon. Above Cantas it is sunk nearly a mile below the level of

the desert upland through which it flows. Along its borders are exposed

basal granites, old sedimentaries, and lavas; inter-bedded with it are

other lavas that lie near the base of the great volcanic series; through

it still project the old granites of the Coast Range; and upon it have

been accumulated additional volcanic rocks, wind-blown deposits, and,

finally, coarse wash formed during the glacial period. From both the

variety of the formations, the small amount of marginal dissection, and

the excellent exposures made possible by the deep erosion and desert

climate, the Majes Valley is one of the most profitable places in Peru

for physiographic and geologic study.

[Illustration: FIG. 171--Generalized sketch section to show the

structural relations of the Maritime Cordillera, the desert pampas, and

the Coast Range.]

The most complete succession of strata (Tertiary) occurs just below

Cantas on the trail to Jaguey (Fig. 171). Upon a floor of

granite-gneiss, and alternating beds of quartzite and shale belonging to

an older series, are deposited heavy beds of red sandstone with many

conglomerate lenses. The sandstone strata are measurably deformed and

their upper surfaces moderately dissected. Upon them have been deposited

unconformably a thicker series of deposits, conglomerates, sandstones,

and finer wind-blown material. The basal conglomerate is very

coarse--much like beach material in both structure and composition, and

similar to that along and south of the present coast at Camaná. Higher

in the section the material is prevailingly sandy and is deposited in

regular beds from a few inches to a few feet in thickness. Near the top

of the section are a few hundred feet of strata chiefly wind deposited.

Unconformably overlying the whole series and in sharp contrast to the

fine wind-blown stuff below it, is a third series of coarse deposits

about five hundred feet thick. The topmost material, that forming the

surface of the desert upland, consists of wind-blown sand now shifted by

the wind and gathered into sand dunes or irregular drifts, banks of

white earth, “tierra blanca,” and a pebble pavement a few inches thick.

If the main facts of the above section are now summarized they will

facilitate an understanding of other sections about to be described,

inasmuch as the summary will in a measure anticipate our conclusions

concerning the origin of the deposits and their subsequent history. The

sediments in the Majes Valley between Cantas and Jaguey consist of three

series separated by two unconformities. The lowermost series is evenly

bedded and rather uniform in composition and topographic expression,

standing forth in huge cliffs several hundred feet high on the eastern

side of the valley. This lower series is overlain by a second series,

which consists of coarse conglomerate grading into sand and ultimately

into very fine fluffy wind-deposited sands and silts. The lower series

is much more deformed than the upper, showing that the deforming

movements of later geologic times have been much less intense than the

earlier, as if there had been a fading out or weakening of the deforming

agents. Finally there is a third series several hundred feet thick which

forms the top of the section.

[Illustration: FIG. 172--Geologic relations of Coast Range, desert

deposits, and Maritime Cordillera at Moquegua, Peru. After G. I. Adams;

Bol. de Minas del Perú, Vol. 2, No. 4, 1906, p. 20.]

[Illustration: FIG. 173--Sketch section to show structural details on

the walls of the Majes Valley near Aplao, looking south.]

Three other sections may now be examined, one immediately below Cantas,

one just above, and one opposite Aplao. The section below Cantas is

shown in Fig. 173, and indicates a lower series of red sandstones

crossed by vertical faults and unconformably overlain by nearly

horizontal conglomerates, sandstones, etc., and the whole faulted again

with an inclined fault having a throw of nearly 25°. A white to gray

sandstone unconformably overlying the red sandstone is shown

interpolated between the lowermost and uppermost series, the only

example of its kind, however. No important differences in

lithographical character may be noted between these and the beds of the

preceding section.

Again just above Cantas on the east side of the valley is a clean

section exposing about two thousand feet of strata in a half mile of

distance. The foundation rocks are old quartzites and shales in

regularly alternating beds. Upon their uneven upper surfaces are several

thousand feet of red sandstones and conglomerates, which are both folded

and faulted with the underlying quartzites. Above the red sandstones is

a thick series of gray sandstones and silts which makes the top of the

section and unconformably overlies the earlier series.

A similar succession of strata was observed at Aplao, still farther up

the Majes Valley, Fig. 174. A greatly deformed and metamorphosed older

series is unconformably overlaid by a great thickness of younger strata.

The younger strata may be again divided into two series, a lower series

consisting chiefly of red sandstones and an upper consisting of gray to

yellow, and only locally red sands of finer texture and more uniform

composition. The two are separated by an erosion surface and only the

upper series is tilted regionally seaward with faint local deformation;

the lower series is both folded and faulted with overthrusts aggregating

several thousand feet of vertical and a half mile of horizontal

displacement.

[Illustration: FIG. 174--The structural relations of the strata on the

border of the Majes Valley at Aplao, looking west. Field sketch from

opposite side of valley. Height of section about 3,000 feet; length

about ten miles.]

The above sections all lie on the eastern side of the Majes Valley. From

the upper edge of the valley extensive views were gained of the strata

on the opposite side, and two sections, though they were not examined at

close range, are at least worth comparing with those already given. From

the narrows below Cantas the structure appears as in Figs. 175-176, and

shows a deforming movement succeeded by erosion in a lower series. The

upper series of sedimentary rock has suffered but slight deformation. A

still more highly deformed basal series occurs on the right of the

section, presumably the older quartzites. At Huancarqui, opposite Aplao,

an extensive view was gained of the western side of the valley, but the

lower Tertiary seems not to be represented here, as the upper undeformed

series rests unconformably upon a tilted series of quartzites and

slates. Farther up the Cantas valley (an hour’s ride above Aplao) the

Tertiary rests upon volcanic flows or older quartzites or the

granite-gneiss exposed here and there along the valley floor.

[Illustration: FIG. 175--Sketch section to show the structural details

of the strata on the south wall of the Majes Valley near Cantas. The

section is two miles long.]

[Illustration: FIG. 176--Composite geologic section to show the

structural relations of the rocks on the western border of the Maritime

Cordillera. The inclined strata at the right bottom represent older

rocks; in places igneous, in other places sedimentary.]

In no part of the sedimentaries in the Majes Valley were fossils found,

save in the now uplifted and dissected sands that overlie the upraised

terraces along the coast immediately south of Camaná and also back of

Mollendo. Like similar coastal deposits elsewhere along the Peruvian

littoral, the terrace sands are of Pliocene or early Pleistocene age.

The age of the deposits back of the Coast Range is clearly greater than

that of the coastal deposits, (1) since they involve two unconformities,

a mile or more of sediments, and now stand at least a thousand feet

above the highest Pliocene (or Pleistocene) in the Camaná Valley, and

(2) because the erosion history of the interior sediments may be

correlated with the physiographic history of the coastal terraces and

the correlation shows that uplift and dissection of the terraces and of

the interior deposits went hand in hand, and that the deposits on the

terraces may similarly be correlated with alluvial deposits in the

valley.

We shall now see what further ground there is for the determination of

the age of these sediments. Just below Chuquibamba, where they first

appear, the sediments rest upon a floor of volcanic and older rock

belonging to the great field now known from evidence in many localities

to have been formed in the early Tertiary, and here known to be

post-Cretaceous from the relations between Cretaceous limestones and

volcanics in the Cotahuasi Valley (see p. 247). Although volcanic flows

were noted interbedded with the desert deposits, these are few in

number, insignificant in volume, and belong to the top of the volcanic

series. The same may be said of the volcanic flows that locally overlie

the desert deposits. We have then definite proof that the sandstones,

conglomerates, and related formations of the Majes Valley and bordering

uplands are older than the Pliocene or early Pleistocene and younger

than the Cretaceous and the older Tertiary lavas. Hence it can scarcely

be doubted that they represent a considerable part of the Tertiary

period, especially in view of the long periods of accumulation which the

thick sediments represent, and the additional long periods represented

by the two well-marked unconformities between the three principal groups

of strata.

If we now trace the physical history of the region we have first of all

a deep depression between the granite range along the coast and the

western flank of the Andes. Here and there, as in the Vitor, the Majes,

and other valleys, there were gaps through the Coast Range. Nowhere did

the relief of the coastal chain exceed 5,000 feet. The depression had

been partly filled in early geologic (probably early Paleozoic) time by

sediments later deformed and metamorphosed so that they are now

quartzites and shales. The greater resistance of the granite of the

Coast Range resulted in superior relief, while the older deformed

sedimentaries were deeply eroded, with the result that by the beginning

of the Tertiary the basin quality of the depression was again

emphasized. All these facts are expressed graphically in Fig. 171. On

the western flanks of the granite range no corresponding sedimentary

deposits are found in this latitude. The sea thus appears to have stood

farther west of the Coast Range in Paleozoic times than at present.

[Illustration: FIG. 177--Composite structure section at Aplao.]

For the later history it is necessary to assemble the various Tertiary

sections described on the preceding pages. First of all we recognize

three quite distinct types of accumulations, for which we shall have to

postulate three sets of conditions and possibly three separate agents.

The first or lowermost consists of even-bedded deposits of red and gray

sandstones, the former color predominating. The material is in general

well-sorted save locally, where lenses and even thin beds of

conglomerate have been developed. There is, however, about the whole

series a uniformity and an orderliness in striking contrast to the

coarse, cross-bedded, and irregular material above the unconformity. On

their northeastern or inner margin the sandstones are notably coarser

and thicker, a natural result of proximity to the mountains, the source

of the material. The general absence of wind-blown deposits is marked;

these occur entirely along the eastern and northern portions of the

deposits and are recognized (1) by their peculiar cross-bedding, and (2)

by the fact that the cross-bedding is directed northeastward in a

direction contrary to the regional dip of the series, a condition

attributable to the strong sea breezes that prevail every afternoon in

this latitude.

The main body of the material is such as might be deposited on the wide

flood plains of piedmont streams during a period of prolonged erosion

on surrounding highlands that served as the feeding grounds of the

streams. The alternations in the character of the deposits, alternations

which, in a general view, give a banded appearance to the rock, are

produced by successions of beds of fine and coarse material, though all

of it is sandstone. Such successions are probably to be correlated with

seasonal changes in the volume and load of the depositing streams.

To gain an idea of the conditions of deposition we may take the

character of the sediments as described above, and from them draw

deductions as to the agents concerned and the manner of their action.

We may also apply to the area the conclusions drawn from the study of

similar deposits now in process of formation. We have between the coast

ranges of northern Chile and the western flanks of the Cordillera

Sillilica, probably the best example of piedmont accumulation in a dry

climate that the west coast of South America affords.

Along the inner edge of the Desert of Tarapacá, roughly between the

towns of Tarapacá and Quillagua, Chile, the piedmont gravels, sands,

silts, and muds extend for over a hundred miles, flanking the western

Andes and forming a transition belt between these mountains and the

interior basins of the coast desert. The silts and muds constitute the

outer fringe of the piedmont and are interrupted here and there where

sands are blown upon them from the higher portions of the piedmont, or

from the desert mountains and plains on the seaward side. Practically no

rain falls upon the greater part of the desert and the only water it

receives is that borne to it by the piedmont streams in the early

summer, from the rains and melted snows of the high plateau and

mountains to the eastward. These temporary streams spread upon the outer

edge of the piedmont a wide sheet of mud and silt which then dries and

becomes cracked, the curled and warped plates retaining their character

until the next wet season or until covered with wind-blown sand. The

wind-driven sand fills the cracks in the muds and is even drifted under

the edges of the upcurled plates, filling the spaces completely. Over

this combined fluvial and æolian deposit is spread the next layer of

mud, which frequently is less extensive than the earlier deposits, thus

giving abundant opportunity for the observation of the exact manner of

burial of the older sand-covered stratum.

Now while the alternations are as marked in Peru as in Chile, it is

noteworthy that the Tertiary material in Peru is not only coarse

throughout, even to the farthest limits of the piedmont, but also that

the alternating beds are thick. Moreover, there are only the most feeble

evidences of wind action in the lowermost Tertiary series. I was

prepared to find curled plates, wind-blown sands, and muds and silts,

but they are almost wholly absent. It is, therefore, concluded that the

dryness was far less extreme than it is today and that full streams of

great competency flowed vigorously down from the mountains and carried

their loads to the inner border of the Coast Range and in places to the

sea.

The fact that the finer material is _sandy_, not clayey or silty, that

it almost equals in thickness the coarser layers, and that its

distribution appears to be co-extensive with the coarser, warrants the

conclusion that it too was deposited by competent streams of a type far

different from the withering streams associated with piedmont deposits

in a thoroughly arid climate like that of today. Both in the second

Tertiary series and on the present surface are such clear examples of

deposits made in a drier climate as to leave little doubt that the

earliest of the Tertiary strata of the Majes Valley were deposited in a

time of far greater rainfall than the present. It is further concluded

that there was increasing dryness, as shown by hundreds of feet of

wind-blown sand near the top of the section. But the growing dryness was

interrupted by at least one period of greater precipitation. Since that

time there has been a return to the dry climate of a former epoch.

Uplift and erosion of the earliest of the Tertiary deposits of the Majes

Valley is indicated in two ways: (1) by the deformed character of the

beds, and (2) by the ensuing coarse deposits which were derived from the

invigorated streams. Without strong deformations it would not be

possible to assign the increased erosion so confidently to uplift; with

the coarse deposits that succeed the unconformity we have evidence of

accumulation under conditions of renewed uplift in the mountains and of

full streams competent to remove the increasing load.

It is in the character of the sediments toward the top of the Tertiary

that we have the clearest evidence of progressive desiccation of the

climate of the region. The amount of wind-blown material steadily

increases and the uppermost five hundred feet is composed predominantly,

and in places exclusively, of this material. The evidences of wind

action lie chiefly in the fine (in places fluffy) nature of the

deposits, their uniform character, and in the tangency of the layers

with respect to the surface on which they were deposited. There are

three diagnostic structural features of great importance: the very steep

dip of the fine laminae; the peculiar and harmonious blending of their

contacts; the manner in which the highly inclined laminae cut off and

succeed each other, whereby quite bewildering changes in the direction

of dip of the inclined beds are brought about on any exposed plane. Some

of these features require further discussion.

It is well known that the front of a sand dune generally consists of

sand deposited on a slope inclined at the angle of repose, say between

30° and 35°, and rolled into place up the long back slope of the dune by

the wind. It has not, however, been generally recognized that the angle

of repose may be exceeded (a) when there exists a strong back eddy or

(b) when the wind blows violently and for a short time in the opposite

direction. In either case sand is carried up the short steep slope of

the dune front and accumulated at an angle not infrequently running up

to 43° and 48° and locally, and under the most favorable circumstances,

in excess of 50°. The conditions under which these steep angles are

attained are undoubtedly not universal, but they can be found in some

parts of almost any desert in the world. They appear not to be present

where the sand grains are of uniform size throughout, since that leads

to rolling. They are found rather where there is a certain limited

variation in size that promotes packing. Packing and the development of

steep slopes are also facilitated in parts of the coastal desert of Peru

by a cloud canopy that hangs over the desert in the early morning, that

in the most favorable places moistens even the dune surfaces and that

has least penetration on the steep semi-protected dune fronts. Sand

later blown up the dune front or rolled down from the dune crest is

encouraged to remain near the cornice on an abnormally steep slope by

the attraction which the slightly moister sand has for the dry grains

blown against it. Since dunes travel and since their front layers,

formed on steep slopes, are cut off to the level of the surface in the

rear of the dune, it follows that the steepest dips in exposed sections

are almost always less than those in existing dunes. Exceptions to the

rule will be noted in filled hollows not re-excavated until deeply

covered by wind-blown material. These, re-exposed at the end of a long

period of wind accumulation, may exhibit even the maximum dips of the

dune cornices. Such will be conspicuously the case in sections in

aggraded desert deposits. On the border of the Majes Valley, from 400 to

500 feet of wind-accumulated deposits may be observed, representing a

long period of successive dune burials.

The peculiar blending of the contact lines of dune laminae, related to

the tangency commonly noted in dune accumulations, is apparently due to

the fact that the wind does not require a graded surface to work on, but

blows uphill as well as down. It is present on both the back-slope and

the front-slope deposits. Its finest expression appears to be in

districts where the dune material was accumulated by a violent wind

whose effects the less powerful winds could not destroy.

It is to the ability of the wind to transport material against, as well

as with, gravity, that we owe the third distinct quality of dune

material, the succession of flowing lines, in contrast to the succession

of now flat-lying now steeply inclined beds characteristic of

cross-bedded material deposited by water. One dune travels across the

face of the country only to be succeeded by another.[54] Even if wind

aggradation is in progress, the plain-like surface in the rear of a dune

may be excavated to the level of steeply inclined beds upon whose

truncated outcrop other inclined beds are laid, Fig. 178. The contrast

to these conditions in the case of aggradation by water is so clearly

and easily inferred that space will not be taken to point them out. It

is also true as a corollary to the above that the greater part of a body

of wind-drifted material will consist of cross-bedded layers, and not a

series of evenly divided and alternating flat-lying and cross-bedded

layers which result from deposition in active and variable currents of

water.

The caution must of course be observed that wind action and water action

may alternate in a desert region, as already described in Tarapacá in

northern Chile, so that the whole of a deposit may exhibit an

alternation of cross-bedded and flat-lying layers; but the former only

are due to wind action, the latter to water action.

Finally it may be noted that the sudden, frequent, and diversified dips

in the cross-bedding are peculiarly characteristic of wind action.

Although one sees in a given cross-section dips apparently directed only

toward the left or the right, excavation will supply a third dimension

from which the true dips may be either observed or calculated. These

show an almost infinite variety of directions of dip, even in restricted

areas, a condition due to the following causes:

(1) the curved fronts of sand dunes, which produce dips concentric with

respect to a point and ranging through 180° of arc; (2) the irregular

character of sand dunes in many places, a condition due in turn to (a)

the changeful character of the strong wind (often not the prevailing

wind) to which the formation of the dunes is due, and (b) the influence

of the local topography upon wind directions within short distances or

upon winds of different directions in which a slight change in wind

direction is followed by a large change in the local currents; (3) the

fact that all combinations are possible between the erosion levels of

the wind in successive generations of dunes blown across a given area,

hence _any_ condition at a given level in a dune may be combined with

_any other_ condition of a succeeding dune; (4) variations in the sizes

of successive dunes will lead to further contrasts not only in the

scale of the features but also in the direction and amount of the dips.

[Illustration: FIG. 178--Plan and cross-sections of superimposed sand

dunes of conventional outline. In the sections, dune _A_ is supposed to

have left only a small basal portion to be covered by dune _B_. In the

same way dune _C_ has advanced to cover both _A_ and _B_. The basal

portions that have remained are exaggerated vertically in order to

display the stratification. It is obviously not necessary that the dunes

should all be of the same size and shape and advancing in the same

direction in order to have the tangential relations here displayed. Nor

need the aggrading material be derived from true dunes. The results

would be the same in the case of sand _drifts_ with their associated

wind eddies. All bedded wind-blown deposits would have the same general

relations. No two successive deposits, no matter from what direction the

successive drifts or dunes travel, would exactly correspond in direction

and amount of dip.]

Finally, we may note that a section of dune deposits has a distinctive

feature not exhibited by water deposits. If the foreset beds of a

cross-bedded water deposit be exposed in a plane parallel to the strike

of the beds, the beds will appear to be horizontal. They could not then

be distinguished from the truly horizontal beds above and below them.

But the conditions of wind deposition we have just noted, and chiefly

the facts expressed by Fig. 178, make it impossible to select a position

in which both tangency and irregular dips are not well developed in a

wind deposit. I believe that we have in the foregoing facts and

inferences a means for the definite separation of these two classes of

deposits. Difficulties will arise only when there is a quick succession

of wind and water action in time, or where the wind produces powerful

and persistent effects without the actual formation of dunes.

The latest known deposits in the coastal region are found surmounting

the terrace tops along the coast between Camaná and Quilca, where they

form deposits several hundred feet thick in places. The age of these

deposits is determined by fossil evidence, and is of extraordinary

interest in the determination of the age of the great terraces upon

which they lie. They consist of alternating beds of coarse and fine

material, the coarser increasing in thickness and frequency toward the

bottom of the section. It is also near the bottom of the section that

fossils are now found; the higher members are locally saline and

throughout there is a marked inclination of the beds toward the present

shore. The deposits appear not to have been derived from the underlying

granite-gneiss. They are distributed most abundantly near the mouths of

the larger streams, as near the Vitor at Quilca, and the Majes at

Camaná. Elsewhere the terrace summit is swept clean of waste, except

where local clay deposits lie in the ravines, as back of Mollendo and

where “tierras blancas” have been accumulated by the wind.

These coastal deposits were laid down upon a dissected terrace up to

five miles in width. The degree of dissection is variable, and depends

upon the relation of the through-flowing streams to the Coast Range. The

Vitor and the Majes have cut down through the Coast Range, and locally

removed the terrace; smaller streams rising on the flanks of the Coast

Range either die out near the foot of the range or cross it in deep and

narrow valleys. The present drainage on the seaward slopes of the Coast

Range is entirely ineffective in reaching the sea, as was seen in 1911,

the wettest season known on the coast in years and one of the wettest

probably ever observed on this coast by man.

In consequence of their deposition on a terrace that ranges in elevation

from zero to 1,500 feet above sea level, the deposits of the coast are

very irregularly disposed. But in consequence of their great bulk they

have a rather smooth upper surface, gradation having been carried to the

point where the irregularities of the dissected terrace were smoothed

out. Their general uniformity is broken where streams cross them, or

where streams crossed them during the wetter Pleistocene. Their

elevation, several hundred feet above sea level, is responsible for the

deep dissection of their coastal margin, where great cliffs have been

cut.

PLEISTOCENE

The broad regional uplift of the Peruvian Andes in late Tertiary and in

Pleistocene times carried their summits above the level of perpetual

snow. It is still an open question whether or not uplift was

sufficiently great in the early Pleistocene to be influenced by the

first glaciations of that period. As yet, there are evidences of only

two glacial invasions, and both are considered late events on account of

the freshness of their deposits and the related topographic forms. The

coarse deposits--nearly 500 feet thick--that form the top of the desert

section described above clearly indicate a wetter climate than prevailed

during the deposition of the several hundred feet of wind-blown deposits

beneath them. But if our interpretation be correct these deposits are of

late Tertiary age, and their character and position are taken to

indicate climatic changes in the Tertiary. They may have been the mild

precursors of the greater climatic changes of glacial times. Certain it

is that they are quite unlike the mass of the Tertiary deposits. On the

other hand they are separated from the deposits of known glacial age by

a time interval of great length--an epoch in which was cut a benched

canyon nearly a mile deep and three miles wide. They must, therefore,

have been formed when the Andes were thousands of feet lower and unable

to nourish glaciers. It was only after the succeeding uplifts had raised

the mountain crests well above the frost line that the records of

oscillating climates were left in erratic deposits, troughed valleys,

cliffed cirques and pinnacled divides.

The glacial forms are chiefly at the top of the country; the glacial

deposits are chiefly in the deep valleys that were carved before the

colder climate set in. The rock waste ground up by the ice was only a

small part of that delivered to the streams in glacial times. Everywhere

the wetter climate resulted in the partial stripping of the residual

soil gathered upon the smooth mature slopes formed during the long

Tertiary cycle of erosion. This moving sheet of waste as well as the

rock fragments carried away from the glacier ends were strewn along the

valley floors, forming a deep alluvial fill. Thereby the canyon floors

were rendered habitable.

In the chapters on human geography we have already called attention to

the importance of the U-shaped valleys carved by the glaciers. Their

floors are broad and relatively smooth. Their walls restrain the live

stock. They are sheltered though lofty. But all the human benefits

conferred by ice action are insignificant beside those due to the

general shedding of waste from the cold upper surfaces to the warm

levels of the valley floors. The alluvium-filled valleys are the seats

of dense populations. In the lowest of them tropical and sub-tropical

products are raised, like sugar-cane and cotton, in a soil that once lay

on the smooth upper slopes of mountain spurs or that was ground fine on

the bed of an Alpine glacier.

[Illustration: FIG. 179--Snow fields on the summit of the Cordillera

Vilcapampa near Ollantaytambo. A huge glacier once lay in the steep

canyon in the background and descended to the notched terminal moraine

at the canyon mouth. In places the glacier was over a thousand feet

thick. From the terminal moraine an enormous alluvial fan extends

forward to the camera and to the opposite wall of the Urubamba Valley.

It is confluent with other fans of the same origin. See Fig. 180. In the

foreground are flowers, shrubs, and cacti. A few miles below Urubamba at

11,500 feet.]

[Illustration: FIG. 180--Urubamba Valley between Ollantaytambo and

Torontoy, showing (1) more moderate upper slopes and steeper lower

slopes of the two-cycle mountain spurs; (2) the extensive alluvial

deposits of the valley, consisting chiefly of confluent alluvial fans

heading in the glaciated mountains on the left. See Fig. 179.]

[Illustration: FIG. 181--Glacial features of the Central Ranges (see

Fig. 204). Huge lateral moraines built by ice streams tributary to the

main valley north of Chuquibambilla. That the tributaries persisted long

after the main valley became free of ice is shown by the descent of the

lateral moraines over the steep border of the main valley and down to

the floor of it.]

The Pleistocene deposits fall into three well-defined groups: (1)

glacial accumulations at the valley heads, (2) alluvial deposits in

the valleys, and (3) lacustrine deposits formed on the floors of

temporary lakes in inclosed basins. Among these the most variable in

form and composition are the true glacier-laid deposits at the valley

heads. The most extensive are the fluvial deposits accumulated as valley

fill throughout the entire Andean realm. Though important enough in some

respects the lacustrine deposits are of small extent and of rather local

significance. Practically none of them fall within the field of the

present expedition; hence we shall describe only the first two classes.

The most important glacial deposits were accumulated in the eastern part

of the Andes as a result of greater precipitation, a lower snowline, and

catchment basins of larger area. In the Cordillera Vilcapampa glaciers

once existed up to twelve and fifteen miles in length, and those several

miles long were numerous both here and throughout the higher portions of

the entire Cordillera, save in the belt of most intense volcanic action,

which coincides with the driest part of the Andes, where the glaciers

were either very short or wanting altogether.

Since vigorous glacial action results in general in the cleaning out of

the valley heads, no deposits of consequence occur in these locations.

Down valley, however, glacial deposits occur in the form of terminal

moraines of recession and ground moraines. The general nature of these

deposits is now so well known that detailed description seems quite

unnecessary except in the case of unusual features.

It is noteworthy that the moraines decrease in size up valley since each

valley had been largely cleaned out by ice action before the retreat of

the glacier began. Each lowermost terminal moraine is fronted by a great

mass of unsorted coarse bowldery material forming a fill in places

several hundred feet thick, as below Choquetira and in the Vilcapampa

Valley between Vilcabamba and Puquiura. This bowldery fill is quite

distinct from the long, gently inclined, and stratified valley train

below it, or the marked ridge-like moraine above it. It is in places a

good half mile in length. Its origin is believed to be due to an

overriding action beyond the last terminal moraine at a time when the

ice was well charged with débris, an overriding not marked by morainal

accumulations, chiefly because the ice did not maintain an extreme

position for a long period.

In the vicinity of the terminal moraines the alluvial valley fill is

often so coarse and so unorganized as to look like till in the cut banks

along the streams, though its alluvial origin is always shown by the

topographic form. This characteristic is of special geologic interest

since the form may be concealed through deposition or destroyed by

erosion, and no condition but the structure remain to indicate the

manner of origin of the deposit. In such an event it would not be

possible to distinguish between alluvium and till. The gravity of the

distinction appears when it is known that such apparently unsorted

alluvium may extend for several miles forward of a terminal moraine, in

the shape of a widespreading alluvial fan apparently formed under

conditions of extremely rapid aggradation. I suppose it would not be

doubted in general that a section of such stony, bowldery, unsorted

material two miles long would have other than a glacial origin, yet such

may be the case. Indeed, if, as in the Urubamba Valley, a future section

should run parallel to the valley across the heads of a great series of

fans of similar composition, topographic form, and origin, it would be

possible to see many miles of such material.

The depth of the alluvial valley fill due to tributary fan accumulation

depends upon both the amount of the material and the form of the valley.

Below Urubamba in the Urubamba Valley a fine series is displayed, as

shown in Fig. 180. The fans head in valleys extending up to snow-covered

summits upon whose flanks living glaciers are at work today. Their heads

are now crowned by terminal moraines and both moraines and alluvial fans

are in process of dissection. The height and extent of the moraines and

the alluvial fans are in rough proportion and in turn reflect the

height, elevation, and extent of the valley heads which served as fields

of nourishment for the Pleistocene glaciers. Where the fans were

deposited in narrow valleys the effect was to increase the thickness of

the deposits at the expense of their area, to dam the drainage lines or

displace them, and to so load the streams that they have not yet

cleared their beds after thousands of years of work under torrential

conditions.

Below Urubamba the alluvial fans entering the main valley from the east

have pushed the river against its western valley wall, so that the river

flows on one side against rock and on the other against a hundred feet

of stratified material. In places, as at the head of the narrows on the

valley trail to Ollantaytambo, a flood plain has been formed in front of

the scarp cut into the alluvium, while the edge of the dissected

alluvial fans has been sculptured into erosion forms resembling

bad-lands topography. On the western side of the valley the alluvial

fans are very small, since they are due to purely local accumulations of

waste from the edge of the plateau. Glaciation has here displaced the

river. Its effects will long be felt in the disproportionate erosion of

the western wall of the valley.

By far the most interesting of the deposits of glacial time are those

laid down on the valley floors in the form of an alluvial fill. Though

such deposits have greater thickness as a rule near the nourishing

moraines or bordering alluvial fans at the lower ends of the valleys,

they are everywhere important in amount, distinctive in topographic

form, and of amazingly wide extent. They reach far into and possibly

across the Amazon basin, they form a distinct though small piedmont

fringe along the eastern base of the Andes, and they are universal

throughout the Andean valleys. That a deposit of such volume--many times

greater than all the material accumulated in the form of high-level

alluvial fans or terminal moraines--should originate in a tropical land

in a region that suffered but limited Alpine glaciation vastly increases

its importance.

[Illustration: FIG. 182--Dissected alluvial fans on the border of the

Urubamba Valley near Hacienda Chinche. A Characteristic feature of the

valleys of the Peruvian Andes below the zone of glaciation but within

the limits of its aggraditional effects. Through alluviation the valleys

and basins of the Andean Cordillera, and vast areas of the great Amazon

plains east of it, felt the effects of the glacial conditions of a past

age.]

The fill is composed of both fine and coarse material laid down by water

in steep valley floors to a depth of many feet. It breaks the steep

slope of each valley, forming terraces with pronounced frontal scarps

facing the river. On the raw bluffs at the scarps made by the

encroaching stream good exposures are afforded. At Chinche in the

Urubamba Valley above Santa Ana, the material is both sand and clay with

an important amount of gravel laid down with steep valleyward

inclination and under torrential conditions; so that within a given bed

there may be an apparent absence of lamination. Almost identical

conditions are exhibited frequently along the railway to Cuzco in the

Vilcanota Valley. The material is mixed sand and gravel, here and there

running to a bowldery or stony mass where accessions have been received

from some source nearby. It is modified along its margin not only in

topographic form but also in composition by small tributary alluvial

fans, though these in general constitute but a small part of the total

mass. At Cotahuasi, Fig. 29, there is a remarkable fill at least four

hundred feet deep in many places where the river has exposed fine

sections. The depth of the fill is, however, not determined by the

height of the erosion bluffs cut into it, since the bed of the river is

made of the same material. The rock floor of the valley is probably at

least an additional hundred feet below the present level of the river.

[Illustration: FIG. 183--Two-cycle slopes and alluvial fill between

Iluichihua and Chuquibambilla. The steep slopes on the inner valley

border are in many places vertical and rock cliffs are everywhere

abundant. Mature slopes have their greatest development here between

13,500 and 15,000 feet (4,110 to 4,570 m.). Steepest mature slopes run

from 15° to 21°. Least steep are the almost level spur summits. The

depths of the valley fill must be at least 300, and may possibly be 500

feet. The break between valley fill and steep slopes is most pronounced

where the river runs along the valley wall or undercuts it; least

pronounced where alluvial fans spread out from the head of some ravine.

It is a bowldery, stony fill almost everywhere terraced and cultivated.]

Similar conditions are well displayed at Huadquiña, where a fine series

of terraces at the lower end of the Torontoy Canyon break the descent of

the environing slopes; also in the Urubamba Valley below Rosalina, and

again at the edge of the mountains at the Pongo de Mainique. It is

exhibited most impressively in the Majes Valley, where the bordering

slopes appear to be buried knee-deep in waste, and where from any

reasonable downward extension of rock walls of the valley there would

appear to be at least a half mile of it. It is doubtful and indeed

improbable that the entire fill of the Majes Valley is glacial, for

during the Pliocene or early Pleistocene there was a submergence which

gave opportunity for the partial filling of the valley with non-glacial

alluvium, upon which the glacial deposits were laid as upon a flat and

extensive floor that gives an exaggerated impression of their depth.

However, the head of the Majes Valley contains at least six hundred feet

and probably as much as eight hundred feet of alluvium now in process of

dissection, whose coarse texture and position indicates an origin under

glacial conditions. The fact argues for the great thickness of the

alluvial material of the lower valley, even granting a floor of Pliocene

or early Pleistocene sediments. The best sections are to be found just

below Chuquibamba and again about halfway between that city and Aplao,

whereas the best display of the still even-floored parts of the valley

are between Aplao and Cantas, where the braided river still deposits

coarse gravels upon its wide flood plain.

CHAPTER XVI

GLACIAL FEATURES

THE SNOWLINE

South America is classical ground in the study of tropical snowlines.

The African mountains that reach above the snowline in the equatorial

belt--Ruwenzori, Kibo, and Kenia--have only been studied recently

because they are remote from the sea and surrounded by bamboo jungle and

heavy tropical forest. On the other hand, many of the tropical mountains

of South America lie so near the west coast as to be visible from it and

have been studied for over a hundred years. From the days of Humboldt

(1800) and Boussingault (1825) down to the present, observations in the

Andes have been made by an increasing number of scientific travelers.

The result is a large body of data upon which comparative studies may

now be profitably undertaken.

Like scattered geographic observations of many other kinds, the earlier

studies on the snowline have increased in value with time, because the

snowline is a function of climatic elements that are subject to periodic

changes in intensity and cannot be understood by reference to a single

observation. Since the discovery of physical proofs of climatic changes

in short cycles, studies have been made to determine the direction and

rate of change of the snowline the world over, with some very striking

results.

It has been found[55] that the changes run in cycles of from thirty to

thirty-five years in length and that the northern and southern

hemispheres appear to be in opposite phase. For example, since 1885 the

snowline in the southern hemisphere has been decreasing in elevation in

nine out of twelve cases by the average amount of nine hundred feet.

With but a single exception, the snowline in the northern hemisphere

has been rising since 1890 with an average increase of five hundred feet

in sixteen cases. To be sure, we must recognize that the observations

upon which these conclusions rest have unequal value, due both to

personal factors and to differences in instrumental methods, but that in

spite of these tendencies toward inequality they should agree in

establishing a general rise of the snowline in the northern hemisphere

and an opposite effect in the southern is of the highest significance.

It must also be realized that snowline observations are altogether too

meager and scattered in view of the abundant opportunities for making

them, that they should be standardized, and that they must extend over a

much longer period before they attain their full value in problems in

climatic variations. Once the possible significance of snowline changes

is appreciated the number and accuracy of observations on the elevation

and local climatic relations of the snowline should rapidly increase.

In 1907 I made a number of observations on the height of the snowline in

the Bolivian and Chilean Andes between latitudes 17° and 20° south, and

in 1911 extended the work northward into the Peruvian Andes along the

seventy-third meridian. It is proposed here to assemble these

observations and, upon comparison with published data, to make a few

interpretations.

From Central Lagunas, Chile, I went northeastward via Pica and the

Huasco Basin to Llica, Bolivia, crossing the Sillilica Pass in May,

1907, at 15,750 feet (4,800 m.). Perpetual snow lay at an estimated

height of 2,000-2,500 feet above the pass or 18,000 feet (5,490 m.)

above the sea. Two weeks later the Huasco Basin, 14,050 feet (4,280 m.),

was covered a half-foot deep with snow and a continuous snow mantle

extended down to 13,000 feet. Light snows are reported from 12,000 feet,

but they remain a few hours only and are restricted to the height of

exceptionally severe winter seasons (June and early July). Three or four

distant snow-capped peaks were observed and estimates made of the

elevation of the snowline between the Cordillera Sillilica and Llica on

the eastern border of the Maritime Cordillera. All observations agreed

in giving an elevation much in excess of 17,000 feet. In general the

values run from 18,000 to 19,000 feet (5,490 to 5,790 m.). Though the

bases of these figures are estimates, it should be noted that a large

part of the trail lies between 14,000 and 16,000 feet, passing mountains

snow-free at least 2,000 to 3,000 feet higher, and that for general

comparisons they have a distinct value.

In the Eastern Cordillera of Bolivia, snow was observed on the summit of

the Tunari group of peaks northwest of Cochabamba. Steinmann, who

visited the region in 1904, but did not reach the summit of the Tunari

group of peaks, concludes that the limit of perpetual snow should be

placed above the highest point, 17,300 (5,270 m.); but in July and

August, 1907, I saw a rather extensive snow cover over at least the

upper 1,000 feet, and what appeared to be a very small glacier. Certain

it is that the Cochabamba Indians bring clear blue ice from the Tunari

to the principal hotels, just as ice is brought to Cliza from the peaks

above Arani. On these grounds I am inclined to place the snowline at

17,000 feet (5,180 m.) near the eastern border of the Eastern

Cordillera, latitude 17° S. At 13,000 feet, in July, 1907, snow occurred

in patches only on the pass called Abre de Malaga, northeast of Colomi,

13,000 feet, and fell thickly while we were descending the northern

slopes toward Corral, so that in the early morning it extended to the

cold timber line at 10,000 feet. In a few hours, however, it had

vanished from all but the higher and the shadier situations.

In the Vilcanota knot above the divide between the Titicaca and

Vilcanota hydrographic systems, the elevation of the snowline was

16,300+ feet (4,970 m.) in September, 1907. On the Cordillera Real of

Bolivia it is 17,000 to 17,500 feet on the northeast, but falls to

16,000 feet on the southwest above La Paz. In the first week of July,

1911, snow fell on the streets of Cuzco (11,000 feet) and remained for

over an hour. The heights north of San Geronimo (16,000 feet) miss the

limit of perpetual snow and are snow-covered only a few months each

year.

In taking observations on the snowline along the seventy-third meridian

I was fortunate enough to have a topographer the heights of whose

stations enabled me to correct the readings of my aneroid barometer

whenever these were taken off the line of traverse. Furthermore, the

greater height of the passes--15,000 to 17,600 feet--brought me more

frequently above the snowline than had been the case in Bolivia and

Chile. More detailed observations were made, therefore, not only upon

the elevation of the snowline from range to range, but also upon the

degree of canting of the snowline on a given range. Studies were also

made on the effect of the outline of the valleys upon the extent of the

glaciers, the influence on the position of the snowline of mass

elevation, precipitation, and cloudiness.

Snow first appears at 14,500 feet (4,320 m.) on the eastern flanks of

the Cordillera Vilcapampa, in 13° south latitude. East of this group of

ridges and peaks as far as the extreme eastern border of the mountain

belt, fifty miles distant, the elevations decrease rapidly to 10,000

feet and lower, with snow remaining on exceptionally high peaks from a

few hours to a few months. In the winter season snow falls now and then

as low as 11,500 feet, as in the valley below Vilcabamba pueblo in early

September, 1911, though it vanishes like mist with the appearance of the

sun or the warm up-valley winds from the forest. Storms gather daily

about the mountain summits and replenish the perpetual snow above 15,000

feet. In the first pass above Puquiura we encountered heavy snow banks

on the northeastern side a hundred feet below the pass (14,500 feet),

but on the southwestern or leeward side it is five hundred feet lower.

This distribution is explained by the lesser insolation on the

southwestern side, the immediate drifting of the clouds from the

windward to the leeward slopes, and to the mutual intensification of

cause and effect by topographic changes such as the extension of

collecting basins and the steeping of the slopes overlooking them with a

corresponding increase in the duration of shade.

It is well known that with increase of elevation and therefore of the

rarity of the air there is less absorption of the sun’s radiant energy,

and a corresponding increase in the degree of insolation. It follows,

therefore, that at high altitudes the contrasts between sun and shade

temperatures will increase. Frankland[56] has shown that the increase

may run as high as 500 per cent between 100 to 10,000 feet above the

sea. I have noted a fall of temperature of 15° F. in six minutes, due to

the obscuring of the sun by cloud at an elevation of 16,000 feet above

Huichihua in the Central Ranges of Peru. Since the sun shines

approximately half the time in the snow-covered portions of the

mountains and since the tropical Andes are of necessity snow-covered

only at lofty elevations, this contrast between shade and sun

temperatures is by far the most powerful factor influencing differences

in elevation of the snowline in Peru.

To the drifting of the fallen snow is commonly ascribed a large portion

of this contrast. I have yet to see any evidence of its action near the

snowline, though I have often observed it, especially under a high wind

in the early morning hours at considerable elevations above the

snowline, as at the summits of lofty peaks. It appears that the lower

ranges bearing but a limited amount of snow are not subject to drifting

because of the wetness of the snow, and the fact that it is compacted by

occasional rains and hail storms. Only the drier snow at higher

elevations and under stronger winds can be effectively dislodged.

The effect of unequal distribution of precipitation on the windward and

leeward slopes of a mountain range is in general to depress the snowline

on the windward slopes where the greater amount falls, but this may be

offset in high altitudes by temperature contrasts as in the westward

trending Cordillera Vilcapampa, where north and south slopes are in

opposition. If the Cordillera Vilcapampa ran north and south we should

have the windward and leeward slopes equally exposed to the sun and the

snowline would lie at a lower elevation on the eastern side. Among all

the ranges the slopes have decreasing precipitation to the leeward, that

is, westerly. The second and third passes, between Arma and Choquetira,

are snow-free (though their elevations equal those of the first pass)

because they are to leeward of the border range, hence receive less

precipitation. The depressive effect of increased precipitation on the

snowline is represented by A-B, Fig. 184; in an individual range the

effect of heavier precipitation may be offset by temperature contrasts

between shady and sunny slopes, as shown by the line a-b in the same

figure.

The degree of canting of the snowline on opposite slopes of the

Cordillera Vilcapampa varies between 5° and 12°, the higher value being

represented four hours southwest of Arma on the Choquetira trail,

looking northeast. A general view of the Cordillera looking east at this

point (Fig. 186), shows the appearance of the snowline as one looks

along the flanks of the range. In detail the snowline is further

complicated by topography and varying insolation, each spur having a

snow-clad and snow-free aspect as shown in the last figure. The degree

of difference on these minor slopes may even exceed the difference

between opposite aspects of the range in which they occur.

[Illustration: FIG. 184--To illustrate the canting of the snowline.

_A-B_ is the snowline depressed toward the north (right) in response to

heavier precipitation. The line _a-b_ represents a depression in the

opposite direction due to the different degree of insolation on the

northern (sunny) and southern (shady) slopes.]

To these diversifying influences must be added the effect of warm

up-valley winds that precede the regular afternoon snow squalls and that

melt the latest fall of snow to exceptionally high elevations on both

the valley floor and the spurs against which they impinge. The influence

of the warmer air current is notably confined to the heads of those

master valleys that run down the wind, as in the valley heading at the

first pass, Cordillera Vilcapampa, and at the heads of the many valleys

terminating at the passes of the Maritime Cordillera. Elsewhere the

winds are dissipated in complex systems of minor valleys and their

effect is too well distributed to be recognized.

It is clear from the conditions of the problem as outlined on preceding

pages that the amount of canting may be expressed in feet of difference

of the snowline on opposite sides of a range or in degrees. The former

method has, heretofore, been employed. It is proposed that this method

should be abolished and degrees substituted, on the following grounds:

Let _A_ and _B_, Fig. 190, represent two mountain masses of unequal area

and unequal elevation. Let the opposite ends of the snowlines of both

figures lie 1,000 feet apart as between the windward and leeward sides

of a broad cordillera (A), or as between the relatively sunnier and

relatively shadier slopes of individual mountains or narrow ranges in

high latitudes or high altitudes (B). With increasing elevation there is

increasing contrast between temperatures in sunshine and in shade, hence

a greater degree of canting (B). Tending toward a still greater degree

of contrast is the effect of the differences in the amounts of snowy

precipitation, which are always more marked on an isolated and lofty

mountain summit than upon a broad mountain mass (1) because in the

former there is a very restricted area where snow may accumulate, and

(2) because with increase of elevation there is a rapid and differential

decrease in both the rate of adiabatic cooling and the amount of water

vapor; hence the snow-producing forces are more quickly dissipated.

[Illustration: FIG. 185--Glacial features in the Peruvian Andes near

Arequipa. Sketched from a railway train, July, 1911. The horizontal

broken lines represent the lower limit of light snow during late June,

1911. There is a fine succession of moraines in U-shaped valleys in all

the mountains of the Arequipa region. _A_ represents a part of Chacchani

northwest of Arequipa; _B_ is looking south by east at the northwest end

of Chacchani near Pampa de Arrieros; _C_ also shows the northwest end of

Chacchani from a more distant point.]

[Illustration: FIG. 186--Canted snowline in the Cordillera Vilcapampa

between Arma and Choquetira. Looking east from 13,500 feet.]

[Illustration: FIG. 187--Glacial topography between Lambrama and

Antabamba in the Central Ranges. A recent fall of snow covers the

foreground. The glaciers are now almost extinct and their action is

confined to the deepening and steepening of the cirques at the valley

heads.]

[Illustration: FIG. 188--Asymmetrical peaks in the Central Ranges

between Antabamba and Lambrama. The snow-filled hollows in the

photograph face away from the sun--that is, south--and have retained

snow since the glacial epoch; while the northern slopes are snow-free.

There is no true glacial ice and the continued cirque recession is due

to nivation.]

[Illustration: FIG. 189--Glacial topography north of the divide on the

seventy-third meridian. Maritime Cordillera. Looking downstream at an

elevation of 16,500 feet (5,030 m.).]

Furthermore, the leeward side of a lofty mountain not only receives much

less snow proportionally than the leeward side of a lower mountain,

but also loses it faster on account of the smaller extent of surface

upon which it is disposed and the proportionally larger extent of

counteractive, snow-free surface about it. Among the volcanoes of

Ecuador are many that show differences of 500 feet in snowline elevation

on windward and leeward (east) slopes and some, as for example

Chimborazo, that exhibit differences of 1,000 feet. The latter figure

also expresses the differences in the broad Cordillera Vilcapampa and in

the Maritime Cordillera, though the _rate_ of canting as expressed in

degrees is much greater in the case of the western mountains.

[Illustration: FIG. 190--To illustrate the difference in the degree of

canting of the snowline on large and on small mountain masses.]

The advantages of the proposed method of indicating the degree of

canting of the snowline lie in the possibility thus afforded of

ultimately separating and expressing quantitatively the various factors

that affect the position of the line. In the Cordillera Vilcapampa, for

example, the dominant canting force is the difference between sun and

shade temperatures, while in the volcanoes of Ecuador, where

_symmetrical volcanoes, almost on the equator, have equal insolation on

all aspects_ and the temperature contrasts are reduced to a minimum--the

differences are owing chiefly to varying exposure to the winds. The

elusive factors in the comparison are related to the differences in area

and in elevation.

The value of arriving finally at close snowline analyses grows out of

(1) the possibility of snowline changes in short cycles and (2)

uncertainty of arriving by existing methods at the snowline of the

glacial period, whose importance is fundamental in refined physiographic

studies in glaciated regions with a complex topography. To show the

application of the latter point we shall now attempt to determine the

snowline of the glacial period in the belt of country along the route of

the Expedition.

In the group of peaks shown in Fig. 188 between Lambrama and Antabamba,

the elevation of the snowline varies from 16,000 to 17,000 feet

(4,880-5,180 m.), depending on the topography and the exposure. The

determination of the limit of perpetual snow was here, as elsewhere

along the seventy-third meridian, based upon evidences of nivation. It

will be observed in Fig. 191 that just under the snow banks to the left

of the center are streams of rock waste which head in the snow. Their

size is roughly proportional to the size of the snow banks, and,

furthermore, they are not found on snow-free slopes. From these facts it

is concluded that they represent the waste products of snow erosion or

nivation, just as the hollows in which the snow lies represent the

topographic products of nivation. On account of the seasonal and annual

variation in precipitation and temperature--hence in the elevation of

the snowline--it is often difficult to make a correct snowline

observation based upon depth and _apparent_ permanence. Different

observers report great changes in the snowline in short intervals,

changes not explained by instrumental variations, since they are

referred to topographic features. It appears to be impossible to rely

upon present records for small changes possibly related to minor

climatic cycles because of a lack of standardization of observations.

Nothing in the world seems simpler at first sight than an observation on

the elevation of the snowline. Yet it can be demonstrated that large

numbers of observers have merely noted the position of temporary snow.

It is strongly urged that evidences of nivation serve henceforth as

proof of permanent snow and that photographic records be kept for

comparison. In this way measurements of changes in the level of the

snowline may be accurately made and the snow cover used as a climatic

gauge.

Farther west in the Maritime Cordillera, the snowline rises to 18,000

feet on the northern slopes of the mountains and to 17,000 feet on the

southern slopes. The top of the pass above Cotahuasi, 17,600 feet (5,360

m.), was snow-free in October, 1911, but the snow extended 500 feet

lower on the southern slope. The degree of canting is extraordinary at

this point, single volcanoes only 1,500 to 2,000 feet above the general

level and with bases but a few miles in circumference exhibit a thousand

feet of difference in the snowline upon northern and southern aspects.

This is to be attributed no less to the extreme elevation of the snow

(and, therefore, stronger contrasts of shade and sun temperatures) than

to the extreme aridity of the region and the high daytime temperatures.

The aridity is a factor, since heavy snowfall means a lengthening of the

period of precipitation in which a cloud cover shuts out the sun and a

shortening of the period of insolation and melting.

Contrasts between shade and sun temperatures increase with altitude but

their effects also increase in _time_. Of two volcanoes of equal size

and both 20,000 feet above sea level, that one will show the greater

degree of canting that is longer exposed to the sun. The high daytime

temperature is a factor, since it tends to remove the thinnest snow,

which also falls in this case on the side receiving the greatest amount

of heat from the sun. The high daytime temperature is phenomenal in this

region, and is owing to the great extent of snow-free land at high

elevations and yet below the snowline, and to the general absence of

clouds and the thinness of vegetation.

On approach to the western coast the snowline descends again to 17,500

feet on Coropuna. There are three chief reasons for this condition.

First, the well-watered Majes Valley is deeply incised almost to the

foot of Coropuna, above Chuquibamba, and gives the daily strong sea

breeze easy access to the mountain. Second, the Coast Range is not only

low at the mouth of the Majes Valley, but also is cut squarely across by

the valley itself, so that heavy fogs and cloud sweep inland nightly and

at times completely cover both valley and desert for an hour after

sunrise. Although these yield no moisture to the desert or the valley

floor except such as is mechanically collected, yet they do increase the

precipitation upon the higher elevations at the valley head.

[Illustration: THE YALE PERUVIAN EXPEDITION OF 1911

HIRAM BINGHAM DIRECTOR

ANTABAMBA QUADRANGLE]

A third factor is the size of Coropuna itself. The mountain is not a

simple volcano but a composite cone with five main summits reaching well

above the snowline, the highest to an elevation of 21,703 feet (6,615

m.). It measures about 20 miles (32 km.) in circumference at the

snowline and 45 miles (72 km.) at its base (measuring at the foot of the

steeper portion), and stands upon a great tributary lava plateau from

15,000 to 17,000 feet above sea level. Compared with El Misti, at

Arequipa, its volume is three times as great, its height two thousand

feet more, and its access to ocean winds at least thirty per cent more

favorable. El Misti, 19,200 feet (5,855 m.) has snow down as far as

16,000 feet in the wet season and rarely to 14,000 feet, though by

sunset a fall of snow may almost disappear whose lower limit at sunrise

was 16,000 feet. Snow may accumulate several thousand feet below the

summit during the wet season, and in such quantities as to require

almost the whole of the ensuing dry season (March to December) for its

melting. Northward of El Misti is the massive and extended range,

Chachani, 20,000 feet (6,100 m.) high; on the opposite side is the

shorter range called Pichu-Pichu. Snow lies throughout the year on both

these ranges, but in exceptional seasons it nearly disappears from

Chachani and wholly disappears from Pichu-Pichu, so that the snowline

then rises to 20,000 feet. It is considered that the mean of a series of

years would give a value between 17,000 and 18,000 feet for the snowline

on all the great mountains of the Arequipa region.[57] This would,

however, include what is known to be temporary snow; the limit of

“perpetual” snow, or the true snowline, appears to lie about 19,000 feet

on Chachani and _above_ El Misti, say 19,500 feet. It is also above the

crest of Pichu-Pichu. The snowline, therefore, appears to rise a

thousand feet from Coropuna to El Misti, owing chiefly to the poorer

exposure of the latter to the sources of snowy precipitation.

It may also be noted that the effect of the easy access of the ocean

winds in the Coropuna region is also seen in the increasing amount of

vegetation which appears in the most favorable situations. Thus, along

the Salamanca trail only a few miles from the base of Coropuna are a few

square kilometers of _quenigo_ woodland generally found in the cloud

belt at high altitudes; for example, at 14,000 feet above Lambrama and

at 9,000 feet on the slope below Incahuasi, east of Pasaje. The greater

part of the growth is disposed over hill slopes and on low ridges and

valley walls. It is, therefore, clearly unrelated as a whole to the

greater amount of ground-water with which a part is associated, as along

the valley floors of the streams that head in the belt of perpetual

snow. The appearance of this growth is striking after days of travel

over the barren, clinkery lava plateau to eastward that has a less

favorable exposure. The _quenigo_ forest, so-called, is of the greatest

economic value in a land so desolate as the vast arid and semi-arid

mountain of western Peru. Every passing traveler lays in a stock of

fire-wood as he rests his beasts at noonday; and long journeys are made

to these curious woodlands from both Salamanca and Chuquibamba to gather

fuel for the people of the towns.

NIVATION

The process of nivation, or snow erosion, does not always produce

visible effects. It may be so feeble as to make no impression upon very

resistant rock where the snow-fall is light and the declivity low.

Ablation may in such a case account for almost the whole of the snow

removed. On strong and topographically varied slopes where the snow is

concentrated in headwater alcoves, there is a more pronounced downward

movement of the snow masses with more prominent effects both of erosion

beneath the snow and of accumulation at the border of the snow. In such

cases the limit of perpetual snow may be almost as definitely known as

the limit of a glacier. Like glaciers these more powerful snow masses

change their limits in response to regional changes in precipitation,

temperature, or both. It would at first sight appear impossible to

distinguish between these changes through the results of nivation. Yet

in at least a few cases it may be as readily determined as the past

limits of glaciers are inferred from the terminal moraines, still

intact, that cross the valley floors far below the present limits of the

ice.

In discussing the process of nivation it is necessary to assume a

sliding movement on the part of the snow, though it is a condition in

Matthes’ original problem in which the nivation idea was introduced that

the snow masses remain stationary. It is believed, however, that

Matthes’ valuable observations and conclusions really involve but half

the problem of nivation; or at the most but one of two phases of it. He

has adequately shown the manner in which that phase of nivation is

expressed which we find _at the border of the snow_. Of the action

_beneath_ the snow he says merely: “Owing to the frequent oscillations

of the edge and the successive exposure of the different parts of the

site to frost action, the area thus affected will have no well-defined

boundaries. The more accentuated slopes will pass insensibly into the

flatter ones, and the general tendency will be to give the drift site a

cross section of smoothly curved outline and ordinarily concave.”[58]

From observations on the effects of nivation in valleys, Matthes further

concludes that “on a grade of about 12 per cent ... névé must attain a

thickness of at least 125 feet in order that it may have motion,”[59]

though as a result of the different line of observations Hobbs

concludes[60] that a somewhat greater thickness is required.

[Illustration: FIG. 191--The “pocked” surface characteristically

developed in the zone of light nivation. Compare with Fig. 194, showing

the effects of heavy nivation.]

[Illustration: FIG. 192--Steep cirque walls and valleys head in the

Central Ranges between Lambrama and Chuquibambilla. The snow is here a

vigorous agent in transporting talus material and soil from all the

upper slopes down to the foot of the cirque wall.]

The snow cover in tropical mountains offers a number of solid advantages

in this connection. Its limits, especially on the Cordillera Vilcapampa,

on the eastern border of the Andes, are subject to _small seasonal

oscillations_ and the edge of the “perpetual” snow is easily determined.

Furthermore, it is known from the comparatively “fixed quality of

tropical climate,” as Humboldt put it, that the variations of the

snowline in a period of years do not exceed rather narrow limits. In

mid-latitudes on the contrary there is an extraordinary shifting of the

margin of the snow cover, and a correspondingly wide distribution of

the feeble effects of nivation.

[Illustration: FIG. 193--Panta Mountain and its glacier system. The

talus-covered mass in the center (B) is a terminal moraine topped by the

dirt-stained glacier that descends from the crest. The separate glaciers

were formerly united to form a huge ice tongue that truncated the

lateral spurs and flattened the valley floor. One of its former stages

is shown by the terminal moraine in the middle distance, breached by a

stream, and impounding a lake not visible from this point of view.]

[Illustration: FIG. 194--Recessed southern slopes of volcanoes whose

northern slopes are practically without glacial modifications. Summit of

the lava plateau, Maritime Cordillera, western Peru, between Antabamba

and Cotahuasi.]

Test cases are presented in Figs. 191, 192, and 193, Cordillera

Vilcapampa, for the determination of the fact of the movement of the

snow long before it has reached the thickness Matthes or Hobbs believes

necessary for a movement of translation to begin. Fig. 191 shows snow

masses occupying pockets on the slope of a ridge that was never covered

with ice. Past glacial action with its complicating effects is,

therefore, excluded and we have to deal with snow action pure and

simple. The pre-glacial surface with smoothly contoured slopes is

recessed in a noteworthy way from the ridge crest to the snowline of the

glacial period at least a thousand feet lower. The recesses of the

figure are peculiar in that not even the largest of them involve the

entire surface from top to bottom; they are of small size and are

scattered over the entire slope. This is believed to be due to the fact

that they represent the limits of variations of the snowline in short

cycles. Below them as far as the snowline of the glacial period are

larger recesses, some of which are terminated by masses of waste as

extensive as the neighboring moraines, but disposed in irregular

scallops along the borders of the ridges or mountain slopes in which the

recesses have been found.

The material accumulated at the lower limit of the snow cover of the

glacial period was derived from two sources: (1) from slopes and cliffs

overlooking the snow, (2) from beneath the snow by a process akin to ice

plucking and abrasion. The first process is well known and resembles the

shedding of waste upon a valley glacier or a névé field from the

bordering cliffs and slopes. Material derived in this manner in many

places rolls down a long incline of snow and comes to rest at the foot

of it as a fringe of talus. The snow is in this case but a substitute

for a normal mass of talus. The second process produces its most clearly

recognizable effects on slopes exceeding a declivity of 20°; and upon

30° and 40° slopes its action is as well-defined as true glacial action

which it imitates. It appears to operate in its simplest form as if

independent of the mass of the snow, small and large snow patches

showing essentially the same results. This is the reverse of Matthes’

conclusion, since he says that though the minimum thickness “must vary

inversely with the percentage of the grade,” “the influence of the grade

is inconsiderable,” and that the law of variation must depend upon

additional observation.[61]

Let us examine a number of details and the argument based upon them and

see if it is not possible to frame a satisfactory law of variation.

In Fig. 193 the chief conditions of the problem are set forth. Forward

from the right-hand peak are snow masses descending to the head of a

talus (_A_) whose outlines are clearly defined by freshly fallen snow.

At (_B_) is a glacier whose tributaries descend the middle and left

slopes of the picture after making a descent from slopes several

thousand feet higher and not visible in this view. The line beneath the

glacier marks the top of the moraine it has built up. Moraines farther

down valley show a former greater extent of the glacier. Clearly the

talus material at (_A_) was accumulated after the ice had retreated to

its present position. It will be readily seen from an inspection of the

photograph that the total amount of material at (_A_) is an appreciable

fraction of that in the moraine. The ratio appears to be about 1:8 or

1:10. I have estimated that the total area of snow-free surface about

the snowfields of the one is to that of the other as 2:3. The gradients

are roughly equivalent, but the volume of snow in the one case is but a

small fraction of that in the other. It will be seen that the snow

masses have recessed the mountain slopes at _A_ and formed deep hollows

and that the hollowing action appears to be most effective where the

snow is thickest.

Summarizing, we note first, that the roughly equivalent factors are

gradient and amount of snow-free surface; second, that the unequal

factors are (a) accumulated waste, (b) degree of recessing, and (c) the

degree of compacting of snow into ice and a corresponding difference in

the character of the glacial agent, and (d) the extent of the snow

cover. The direct and important relation of the first two unequal

factors to the third scarcely need be pointed out.

We have then an inequality in amount of accumulated material to be

explained by either an inequality in the extent of the snow and

therefore an inequality of snow action, or an inequality due to the

presence of ice in one valley and not in the other, or by both. It is at

once clear that if ice is absent above (_A_) and the mountain slopes are

recessed that snow action is responsible for it. It is also recognized

that whatever rate of denudation be assigned to the snow-free surfaces

this rate must be exceeded by the rate of snow action, else the

inequalities of slope would be decreased rather than increased. The

accumulated material at (_A_) is, therefore, partly but not chiefly due

to denudation of snow-free surfaces. It is due chiefly to _erosion_

beneath the snow. Nor can it be argued that the hollows now occupied by

snow were formed at some past time when ice not snow lay in them. They

are not ice-made hollows for they are on a steep spur above the limits

of ice action even in the glacial period. Any past action is, therefore,

represented here in _kind_ by present action, though there would be

differences in _degree_ because the heavier snows of the past were

displaced by the lighter snows of today.

While it appears that the case presents clear proof of degradation by

snow it is not so clear how these results were accomplished. Real

abrasion on a large scale as in bowlder-shod glaciers is ruled out,

since glacial striæ are wholly absent from nivated surfaces according to

both Matthes’ observations and my own. Yet all nivated surfaces have

very distinctive qualities, delicately organized slopes which show a

marked change from any original condition related to water-carving. In

the absence of striæ, the general absence of all but a thin coating of

waste _even in rock hollows_, and the accumulation of waste up to

bowlders in size at the lower edge of the nivated zone, I conclude that

compacted snow or névé of sufficient thickness and gradient may actually

pluck rock outcrops in the same manner though not at the rate which ice

exhibits. That the products of nivation may be bowlders as well as fine

mud would seem clearly to follow increase in effectiveness, due to

increase in amount of the accumulated snow; that bowlders are actually

transported by snow is also shown by their presence on the lower margins

of nivated tracts.

Our argument may be made clearer by reference to the observed action of

snow in a particular valley. Snow is shed from the higher, steeper

slopes to the lower slopes and eventually accumulates to a marked degree

on the bottoms of the depressions, whence it is avalanched down valley

over a series of irregular steps on the valley floor. An avalanche takes

place through the breaking of a section of snow just as an iceberg

breaks off the end of a tide-water glacier. Evidently there must be

pressure from behind which crowds the snow forward and precipitates it

to a lower level.

As a snow mass falls it not only becomes more consolidated, beginning at

the plane of impact, but also gives a shock to the mass upon which it

falls that either starts it in motion or accelerates its rate of motion.

The action must therefore be accompanied by a drag upon the floor and if

the rock be close-jointed and the blocks, defined by the joint planes,

small enough, they will be transported. Since snow is not so compact as

ice and permits included blocks easily to adjust themselves to new

resistances, we should expect the detached blocks included in the snow

to change their position constantly and to form irregular scratches, but

not parallel striæ of the sort confidently attributed to stone-shod ice.

It is to the plasticity of snow that we may look for an explanation of

the smooth-contoured appearance of the landscape in the foreground of

Fig. 135. The smoothly curved lines are best developed where the entire

surface was covered with snow, as in mid-elevations in the larger

snowfields. At higher elevations, where the relief is sharper, the snow

is shed from the steeper declivities and collected in the minor basins

and valley heads, where its action tends to smooth a floor of limited

area, while snow-free surfaces retain all their original irregularities

of form or are actually sharpened.

The degree of effectiveness of snow and névé action may be estimated

from the reversed slopes now marked by ponds or small marshy tracts

scattered throughout the former névé fields, and the many niched

hollows. They are developed above Pampaconas in an admirable manner,

though their most perfect and general development is in the summit belt

of the Cordillera Vilcapampa between Arma and Choquetira, Fig. 135. It

is notable in _all_ cases where nivation was associated with the work of

valley glaciers that the rounded nivated slopes break rather sharply

with the steep slopes that define an inner valley, whose form takes on

the flat floor and under-cut marginal walls normal to valley glaciation.

A classification of numerous observations in the Cordillera Vilcapampa

and in the Maritime Cordillera between Lambrama and Antabamba may now be

presented as the basis for a tentative expression of the law of

variation respecting snow motion. The statement of the law should be

prefaced by the remark that thorough checking is required under a wider

range of conditions before we accept the law as final. Near the lower

border of the snow where rain and hail and alternate freezing and

thawing take place, the snow is compacted even though but fifteen to

twenty feet thick, and appears to have a down-grade movement and to

exercise a slight drag upon its floor when the gradient does not fall

below 20°. Distinct evidences of nivation were observed on slopes with a

declivity of 5° near summit areas of past glacial action, where the snow

did not have an opportunity to be alternately frozen and thawed.

The _thickness_ of the former snow cover could, however, not be

accurately determined, but was estimated from the topographic

surroundings to have been at least several hundred feet. Upon a 40°

slope a snow mass 50 feet thick was observed to be breaking off at a

cliff-face along the entire cross-section as if impelled forward by

thrust, and to be carrying a small amount of waste--enough distinctly to

discolor the lowermost layers--which was shed upon the snowy masses

below. With increase in the degree of compactness of the snow at

successively lower elevations along a line of snow discharge, gradients

down to 25° were still observed to carry strongly crevassed, waste-laden

snow down to the melting border. It appeared from the clear evidences of

vigorous action--the accumulation of waste, the strong crevassing, the

stream-like character of the discharging snow, and the pronounced

topographic depression in which it lay--that much flatter gradients

would serve, possibly not more than 15°, for a snow mass 150 feet wide,

30 to 40 feet thick, and serving as the outlet for a set of tributary

slopes about a square mile in area and with declivities ranging from

small precipices to slopes of 30°.

We may say, therefore, that the factors affecting the rate of motion are

(1) thickness, (2) degree of compactness, (3) diurnal temperature

changes, and (4) gradient. Among these, diurnal temperature changes

operate indirectly by making the snow more compact and also by inducing

motion directly. At higher elevations above the snowline, temperature

changes play a decreasingly important part. The thickness required

varies inversely as the gradient, and upon a 20° slope is 20 feet for

wet and compact snow subjected to alternate freezing and thawing. For

dry snow masses above the zone of effective diurnal temperature changes,

an increasing gradient is required. With a gradient of 40°, less than 50

feet of snow will move _en masse_ if moderately compacted under its own

weight; if further compacted by impact of falling masses from above, the

required thickness may diminish to 40 feet and the required declivity to

15°. The gradient may decrease to 0° or actually be reversed and motion

still continue provided the compacting snow approach true névé or even

glacier ice as a limit.

From the sharp topographic break between the truly glaciated portions of

the valley in regions subjected to temporary glaciation, it is concluded

that the eroding power of the moving mass is suddenly increased at the

point where névé is finally transformed into true ice. This

transformation must be assumed to take place suddenly to account for so

sudden a change of function as the topographic break requires. Below the

point at which the transformation occurs the motion takes place under a

new set of conditions whose laws have already been formulated by

students of glaciology.

[Illustration: FIG. 195--Curve of snow motion. Based on many

observations of snow motion to show minimum thickness of snow required

to move on a given gradient. Figures on the left represent thickness of

snow in feet. The degrees represent the gradient of the surface. The

gradients have been run in sequence down to 0° for the sake of

completing the accompanying discussion. Obviously no glacially

unmodified valley in a region of mountainous relief would start with so

low a gradient, though glacial action would soon bring it into

existence. Between +5° and -5° the curve is based on the gradients of

nivated surfaces.]

The foregoing readings of gradient and depth of snow are typical of a

large number which were made in the Peruvian Andes and which have served

as the basis of Fig. 195. It will be observed that between 15° and 20°

there is a marked change of function and again between +5° and -5°

declivity, giving a double reversed curve. The meaning of the change

between 15° and 20° is inferred to be that, with gradients over 20°,

snow cannot wholly resist gravity in the presence of diurnal temperature

changes across the freezing point and occasional snow or hail storms.

With increase of thickness compacting appears to progress so rapidly as

to permit the transfer of thrust for short distances before absorption

of thrust takes place in the displaced snow. At 250 feet thorough

compacting appears to take place, enabling the snow to move out under

its own weight on even the faintest slopes; while, with a thickness

still greater, the resulting névé may actually be forced up slight

inclines whose declivity appears to approach 5° as a limit. I have

nowhere been able to find in truly nivated areas reversed curves

exceeding 5°, though it should be added that depressions whose leeward

slopes were reversed to 2° and 3° are fairly common. If the curve were

continued we should undoubtedly find it again turning to the left at the

point where the thickness of the snow results in the transformation of

snow to ice. From the sharp topographic break observed to occur in a

narrow belt between the névé and the ice, it is inferred that the

erosive power of the névé is to that of the ice as 2:4 or 1:5 _for equal

areas_; and that reversed slopes of a declivity of 10° to 15° may be

formed by glaciers is well known. Precisely what thickness of snow or

névé is necessary and what physical conditions effect its transformation

into ice are problems not included in the main theme of this chapter.

It is important that the proposed curve of snow motion under minimum

conditions be tested under a large variety of circumstances. It may

possibly be found that each climatic region requires its special

modifications. In tropical mountains the sudden alternations of freezing

and thawing may effect such a high degree of compactness in the snow

that lower minimum gradients are required than in the case of

mid-latitude mountains where the perpetual snow of the high and cold

situations is compacted through its own weight. Observations of the

character introduced here are still unattainable, however. It is hoped

that they will rapidly increase as their significance becomes apparent;

and that they have high significance the striking nature of the curve of

motion seems clearly to establish.

BERGSCHRUNDS AND CIRQUES

The facts brought out by the curve of snow-motion (Fig. 195) have an

immediate bearing on the development of cirques, whose precise mode of

origin and development have long been in doubt. Without reviewing the

arguments upon which the various hypotheses rest, we shall begin at once

with the strongest explanation--W. D. Johnson’s famous bergschrund

hypothesis. The critical condition of this hypothesis is the diurnal

migration across the freezing point of the air temperature at the bottom

of the schrund. Alternate freezing and thawing of the water in the

joints of the rock to which the schrund leads, exercise a quarrying

effect upon the rock and, since this effect is assumed to take place at

the foot of the cirque, the result is a steady retreat of the steep

cirque wall through basal sapping.

While Johnson’s hypothesis has gained wide acceptance and is by many

regarded as the final solution of the cirque problem it has several

weaknesses in its present form. In fact, I believe it is but one of two

factors of equal importance. In the first place, as A. C. Andrews[62]

has pointed out, it is extremely improbable that the bergschrund of

glacial times under the conditions of a greater volume of snow could

have penetrated to bedrock at the base of the cirque where the present

change of slope takes place. In the second place, the assumption is

untenable that the bergschrund in all cases reaches to or anywhere near

the foot of the cirque wall. A third condition outside the hypothesis

and contradictory to it is the absence of a bergschrund in snowfields at

many valleys heads where cirques are well developed!

Johnson himself called attention to the slender basis of observation

upon which his conclusions rest. In spite of his own caution with

respect to the use of his meager data, his hypothesis has been applied

in an entirely too confident manner to all kinds of cirques under all

kinds of conditions. Though Johnson descended an open bergschrund to a

rock floor upon which ice rested, his observations raise a number of

proper questions as to the application of these valuable data: How long

are bergschrunds open? How often are they open? Do they everywhere open

to the foot of the cirque wall? Are they present for even a part of the

year in all well-developed cirques? Let us suppose that it is possible

to find many cirques filled with snow, not ice, surrounded by truly

precipitous walls and with an absence of bergschrunds, how shall we

explain the topographic depressions excavated underneath the snow? If

cirque formation can be shown to take place without concentrated frost

action at the foot of the bergschrund, then is the bergschrund not a

secondary rather than a primary factor? And must we not further conclude

that when present it but hastens an action which is common to all

snow-covered recesses?

It is a pleasure to say that we may soon have a restatement of the

cirque problem from the father of the bergschrund idea. The argument in

this chapter was presented orally to him after he had remarked that he

was glad to know that some one was finding fault with his hypothesis.

“For,” he said, with admirable spirit, “I am about to make a most

violent attack upon the so-called Johnson hypothesis.” I wish to say

frankly that while he regards the following argument as a valid addition

to the problem, he does not think that it solves the problem. There are

many of us who will read his new explanation with the deepest interest.

[Illustration: FIG. 196--Relation of cirque wall to trough’s end at the

head of a glaciated valley. The ratio of the inner to the outer radius

is 1:4.]

[Illustration: FIG. 197--Mode of cirque formation. Taking the facts of

snow depth represented in the curve, Fig. 195, and transposing them over

a profile (the heavy line) which ranges from 0° declivity to 50°, we

find that the greatest excess of snow occurs roughly in the center. Here

ice will first form at the bottom of the snow in the advancing hemicycle

of glaciation, and here it will linger longest in the hemicycle of

retreat. Here also there will be the greatest mass of névé. All of these

factors are self-stimulating and will increase in time until the floor

of the cirque is flattened or depressed sufficiently to offset through

uphill ice-flow the augmented forces of erosion. The effects of

self-stimulation are shown by “snow increase”; the ice shoe at the

bottom of the cirque is expressed by “ice factor.” The form accompanying

both these terms is merely suggestive. The top of “excess snow” has a

gradient characteristic of the surface of snow fields. A preglacial

gradient of 0° is not permissible, but I have introduced it to complete

the discussion in the text and to illustrate the flat floor of a cirque.

A bergschrund is not required for any stage of this process, though the

process is hastened wherever bergschrunds exist.]

We shall begin with the familiar fact that many valleys, now without

perpetual snow, formerly contained glaciers from 500 to 1,000 feet thick

and that their snowfields were of wide extent and great depth. At the

head of a given valley where the snow is crowded into a small

cross-section it is compacted and suffers a reduction in its volume. At

first nine times the volume of ice, the gradually compacting névé

approaches the volume of ice as a limit. At the foot of the cirque wall

we may fairly assume in the absence of direct observations, a volume

reduction of one-half due to compacting. But this is offset in the case

of a well-developed cirque by volume increases due to the convergence of

the snow from the surrounding slopes, as shown in Fig. 196. Taking a

typical cirque from a point above Vilcabamba pueblo I find that the

radius of the trough’s end is to the radius of the upper wall of the

cirque as 1:4; and since the corresponding surfaces are to one another

as the squares of their similar dimensions we have 1:4 or 1:16 as the

ratio of their snow areas. If no compacting took place, then to

accommodate all the snow in the glacial trough would require an increase

in thickness in the ratio of 1:4. If the snow were compacted to half its

original volume then the ratio would be 1:2. Now, since the volume ratio

of ice to snow is 1:9 and the thickness of the ice down valley is, say

400 feet, the equivalent of loose snow at the foot of the cirque must be

more than 1:4 over 1:9 or more than two and one-quarter times thicker,

or 400 feet thick; and would give a pressure of (900 ÷ 10) × 62.5

pounds, or 5,625 pounds, or a little less than three tons per square

foot. Since a pressure of 2,500 pounds per square foot will convert snow

into ice at freezing temperature, it is clear that ice and not snow was

the state at the bottom of the mass in glacial times. Further, between

the surface of the snow and the surface of the bottom layer of the ice

there must have been every gradation between loose snow and firm ice,

with the result that a thickness much less than 900 feet must be

assumed. Precisely what thickness would be found at the foot of the

cirque wall is unknown. But granting a thickness of 400 feet of ice an

additional 300 feet for névé and snow would raise the total to 700 feet.

The application of the facts in the above paragraph is clearly seen when

we refer to Fig. 197. The curve of snow motion of Fig. 195 is applied to

an unglaciated mountain valley. Taking a normal snow surface and filling

the valley head it is seen that the excess of snow depth over the amount

required to give motion is a measure at various points in the valley

head and at different gradients of the erosive force of the snow. It is

strikingly concentrated on the 15°-20° gradient which is precisely where

the so-called process of basal sapping is most marked. If long continued

the process will lead to the developing of a typical cirque for it is a

process that is self-stimulating. The more the valley is changed in form

the more it tends to change still further in form because of deepening

snowfields until cliffed pinnacles and matterhorns result.

By further reference to the figure it is clear that a schrund 350 feet

deep could not exist on a cirque wall with a declivity of even 20°

without being closed by flow, unless we grant _more rapid flow_ below

the crevasse. In the case of a glacier flowing over a nearly flat bed

away from the cirque it is difficult to conceive of a rate of flow

greater than that of snow and névé on the steep lower portion of the

cirque wall, when movement on that gradient _begins_ with snow but 20

feet thick.

In contrast to this is the view that the schrund line should lie well up

the cirque wall where the snow is comparatively thin and where there is

an approach to the lower limits of movement. The schrund would appear to

open where the bottom material changes its form, i.e., where it first

has its motion accelerated by transformation into névé. In this view

the schrund opens not at the foot of the cirque wall but well above it

as in Fig. 198, in which _C_ represents snow from top to bottom; _B_,

névé; and _A_, ice. The required conditions are then (1) that the

steepening of the cirque wall from _x_ to _y_ should be effected by

sapping originated at _y_ through the agencies outlined by Johnson; (2)

that the steepening from _x_ to _y_ should be effected by sapping

originated at _x_ through the change of the agent from névé to ice with

a sudden change of function; (3) and that the essential unity of the

wall _x-y-z_ be maintained through the erosive power of the névé, which

would tend to offset the formation of a shelf along a horizontal plane

passed through _y_. The last-named process not only appears entirely

reasonable from the conditions of gradient and depth outlined on pp. 296

to 298, but also meets the actual field conditions in all the cases

examined in the Peruvian Andes. This brings up the second and third of

our main considerations, that the bergschrund does not always or even in

many cases reach the foot of the cirque wall, and that cirques exist in

many cases where bergschrunds are totally absent.

It is a striking fact that frost action at the bottom of the bergschrund

has been assumed to be the only effective sapping force, in spite of the

common observation that bergschrunds lie in general well toward the

upper limits of snowfields--so far, in fact, that their bottoms in

general occur several hundred feet above the cirque floors. Is the

cirque under these circumstances a result of the schrund or is the

schrund a result of the cirque? _In what class of cirques do schrunds

develop?_ If cirque development in its early stages is not marked by the

development of bergschrunds, then are bergschrunds an _essential_

feature of cirques in their later stages, however much the sapping

process may be hastened by schrund formation?

Our questions are answered at once by the indisputable facts that many

schrunds occur well toward the upper limit of snow, and that many

cirques exist whose snowfields are not at all broken by schrunds. It was

with great surprise that I first noted the bergschrunds of the Central

Andes, especially after becoming familiar with Johnson’s apparently

complete proof of their genetic relation to the cirques. But it was less

surprising to discover the position of the few observed--high up on the

cirque walls and always near the upper limit of the snowfields.

A third fact from regions once glaciated but now snow-free also combined

with the two preceding facts in weakening the wholesale application of

Johnson’s hypothesis. In many headwater basins the cirque whose wall at

a distance seemed a unit was really broken into two unequal portions; a

lower, much grooved and rounded portion and an upper unglaciated,

steep-walled portion. This condition was most puzzling in view of the

accepted explanation of cirque formation, and it was not until the two

first-named facts and the applications of the curves of snow motion were

noted that the meaning of the break on the cirque became clear.

Referring to Fig. 198 we see at once that the break occurs at _y_ and

means that under favorable topographic and geologic conditions sapping

at _y_ takes place faster than at _x_ and that the retreat of _y-z_ is

faster than _x-y_. It will be clear that when these conditions are

reversed or sapping at _x_ and at _y_ are equal a single wall will

result. On reference to the literature I find that Gilbert recently

noted this feature and called it the _schrundline_.[63] He believes that

it marks the base of the bergschrund _at a late stage in the excavation

of the cirque basin_. He notes further that the lower less-steep slope

is glacially scoured and that it forms “a sort of shoulder or terrace.”

[Illustration: FIG. 198--The development of cirques. See text, p. 209,

and Fig. 199.]

If all the structural and topographic conditions were known in a great

variety of gathering basins we should undoubtedly find in them, and not

in special forms of ice erosion, an explanation of the various forms

assumed by cirques. The limitations inherent in a high-altitude field

and a limited snow cover prevented me from solving the problem, but it

offered sufficient evidence at least to indicate the probable lines of

approach to a solution. For example it is noteworthy that in _all_ the

cases examined the schrundline was better developed the further glacial

erosion had advanced. So constantly did this generalization check up,

that if at a distance a short valley was observed to end in a cirque, I

knew at once and long before I came to the valley head that a shoulder

below the schrundline did not exist. At the time this observation was

made its significance was a mystery, but it represents a condition so

constant that it forms one of the striking features of the glacial forms

in the headwater region.

[Illustration: FIG. 199--Further stages in the development of cirques.

See p. 299 and Fig. 198.]

The meaning of this feature is represented in Fig. 199, in which three

successive stages in cirque development are shown. In _A_, as displayed

in small valleys or mountainside alcoves which were but temporarily

occupied by snow and ice, or as in all higher valleys during the earlier

stages of the advancing hemicycle of glaciation, snow collects, a short

glacier forms, and a bergschrund develops. As a result of the

concentrated frost action at the base of the bergschrund a rapid

deepening and steepening takes place at _a_. As long as the depth of

snow (or snow and névé) is slight the bergschrund may remain open. But

its existence at this particular point is endangered as the cirque

grows, since the increasing steepness of the slope results in more rapid

snow movement. Greater depth of snow goes hand in hand with increasing

steepness and thus favors the formation of névé and even ice at the

bottom of the moving mass and a constantly accelerated rate of motion.

At the same time the bergschrund should appear higher up for an

independent reason, namely, that it tends to form between a mass of

slight movement and one of greater movement, which change of function,

as already pointed out, would appear to be controlled by change from

snow to névé or ice on the part of the bottom material.

The first stages in the upward migration of the bergschrund will not

effect a marked change from the original profile, since the converging

slopes, the great thickness of névé and ice at this point, and the steep

gradient all favor powerful erosion. When, however, stage _C_ is

reached, and the bergschrund has retreated to _c″_, a broader terrace

results below the schrundline, the gradient is decreased, the ice and

névé (since they represent a constant discharge) are spread over a

greater area, hence are thinner, and we have the cirque taking on a

compound character with a lower, less steep and an upper, precipitous

section.

It is clear that a closely jointed and fragile rock might be quarried by

moving ice at _c′-c″_ and the cirque wall extended unbroken to _x_; it

is equally clear that a homogeneous, unjointed granite would offer no

opportunities for glacial plucking and would powerfully resist the much

slower process of abrasion. Thus Gilbert[64] observed the schrundline in

the granites of the Sierra Nevada, which are “in large part

structureless” and my own observations show the schrundline well

developed in the open-jointed granites of the Cordillera Vilcapampa and

wholly absent in the volcanoes of the Maritime Cordillera, where ashes

and cinders, the late products of volcanic action, form the easily

eroded walls of the steep cones. Somewhere between these extremes--lack

of a variety of observations prevents our saying where--the resistance

and the internal structure of the rock will just permit a cirque wall to

extend from _x_ to _c′ ″_ of Fig. 199.

A common feature of cirques that finds an explanation in the proposed

hypothesis is the notch that commonly occurs at some point where a

convergence of slopes above the main cirque wall concentrates snow

discharge. It is proposed to call this type the notched cirque. It is

highly significant that these notches are commonly marked by even

steeper descents at the point of discharge into the main cirque than the

remaining portion of the cirque wall, even when the discharge was from a

very small basin and in the form of snow or at the most névé. The excess

of discharge at a point on the basin rim ought to produce the form we

find there under the conditions of snow motion outlined in earlier

paragraphs. It is also noteworthy that it is at such a point of

concentrated discharge that crevasses no sooner open than they are

closed by the advancing snow masses. To my mind the whole action is

eminently representative of the action taking place elsewhere along the

cirque wall on a smaller scale.

What seems a good test of the explanation of cirques here proposed was

made in those localities in the Maritime Cordillera, where large

snowbanks but not glaciers affect the form of the catchment basins. A

typical case is shown in Fig. 201. As in many other cases we have here a

great lava plateau broken frequently by volcanic cones of variable

composition. Some are of lava, others consist of ashes, still others of

tuff and lava and ashes. At lower elevations on the east, as at 16,000

feet between Antabamba and Huancarama, evidences of long and powerful

glaciers are both numerous and convincing. But as we rise still higher

the glaciated topography is buried progressively deeper under the

varying products of volcanic action, until finally at the summit of the

lava fields all evidences of glaciation disappear in the greater part of

the country between Huancarama and the main divide. Nevertheless, the

summit forms are in many cases as significantly altered as if they had

been molded by ice. Precipitous cirque walls surround a snow-filled

amphitheater, and the process of deepening goes forward under one’s

eyes. No moraines block the basin outlets, no U-shaped valleys lead

forward from them. We have here to do with post-glacial action pure and

simple, the volcanoes having been formed since the close of the

Pleistocene.

Likewise in the pass on the main divide, the perpetual snow has begun

the recessing of the very recent volcanoes bordering the pass. The

products of snow action, muds and sands up to very coarse gravel,

glaciated in texture with an intermingling of blocks up to six inches in

diameter in the steeper places, are collected into considerable masses

at the snowline, where they form broad sheets of waste so boggy as to be

impassable except by carefully selected routes. No ice action whatever

is visible below the snowline and the snow itself, though wet and

compact, is not underlain by ice. Yet the process of hollowing goes

forward visibly and in time will produce serrate forms. In neither case

is there the faintest sign of a bergschrund; the gradients seem so well

adjusted to the thickness and rate of movement of the snow from point to

point that the marginal crack found in many snowfields is absent.

The absence of bergschrunds is also noteworthy in many localities where

formerly glaciation took place. This is notoriously the case in the

summit zone of the Cordillera Vilcapampa, where the accumulating snows

of the steep cirque walls tumble down hundreds of feet to gather into

prodigious snowbanks or to form névé fields or glaciers. From the

converging walls the snowfalls keep up an intermittent bombardment of

the lower central snow masses. It is safe to say that if by magic a

bergschrund could be opened on the instant, it would be closed almost

immediately by the impetus supplied by the falling snow masses. The

explanation appears to be that the thicker snow and névé concentrated at

the bottom of the cirque results in a corresponding concentration of

action and effect; and cirque development goes on without reference to a

bergschrund. The chief attraction of the bergschrund hypothesis lies in

the concentration of action at the foot of the cirque wall. But in the

thickening of the snow far beyond the minimum thickness required for

motion at the base of the cirque wall and its change of function with

transformation into névé, we need invoke no other agent. If a

bergschrund forms, its action may take place at the foot of the cirque

wall or high up on the wall, and yet _sapping at the foot of the wall_

continue.

[Illustration: THE YALE PERUVIAN EXPEDITION OF 1911

HIRAM BINGHAM DIRECTOR

LAMBRAMA QUADRANGLE]

From which we conclude (1) that where frost action occurs at the bottom

of a bergschrund opening to the foot of the cirque wall it aids in the

retreat of the wall; (2) that a sapping action takes place at this point

whether or not a bergschrund exists and that bergschrund action is not a

_necessary_ part of cirque formation; (3) that when a more or less

persistent bergschrund opens on the cirque wall above its foot it tends

to develop a schrundline with a marked terrace below it; (4) that

schrundlines are best developed in the mature stages of topographic

development in the glacial cycle; (5) that the varying rates of snow,

névé, and ice motion at a valley head are the _persistent_ features to

which we must look for topographic variations; (6) that the hypothesis

here proposed is applicable to all cases whether they involve the

presence of snow or névé or ice or any combination of these, and whether

bergschrunds are present or not; and (7) at the same time affords a

reasonable explanation for such variations in forms as the compound

cirque with its schrundline and terrace, the unbroken cirque wall, the

notched cirque, and the recessed, snow-covered mountain slopes

unaffected by ice.

ASYMMETRICAL CREST LINES AND ABNORMAL VALLEY PROFILES IN THE CENTRAL

ANDES

To prove that under similar conditions glacial erosion may be greater

than subaërial denudation quantitative terms must be sought. Only these

will carry conviction to the minds of many opponents of the theory that

ice is a vigorous agent of erosion. Gilbert first showed in the Sierra

Nevada that headwater glaciers eroded more rapidly than nonglacial

agents under comparable topographic and structural conditions.[65] Oddly

enough none of the supporters of opposing theories have replied to his

arguments; instead they have sought evidence from other regions to show

that ice cannot erode rock to an important degree. In this chapter

evidence from the Central Andes, obtained in 1907 and 1911, will be

given to show the correctness of Gilbert’s proposition.

The data will be more easily understood if Gilbert’s argument is first

outlined. On the lower slopes of the glaciated Sierra Nevada asymmetry

of form resulted from the presence of ice on one side of each ridge and

its absence on the other (Fig. 200). The glaciers of these lower ridges

were the feeblest in the entire region and were formed on slopes of

small extent; they were also short-lived, since they could have existed

only when glacial conditions had reached a maximum. Let the broken line

in the upper part of the figure represent the preglacial surface and

the solid line beneath it the present surface. It will not matter what

value we give the space between the two lines on the left to express

nonglacial erosion, since had there been no glaciers it would be the

same on both sides of the ridge. The feeble glacier occupying the

right-hand slope was able in a very brief period to erode a depression

far deeper than the normal agents of denudation were able to erode in a

much longer period, i.e., during all of interglacial and postglacial

time. Gilbert concludes: “The visible ice-made hollows, therefore,

represent the local excess of glacial over nonglacial conditions.”

[Illustration: FIG. 200--Diagrammatic cross-section of a ridge glaciated

on one side only; with hypothetical profile (broken line) of preglacial

surface.]

[Illustration: FIG. 201--Postglacial volcano recessed on shady southern

side by the process of nivation. Absolute elevation 18,000 feet (5,490

m.), latitude 14° S., Maritime Cordillera, Peru.]

In the Central Andes are many volcanic peaks and ridges formed since the

last glacial epoch and upon them a remarkable asymmetry has been

developed. Looking southward one may see a smoothly curved, snow-free,

northward-facing slope rising to a crest line which appears as regular

as the slope leading to it. Looking northward one may see by contrast

(Fig. 194) sharp ridges, whose lower crests are serrate, separated by

deeply recessed, snow-filled mountain hollows. Below this highly

dissected zone the slopes are smooth. The smooth slope represents the

work of water; the irregular slopes are the work of snow and ice. The

relation of the north and south slopes is diagrammatically shown in Fig.

201.

To demonstrate the erosive effects of snow and ice it must be shown: (1)

that the initial slopes of the volcanoes are of postglacial age; (2)

that the asymmetry is not structural; (3) that the snow-free slopes have

not had special protection, as through a more abundant plant cover, more

favorable soil texture, or otherwise.

Proof of the postglacial origin of the volcanoes studied in this

connection is afforded: (1) by the relation of the flows and the ash and

cinder beds about the bases of the cones to the glacial topography; (2)

by the complete absence of glacial phenomena below the present snowline.

Ascending a marginal valley (Fig. 202), one comes to its head, where two

tributaries, with hanging relations to the main valley, come down from a

maze of lesser valleys and irregular slopes. Glacial features of a

familiar sort are everywhere in evidence until we come to the valley

heads. Cirques, reversed grades, lakes, and striæ are on every hand. But

at altitudes above 17,200 feet, recent volcanic deposits have over large

areas entirely obscured the older glacial topography. The glacier which

occupied the valley of Fig. 202 was more than one-quarter of a mile

wide, the visible portion of its valley is now over six miles long, but

the extreme head of its left-hand tributary is so concealed by volcanic

material that the original length of the glacier cannot be determined.

It was at least ten miles long. From this point southward to the border

of the Maritime Cordillera no evidence of past glaciation was observed,

save at Solimana and Coropuna, where slight changes in the positions of

the glaciers have resulted in the development of terminal moraines a

little below the present limits of the ice.

From the wide distribution of glacial features along the northeastern

border of the Maritime Cordillera and the general absence of such

features in the higher country farther south, it is concluded that the

last stages of volcanic activity were completed in postglacial time. It

is equally certain, however, that the earlier and greater part of the

volcanic material was ejected before glaciation set in, as shown by the

great depth of the canyons (over 5,000 feet) cut into the lava flows, as

contrasted with the relatively slight filling of coarse material which

was accumulated on their floors in the glacial period and is now in

process of dissection. Physiographic studies throughout the Central

Andes demonstrate both the general distribution of this fill and its

glacial origin.

So recent are some of the smaller peaks set upon the lava plateau that

forms the greater part of the Maritime Cordillera, that the snows massed

on their shadier slopes have not yet effected any important topographic

changes. The symmetrical peaks of this class are in a few cases so very

recent that they are entirely uneroded. Lava flows and beds of tuff

appear to have originated but yesterday, and shallow lava-dammed lakes

retain their original shore relations. In a few places an older

topography, glacially modified, may still be seen showing through a

veneer of recent ash and cinder deposits, clear evidence that the

loftier parts of the lava plateau were glaciated before the last

volcanic eruption.

The asymmetry of the peaks and ridges in the Maritime Cordillera cannot

be ascribed to the manner of eruption, since the contrast in declivity

and form is persistently between northern and southern slopes. Strong

and persistent winds from a given direction undoubtedly influence the

form of volcanoes to at least a perceptible degree. In the case in hand

the ejectamenta are ashes, cinders, and the like, which are blown into

the air and have at least a small component of motion down the wind

during both their ascent and descent. The _prevailing_ winds of the high

plateaus are, however, easterly and the strongest winds are from the

west and blow daily, generally in the late afternoon. Both wind

directions are at right angles to the line of asymmetry, and we must,

therefore, rule out the winds as a factor in effecting the slope

contrasts which these mountains display.

It remains to be seen what influence a covering of vegetation on the

northern slopes might have in protecting them from erosion. The northern

slopes in this latitude (14° S.) receive a much greater quantity of heat

than the southern slopes. Above 18,000 feet (5,490 m.) snow occurs on

the shady southern slopes, but is at least a thousand feet higher on the

northern slopes. It is therefore absent from the northern side of all

but the highest peaks. Thus vegetation on the northern slopes is not

limited by snow. Bunch grass--the characteristic _ichu_ of the mountain

shepherds--scattered spears of smaller grasses, large ground mosses

called _yareta_, and lichens extend to the snowline. This vegetation,

however, is so scattered and thin above 17,500 feet (5,330 m.) that it

exercises no retarding influence on the run-off. Far more important is

the porous nature of the volcanic material, which allows the rainfall to

be absorbed rapidly and to appear in springs on the lower slopes, where

sheets of lava direct it to the surface.

The asymmetry of the north and south slopes is not, then, the result of

preglacial erosion, of structural conditions, or of special protection

of the northern slopes from erosion. It must be concluded, therefore,

that it is due to the only remaining factor--snow distribution. The

southern slopes are snow-clad, the northern are snow-free--in harmony

with the line of asymmetry. The distribution of the snow is due to the

contrasts between shade and sun temperatures, which find their best

expression in high altitudes and on single peaks of small extent.

Frankland’s observations with a black-bulb thermometer _in vacuo_ show

an increase in shade and sun temperatures contrasts of over 40° between

sea level and an elevation of 10,000 feet. Violle’s experiments show an

increase of 26 per cent in the intensity of solar radiation between 200

feet and 16,000 feet elevation. Many other observations up to 16,000

feet show a rapid increase in the difference between sun and shade

temperatures with increasing elevation. In the region herein described

where the snowline is between 18,000 and 19,000 feet (5,490 to 5,790 m.)

these contrasts are still further heightened, especially since the

semi-arid climate and the consequent long duration of sunshine and low

relative humidity afford the fullest play to the contrasting forces. The

coefficient of absorption of radiant energy by water vapor is 1,900

times that of air, hence the lower the humidity the more the radiant

energy expended upon the exposed surface and the greater the sun and

shade contrasts. The effect of these temperature contrasts is seen in a

canting of the snowline on individual volcanoes amounting to 1,500 feet

in extreme instances. The average may be placed at 1,000 feet.

The minimum conditions of snow motion and the bearing of the conclusions

upon the formation of cirques have been described in the chapters

immediately preceding. It is concluded that snow moves upon 20° slopes

if the snow is at least forty feet deep, and that through its motion

under more favorable conditions of greater depth and gradient and the

indirect effects of border melting there is developed a hollow occupied

by the snow. Actual ice is not considered to be a necessary condition of

either movement or erosion. We may at once accept the conclusion that

the invariable association of the cirques and steepened profiles with

snowfields proves that snow is the predominant modifying agent.

An argument for glacial erosion based on profiles and steep cirque walls

in a volcanic region has peculiar appropriateness in view of the

well-known symmetrical form of the typical volcano. Instead of varied

forms in a region of complex structure long eroded before the appearance

of the ice, we have here simple forms which immediately after their

development were occupied by snow. _Ever since their completion these

cones have been eroded by snow on one side and by water on the other._

If snow cannot move and if it protects the surface it covers, then this

surface should be uneroded. All such surfaces should stand higher than

the slopes on the opposite aspect eroded by water. But these assumptions

are contrary to fact. The slopes underneath the snow are deeply

recessed; so deeply eroded indeed, that they are bordered by steep

cliffs or cirque walls. The products of erosion also are to some extent

displayed about the border of the snow cover. In strong contrast the

snow-free slopes are so slightly modified that little of their original

symmetry is lost--only a few low hills and shallow valleys have been

formed.

The measure of the excess of snow erosion over water erosion is

therefore the difference between a northern or water-formed and a

southern or snow-formed profile, Fig. 200. This difference is also shown

in Fig. 201 and from it and the restored initial profiles we conclude

that the rate of water erosion is to that of nivation as 1:3. This ratio

has been derived from numerous observations on cones so recently formed

that the interfluves without question are still intact.

Thus far only those volcanoes have been considered which have been

modified by nivation. There are, however, many volcanoes which have been

eroded by ice as well as by snow and water. It will be seen at once that

where a great area of snow is tributary to a single valley, the snow

becomes compacted into névé and ice, and that it then erodes at a much

faster rate. Also a new force--plucking--is called into action when ice

is present, and this greatly accelerates the rate of erosion. While it

lies outside the limits of my subject to determine quantitatively the

ratio between water and ice action, it is worth pointing out that by

this method a ratio much in excess of 1:3 is determined, which even in

this rough form is of considerable interest in view of the arguments

based on the protecting influence of both ice and snow. I have, indeed,

avoided the question of ice erosion up to this point and limited myself

to those volcanoes which have been modified by nivation only, since the

result is more striking in view of the all but general absence of data

relating to this form of erosion.

[Illustration: FIG. 202--Graphic representation of amount of glacial

erosion during the glacial period. In the background are mature slopes

surmounted by recessed asymmetrical peaks. The river entrenched itself

below the mature slopes before it began to aggrade, and, when

aggradation set in, had cut its valley floor to a′-b′-c. By aggradation

the valley floor was raised to a-b while ice occupied the valley head.

By degradation the river has again barely lowered its channel to a′-b′,

the ice has disappeared, and the depression of the profile represents

the amount of glacial erosion.

a′-b′-c = preglacial profile.

a-b-d-c = present profile.

b′-d-c-b = total ice erosion in the glacial period.

a-b = surface of an alluvial valley fill due to

excessive erosion at valley head.

b-b′ = terminal moraine.

d-c = cirque wall.

e, e′ e″ = asymmetrical summits.]

[Illustration: FIG. 203--A composite sketch to represent general

conditions in the Peruvian Andes. In order to have the actual facts

represented the profiles of this figure were taken from the accompanying

topographic sheets. The main depression on the right and the

corresponding depression of the tributary profiles bear out most

strikingly the conclusions concerning the erosive power of the ice. At

_A_ and _B_ the spurs have been cut off to exhibit the profiles of

tributary valleys. At _2_ and _3_ were tributary glaciers of such size

that they entered the main valley at grade. Lesser tributaries had

floors elevated above those they joined and now have a hanging

character, as just above _2_. _D_ is a matterhorn; _C_ is deeply

recessed by cirques; _E_ represents a peak just below the limit of

glaciation. At _F_ are the undissected post-mature slopes of an earlier

cycle of erosion. _G_ lies on the steep lower slopes formed during the

canyon cycle of erosion. The down-cutting of the stream in the canyon

cycle was generally checked by glaciation and was superseded by

aggradation.]

If we now turn to the valley profiles of the glaciated portions of the

Peruvian Andes, we shall see the excess of ice over water erosion

expressed in a manner equally convincing. To a thoughtful person it is

one of the most remarkable features of any glaciated region that the

flattest profiles, the marshiest valley flats, and the most strongly

meandering stretches of the streams should occur near the heads of the

valleys. The mountain shepherds recognize this condition and drive

their flocks up from the warmer valley into the mountain recesses,

confident that both distance and elevation will be offset by the

extensive pastures of the finest _ichu_ grass. Indeed, to be near the

grazing grounds of sheep and llamas which are their principal means of

subsistence, the Indians have built their huts at the extraordinarily

lofty elevations of 16,000 to 17,000 feet.

An examination of a large number of these valleys and the plotting of

their gradients discloses the striking fact that the heads of the

valleys were deeply sunk into the mountains. It is thus possible by

restoring the preglacial profiles to measure with considerable certainty

the excess of ice over water erosion.

The results are graphically expressed in Fig. 202. It will be seen that

until glacial conditions intervened the stream was flowing on a rock

floor. During the whole of glacial time it was aggrading its rock floor

below _b′_ and forming a deep valley fill. A return to warmer and drier

conditions led to the dissection of the fill and this is now in

progress. The stream has not yet reached its preglacial profile, but it

has almost reached it. We may, therefore, say that the preglacial valley

profile below _b′_ fixes the position of the present profile just as

surely as if the stream had been magically halted in its work at the

beginning of the period of glaciation. There, _b′-d-c-b_ represents the

amount of ice erosion. To be sure the line _b-c_ is inference, but it is

reasonable inference and, whatever position is assigned to it, it cannot

be coincident with _b′-d_, nor can it be anywhere near it. The break in

the valley profile at _b′_ is always marked by a terminal moraine,

regardless of the character of the rock. This is not an accidental but a

causal association. It proves the power of the ice to erode. In glacial

times it eroded the quantity _b-c-d-b′_. This is not an excess of ice

over water erosion, but an absolute measure of ice erosion, since

_a′-b′_ has remained intact. The only possible error arises from the

position assigned _b-c_, and even if we lower it to _b-c′_ (for which we

have no warrant but extreme conservatism) we shall still have left

_b′-c′-d-b_ as a striking value for rock erosion (plucking and abrasion)

by a valley glacier.

A larger diagram, Fig. 203, represents in fuller detail the topographic

history of the Andes of southern Peru and the relative importance of

glaciation. The broad spurs with grass-covered tops that end in steep

scarps are in wonderful contrast to the serrate profiles and truncated

spurs that lie within the zone of past glaciation. In the one case we

have minute irregularities on a canyon wall of great dimensions; in the

other, more even walls that define a glacial trough with a flat floor.

Before glaciation on a larger scale had set in the right-hand section of

the diagram had a greater relief. It was a residual portion of the

mountain and therefore had greater height also. Glaciers formed upon it

in the Ice Age and glaciation intensified the contrast between it and

the left-hand section; not so much by intensifying the relief as by

diversifying the topographic forms.

[Illustration: FIG. 204--Topographic map of the Andes between Abancay

and the Pacific Coast at Camaná. Compiled from the seven accompanying

topographic sheets (see Contents, p. xi). Scale 1:1,000,000. Contour

interval 1,000 feet. Longitude west of Greenwich. The Central Ranges of

the Maritime Cordillera are not confined to the area covered by these

names. In the one case the term includes all the ranges between Lambrama

and Huichihua; in the other case, the peaks and ranges from 14° 30′ S.

to Mt. Coropuna.]

APPENDIX A

SURVEY METHODS EMPLOYED IN THE CONSTRUCTION OF THE SEVEN ACCOMPANYING

TOPOGRAPHIC SHEETS

BY KAI HENDRIKSEN, TOPOGRAPHER

The main part of the topographical outfit consisted of (1) a 4-inch

theodolite, Buff and Buff, the upper part detachable, (2) an 18 x 24

inch plane-table with Johnson tripod and micro-meteralidade. These

instruments were courteously loaned the expedition by the U. S. Coast

and Geodetic Survey and the U. S. Geological Survey respectively.

The method of survey planned was a combination of graphic triangulation

and traverse with the micro-meteralidade. All directions were plotted on

the plane-table which was oriented by backsight; distances were

determined by the micro-meteralidade or triangulation, or both combined;

and elevations were obtained by vertical angles. Finally, astronomical

observations, usually to the sun, were taken at intervals of about 60

miles for latitude and azimuth to check the triangulation. No

observations were made for differences in longitude because this would

probably not have given any reliable result, considering the time and

instruments at our disposal. Because the survey was to follow very

closely the seventy-third meridian west of Greenwich, directions and

distances, checked by latitude and azimuth observations, undoubtedly

afforded far better means of determining the longitude than time

observations. In other words, the time observations made in connection

with azimuth observations were not used for computing longitudinal

differences. Absolute longitude was taken from existing observations of

principal places.

Principal topographical points were located by from two to four

intersections from the triangulation and plane-table stations; and

elevations were determined by vertical angle measurements. Whenever

practicable, the contours were sketched in the field; the details of the

topography otherwise depend upon a great number of photographs taken by

Professor Bowman from critical stations or other points which it was

possible to locate on the maps.

CROSS-SECTION MAP FROM ABANCAY TO CAMANÁ AT THE PACIFIC OCEAN

Seven sheets. Scale, 1:125,000; contour interval, 200 feet. Datum is

mean sea level. Astronomical control: 5 latitude and 5 azimuth

observations as indicated on the accompanying topographic sheets.

On September 10th, returning from a reconnaissance survey of the

Pampaconas River, I joined Professor Bowman’s party, Dr. Erving acting

as my assistant. We crossed the Cordillera Vilcapampa and the Canyon of

the Apurimac and after a week’s rest at Abancay started the topographic

work near Hacienda San Gabriel south of Abancay. Working up the deep

valley of Lambrama, observations for latitude and azimuth were made

midway between Hacienda Matara and Caypi.

On October 4th we made our camp in newly fallen snow surrounded by

beautiful glacial scenery. The next day on the high plateau, we passed

sharp-crested glaciated peaks; a heavy thunder and hail storm broke out

while I occupied the station at the pass, the storm continuing all the

afternoon--a frequent occurrence. The camp was made 6 miles farther on,

and the next morning I returned to finish the latter station. I

succeeded in sketching the detailed topography just south of the pass,

but shortly after noon, a furious storm arose similar to the one the day

before, and made further topographic work impossible; to get connection

farther on I patiently kept my eye to the eye-piece for more than an

hour after the storm had started, and was fortunate to catch the station

ahead in a single glimpse. I had a similar experience some days later at

station 16,079, Antabamba Quadrangle, on the rim of the high-level puna,

the storm preventing all topographic work and barely allowing a single

moment in which to catch a dim sight of the signals ahead while I kept

my eye steadily at the telescope to be ready for a favorable break in

the heavy clouds and hail.

At Antabamba we got a new set of Indian carriers, who had orders to

accompany us to Cotahuasi, the next sub-prefectura. Raimondi’s map

indicates the distance between the two cities to be 35 miles, but

although nothing definite was stated, we found out in Antabamba that the

distance was considerably longer, and moreover that the entire route lay

at a high altitude.

From the second day out of Antabamba until Huaynacotas was in sight in

the Cotahuasi Canyon, a distance of 50 miles, the route lay at an

altitude of from 16,000 to 17,630 feet, taking in 5 successive camps at

an altitude from 15,500 to 17,000 feet; 12 successive stations had the

following altitudes:

16,379 feet

16,852 "

17,104 "

17,559 "

17,675 " --highest station occupied.

17,608 "

17,633 "

16,305 "

17,630 "

17,128 "

16,794 "

16,260 "

The occupation of these high stations necessitated a great deal of

climbing, doubly hard in this rarefied air, and often on volcanoes with

a surface consisting of bowlders and ash and in the face of violent

hailstorms that made extremely difficult the task of connecting up

observations at successive stations.

At Cotahuasi a new pack-train was organized, and on October 25th I

ventured to return alone to the high altitudes in order to continue the

topography at the station at 17,633 feet on the summit of the Maritime

Cordillera. Dr. Erving was obliged to leave on October 18th and

Professor Bowman left a week later in order to carry out his plans for a

physiographic study of the coast between Camaná and Mollendo. Philippi

Angulo, a native of Taurisma, a town above Cotahuasi, acted as majordomo

on this journey. Knowing the trail and the camp sites, I was able to

pick out the stations ahead myself, and made good progress, returning to

Cotahuasi on October 29th, three or four days earlier than planned. From

Cotahuasi to the coast I had the assistance of Mr. Watkins. The most

trying part of the last section of high altitude country was the great

Pampa Colorada, crowned by the snow-capped peaks of Solimana and

Coropuna, reaching heights of 20,730 and 21,703 feet respectively. The

passing of this pampa took seven days and we arrived at Chuquibamba on

November 9th. Two circumstances made the work on this stretch peculiarly

difficult--the scarcity of camping places and the high temperature in

the middle of the day, which heated the rarefied air to a degree that

made long-distance shots very strenuous work for the eyes. Although our

base signals were stone piles higher than a man, I was often forced to

keep my eye to the telescope for hours to catch a glimpse of the

signals; lack of time did not allow me to stop the telescope work in the

hottest part of the day.

The top of Coropuna was intersected from the four stations: 16,344,

15,545, 16,168, and 16,664 feet elevation, the intersections giving a

very small triangular error. The elevation of Mount Coropuna’s high peak

as computed from these 4 stations is:

21,696 feet

21,746 "

21,714 "

21,657 "

------

Mean elevation 21,703 feet above sea level.

The elevation of Coropuna as derived from these four stations has thus a

mean error of 18 feet (method of least squares) while the elevation of

each of the four stations as carried up from mean sea level through 25

stations--vertical angles being observed in both directions--has an

estimated mean error of 30 feet. The result of this is a mean error of

35 feet in Coropuna’s elevation above mean sea level.

The latitude is 15° 31′ 00″ S.; the longitude is 72° 42′ 40″ W. of

Greenwich, the checking of these two determinations giving a result

unexpectedly close.

On November 11th azimuth and latitude observations were taken at

Chuquibamba and two days later we arrived at Aplao in the bottom of the

splendid Majes Valley. In the northern part of this valley I was

prevented from doing any plane-table work in the afternoons of four

successive days. A strong gale set in each noon raising a regular

sandstorm, that made seeing almost impossible, and blowing with such a

velocity that it was impossible to set up the plane-table.

From Hacienda Cantas to Camaná we had to pass the western desert for a

distance of 45 miles. We were told that on the entire distance there was

only one camping place. This was at Jaguey de Majes, where there was a

brook with just enough water for the animals but no fodder. Thus we

faced the necessity of carrying water for ten men and fodder for 14

animals in excess of the usual cargo; and we were unable to foretell how

many days the topography over the hot desert would require.

Although plane-table work in the desert was impossible at all except in

the earliest and latest hours of the day, we made regular progress. We

camped three nights at Jaguey and arrived on the fourth day at Las

Lomas.

The next morning, on November 23rd, at an elevation of 2178 feet near

the crest of the Coast Range, we were repaid for two months of laborious

work by a glorious view of the Pacific Ocean and of the city of Camaná

with her olive gardens in the midst of the desert sand.

The next day I observed latitude and azimuth at Camaná and in the night

my companion and assistant Mr. Watkins and I returned across the desert

to the railroad at Vitor.

CONCLUSIONS

The planned methods were followed very closely. In two cases only the

plane-table had to be oriented by the magnetic needle, the backsights

not being obtainable because of the impossibility of locating the last

station, passing Indians having removed the signals.

In one case only the distance between two stations had to be determined

by graphic triangulation exclusively, the base signals having been

destroyed. Otherwise graphic triangulation was used as a check on

distances.

Vertical angles were always measured in both directions with the

exception of the above-mentioned cases.

Observations for azimuth were always taken to the sun before and after

noon. The direction used in the azimuth observation was also taken with

the prismatic compass. The mean of the magnetic declination thus found

is: East 8° 30′ plus.

Observations for latitude were taken to the sun by the method of

circum-meridian altitudes, except at the town of Vilcabamba where star

observations were taken.

As a matter of course, observations to the sun are not so exact as star

observations, especially in low latitudes where one can expect to

observe the near zenith. However, working in high altitudes for long

periods, moving camp every day and often arriving at camp 2 to 4 hours

after sunset, I found it essential to have undisturbed rest at night. It

was beyond my capacity to spend an hour or two of the night in finding

the meridian and in making the observation. Furthermore, the astronomic

observations were to check the topography mainly, the latter being the

most exact method with the outfit at hand.

The following table contains the comparisons between the latitude

stations as located on the map and by observation:

Map Observation

Camaná Quadrangle S 16° 37′ 34″ 16° 37′ 34″[66]

Coropuna, station 9,691S 15° 48′ 30″ (15° 51′ 44″)

Cotahuasi, " 12,588S 15° 11′ 40″ 15° 12′ 30″

La Cumbre, " 16,852S 14° 28′ 10″ 14° 29′ 46″

Lambrama, " 8,341S 13° 43′ 18″ 13° 43′ 14″

The other observations, with the exception of the one on the Coropuna

Quadrangle, check probably as well as can be expected with the small and

light outfit which we used, and under the exceptionally hard conditions

of work. The observation on the Coropuna Quadrangle just south of

Chuquibamba is, however, too much out. An explanation for this is that

the meridian zenith distance was 1° 23′ 12″ only (in this case the exact

formula was used in computing). Of course, an error or an accumulation

of errors might have been made in the distances taken by the

micrometer-alidade, but the first cause of error mentioned is the more

probable, and this is indicated also by the fact that the location on

the top of Mount Coropuna checks closely with the one determined in an

entirely independent way by the railroad engineers.

For the cross-section map from Abancay to Camaná, the following

statistics are desirable:

Micrometer traverse and graphic triangulation, with contours, field

scale 1:90,000.