Cordillera a little south of Antabamba. Afternoon storms are one of its
most significant features. Cotahuasi, 9,100 feet (2,775 m.) is near the
head of a west-coast valley. Its low humidity is worthy of note. That
for Salamanca, 12,700 feet (3,870 m.), is similar but not so marked.
Aplao, 3,100 feet (945 m.), and Camaná at the seacoast are stations in
the west-coast desert. The interior location of the former gives it a
greater range of temperature than Camaná, yet even here the range is
small in comparison with the diurnal extremes of the montaña, and the
tempering effect of the sea-breeze is clearly apparent. Camaná shows a
diurnal temperature range of under 10° F. and also the high relative
humidity, over 70 per cent, characteristic of the coast.
PART II
PHYSIOGRAPHY OF THE PERUVIAN ANDES
CHAPTER XI
THE PERUVIAN LANDSCAPE
From the west coast the great Andean Cordillera appears to have little
of the regularity suggested by our relief maps. Steep and high cliffs in
many places form the border of the land and obstruct the view; beyond
them appear distant summits rising into the zone of clouds. Where the
cliffs are absent or low, one may look across a sun-baked, yellow
landscape, generally broken by irregular foothills that in turn merge
into the massive outer spurs and ranges of the mountain zone. The plain
is interrupted by widely separated valleys whose green lowland meadows
form a brilliant contrast to the monotonous browns and yellows of the
shimmering desert. In rare situations the valley trenches enable one to
look far into the Cordillera and to catch memorable glimpses of lofty
peaks capped with snow.
If the traveler come to the west-coast landscape from the well-molded
English hills or the subdued mountains of Vermont and New Hampshire with
their artistic blending of moderate profiles, he will at first see
nothing but disorder. The scenery will be impressive and, in places,
extraordinary, but it is apparently composed of elements of the greatest
diversity. All the conceivable variations of form and color are
expressed, with a predominance of bold rugged aspects that give a
majestic appearance to the mountain-bordered shore. One looks in vain
for some sign of a quiet view, for some uniformity of features, for some
landscape that will remind him of the familiar hills of home. The Andes
are aggressive mountains that front the sea in formidable spurs or
desert ranges. Could we see in one view their entire elevation from
depths of over 20,000 feet beneath sea level to snowy summits, a total
altitude of 40,000 feet (12,200 m.), their excessive boldness would be
more apparent. No other mountains in the world are at once so
continuously lofty and so near a coast which drops off to abyssal
depths.
The view from the shore is, however, but one of many which the Andes
exhibit. Seen from the base the towering ranges display a stern aspect,
but, like all mountains, their highest slopes and spurs must be crossed
and re-crossed before the student is aware of other aspects of a quite
different nature. The Andes must be observed from at least three
situations: from the floors of the deep intermontane valleys, from the
intermediate slopes and summits, and from the uppermost levels as along
the range crests and the highest passes. Strangely enough it is in the
summit views that one sees the softest forms. At elevations of 14,000 to
16,000 feet (4,270 to 4,880 m.), where one would expect rugged spurs,
serrate chains, and sharp needles and horns, one comes frequently upon
slopes as well graded as those of a city park--grass-covered,
waste-cloaked, and with gentle declivity (Figs. 121-124).
The graded, waste-cloaked slopes of the higher levels are interpreted as
the result of prolonged denudation in an erosion cycle which persisted
through the greater part of the Tertiary period, and which was closed by
uplifts aggregating at least several thousands of feet. Above the level
of the mature slopes rise the ragged profiles and steep, naked
declivities of the snow-capped mountains which bear residual relations
to the softer forms at their bases. They are formed upon rock masses of
greater original elevation and of higher resistance to denudation.
Though they are dominating topographic features, they are much less
extensive and significant than the tame landscape which they surmount.
[Illustration: FIG. 121--Looking north from the hill near Anta in the
Anta basin north of Cuzco. Typical composition of slopes and intermont
basins in the Central Andes. Alluvial fill in the foreground; mature
slopes in the background; in the extreme background the snow-capped
crests of the Cordillera Vilcapampa.]
[Illustration: FIG. 122--Showing topographic conditions before the
formation of the deep canyons in the Maritime Cordillera. The view,
looking across a tributary canyon of the Antabamba river, shows in the
background the main canyon above Huadquirca. Compare with Fig. 60.]
Below the level of the mature slopes are topographic features of equal
prominence: gorges and canyons up to 7,000 feet deep. The deeply
intrenched streams are broken by waterfalls and almost continuous
rapids, the valley walls are so abrupt that one may, in places, roll
stones down a 4,000 foot incline to the river bed, and the tortuous
trail now follows a stream in the depths of a profound abyss, now scales
the walls of a labyrinthine canyon.
[Illustration: FIG. 123--Mature slopes between Ollantaytambo and
Urubamba.]
[Illustration: FIG. 124--Dissected mature slopes north of Anta in the
Anta basin north of Cuzco.]
[Illustration: FIG. 125--Mature upper and young lower slopes at the
outlet of the Cuzco basin.]
The most striking elements of scenery are not commonly the most
important in physiography. The oldest and most significant surface may
be at the top of the country, where it is not seen by the traveler or
where it cannot impress him, except in contrast to features of greater
height or color. The layman frequently seizes on a piece of bad-land
erosion or an outcrop of bright-colored sandstone or a cliff of
variegated clays or a snow-covered mountain as of most interest. All we
can see of a beautiful snow-clad peak is mere entertainment compared
with what subdued waste-cloaked hill-slopes may show. We do not wish to
imply that everywhere the tops of the Andes are meadows, that there are
no great scenic features in the Peruvian mountains, or that they are not
worth while. But we do wish to say that the bold features are far less
important in the interpretation of the landscape.
Amid all the variable forms of the Peruvian Cordillera certain strongly
developed types recur persistently. That their importance and relation
may be appreciated we shall at once name them categorically and
represent them in the form of a block diagram (Fig. 126). The principal
topographic types are as follows:
1. An extensive system of high-level, well-graded, mature slopes,
below which are:
2. Deep canyons with steep, and in places, cliffed sides and narrow
floors, and above which are:
3. Lofty residual mountains composed of resistant, highly deformed
rock, now sculptured into a maze of serrate ridges and sharp
commanding peaks.
4. Among the forms of high importance, yet causally unrelated to
the other closely associated types, are the volcanic cones and
plateaus of the western Cordillera.
5. At the valley heads are a full complement of glacial features,
such as cirques, hanging valleys, reversed slopes, terminal
moraines, and valley trains.
6. Finally there is in all the valley bottoms a deep alluvial fill
formed during the glacial period and now in process of dissection.
Though there are in many places special features either remotely related
or quite unrelated to the principal enumerated types, they belong to the
class of minor forms to which relatively small attention will be paid,
since they are in general of small extent and of purely local interest.
[Illustration: FIG. 126--Block diagram of the typical physiographic
features of the Peruvian Andes.]
The block diagram represents all of these features, though of necessity
somewhat more closely associated than they occur in nature. Reference to
the photographs, Figs. 121-124, will make it clear that the diagram is
somewhat ideal: on the other hand the photographs together include all
the features which the diagram displays. In descending from any of the
higher passes to the valley floor one passes in succession down a steep,
well-like cirque at a glaciated valley head, across a rocky terminal
moraine, then down a stair-like trail cut into the steep scarps which
everywhere mark the descent to the main valley floors, over one after
another of the confluent alluvial fans that together constitute a large
part of the valley fill, and finally down the steep sides of the inner
valley to the boulder-strewn bed of the ungraded river.
We shall now turn to each group of features for description and
explanation, selecting for first consideration the forms of widest
development and greatest significance--the high-level mature slopes
lying between the lofty mountains which rise above them and the deep,
steep-walled valleys sunk far below them. These are the great pasture
lands of the Cordillera; their higher portions constitute the typical
_puna_ of the Indian shepherds. In many sections it is possible to
pasture the vagrant flocks almost anywhere upon the graded slopes,
confident that the _ichu_, a tufted forage grass, will not fail and that
scattered brooks and springs will supply the necessary water. At
nightfall the flocks are driven down between the sheltering walls of a
canyon or in the lee of a cliff near the base of a mountain, or, failing
to reach either of these camps, the shepherd confines his charge within
the stone walls of an isolated corral.
In those places where the graded soil-covered slopes lie within the zone
of agriculture--below 14,000 feet--they are cultivated, and if the soil
be deep and fertile they are very intensively cultivated. Between Anta
and Urubamba, a day’s march north of Cuzco, the hill slopes are covered
with wheat and barley fields which extend right up to the summits (Fig.
134). In contrast are the uncultivated soil-less slopes of the mountains
and the bare valley walls of the deeply intrenched streams. The
distribution of the fields thus brings out strongly the principal
topographic relations. Where the softer slopes are at too high a level,
the climatic conditions are extreme and man is confined to the valley
floors and lower slopes where a laborious system of terracing is the
first requirement of agriculture.
The appearance of the country after the mature slopes had been formed is
brought out in Fig. 122. The camera is placed on the floor of a still
undissected, mature valley which shows in the foreground of the
photograph. In the middle distance is a valley whose great depth and
steepness are purposely hidden; beyond the valley are the smoothly
graded, catenary curves, and interlocking spurs of the mature upland. In
imagination one sees the valleys filled and the valley slopes confluent
on the former (now imaginary) valley floor which extends without
important change of expression to the border of the Cordillera. No
extensive cliffs occur on the restored surface, and none now occur on
large tracts of the still undissected upland. Since the mature slopes
represent a long period of weathering and erosion, their surfaces were
covered with a deep layer of soil. Where glaciation at the higher levels
and vigorous erosion along the canyons have taken place, the former soil
cover has been removed; elsewhere it is an important feature. Its
presence lends a marked softness and beauty to these lofty though
subdued landscapes.
The graded mountain slopes were not all developed (1) at the same
elevation, nor (2) upon rock of the same resistance to denudation, nor
(3) at the same distance from the major streams, nor (4) upon rock of
the same structure. It follows that they will not all display precisely
the same form. Upon the softer rocks at the lowest levels near the
largest streams the surface was worn down to extremely moderate slopes
with a local relief of not more than several hundred feet. Conversely,
there are quite unreduced portions whose irregularities have mountainous
proportions, and between these extremes are almost all possible
variations. Though the term _mature_ in a broad way expresses the stage
of development which the land had reached, _post mature_ should be
applied to those portions which suffered the maximum reduction and now
exhibit the softest profiles. At no place along the 73rd meridian was
denudation carried to the point of even local peneplanation. All of the
major and some of the minor divides bear residual elevations and even
approximately plane surfaces do not exist.
[Illustration: THE YALE PERUVIAN EXPEDITION OF 1911
HIRAM BINGHAM, DIRECTOR
COROPUNA QUADRANGLE
(_Cotahuasi_)]
Among the most important features of the mature slopes are (1) their
great areal extent--they are exhibited throughout the whole Central
Andes, (2) their persistent development upon rocks of whatever structure
or degree of hardness, and (3) their present great elevation in spite of
moderate grades indicative of their development at a much lower
altitude. Mature slopes of equivalent form are developed in widely
separated localities in the Central Andes: in every valley about
Cochabamba, Bolivia, at 10,000 feet (3,050 m.); at Crucero Alto in
southern Peru at 14,600 feet (4,450 m.); several hundred miles farther
north at Anta near Cuzco, 11,000 feet to 12,000 feet (3,600 to 3,940
m.), and Fig. 129 shows typical conditions in the Vilcabamba Valley
along the route of the Yale Peruvian Expedition of 1911. The
characteristic slopes so clearly represented in these four photographs
are the most persistent topographic elements in the physiography of the
Central Andes.
[Illustration: FIG. 127--Topographic profiles across typical valleys of
southern Peru. They are drawn to scale and the equality of gradient of
the gentler upper slopes is so close that almost any curve would serve
as a composite of the whole. These curves form the basis of the diagram,
Fig. 128, whereby the amount of elevation of the Andes in late geologic
time may be determined. The approximate locations of the profiles are as
follows: 1, Antabamba; 2, Chuquibambilla; 3, upland south of Antabamba;
4, Apurimac Canyon above Pasaje; 5, Abancay; 6, Arma (Cordillera
Vilcapampa); 7, divide above Huancarama; 8, Huascatay; 9, Huasentay,
farther downstream; 10, Rio Pampas. The upper valley in 8 is still
undissected; 7 is practically the same; 8a is at the level which 8 must
reach before its side slopes are as gentle as at the end of the
preceding interrupted cycle.]
The rock masses upon which the mature slopes were formed range from soft
to hard, from stratified shales, slates, sandstones, conglomerates, and
limestones to volcanics and intrusive granites. While these variations
impose corresponding differences of form, the graded quality of the
slopes is rarely absent. In some places the highly inclined strata are
shown thinly veiled with surface débris, yet so even as to appear
artificially graded. The rock in one place is hard granite, in another a
moderately hard series of lava flows, and again rather weak shales and
sandstones.
Proof of the rapid and great uplift of certain now lofty mountain ranges
in late geologic time is one of the largest contributions of
physiography to geologic history. Its validity now rests upon a large
body of diversified evidence. In 1907 I crossed the Cordillera Sillilica
of Bolivia and northern Chile and came upon clear evidences of recent
and great uplift. The conclusions presented at that time were tested in
the region studied in 1911, 500 miles farther north, with the result
that it is now possible to state more precisely the dates of origin of
certain prominent topographic forms, and to reconstruct the conditions
which existed before the last great uplift in which the Central Andes
were born. The relation to this general problem of the forms under
discussion will now be considered.
The gradients of the mature slopes, as we have already seen, are
distinctly moderate. In the Anta region, over an area several hundred
square miles in extent, they run from several degrees to 20° or 30°.
Ten-degree slopes are perhaps most common. If the now dissected slopes
be reconstructed on the basis of many clinometer readings, photographs,
and topographic maps, the result is a series of profiles as in Fig. 127.
If, further, the restored slopes be coördinated over an extensive area
the gradients of the resulting valley floors will run from 3° to 10°.
Finally, if these valley floors be extended westward to the Pacific and
eastward to the Amazon basin, they will be found about 5,000 feet above
sea level and 4,000 feet above the eastern plains. (For explanation of
method and data employed, see the accompanying figures 127-128). It is,
therefore, a justifiable conclusion that since the formation of the
slopes the Andes have been uplifted at least a mile, or, to put it in
another way, the Andes at the time of formation of the mature slopes
were at least a mile lower than they are at present.
[Illustration: FIG. 128--Composition of slopes and profiles in the
Peruvian Andes. By superimposing the cross profiles of typical valleys
as shown in Fig. 127 a restoration is possible of the longitudinal
profiles of the earlier cycle of erosion. The difference in elevation of
the two profiles gives less than the minimum amount of uplift that must
have occurred. Case A represents a valley in which recent cutting has
not yet reached the valley head. Below the point 1 the profile has been
steepened and lowered by erosion in the current cycle. Above point 1 the
profile is still in the stage it reached in the preceding cycle. In case
B the renewed erosion of the current cycle has reached to the valley
head. Case C represents conditions similar to those in the preceding
cases save that the stream is typical of those that lie nearest the
steep flexed or faulted margins of the Cordillera and discharge to the
low levels of the desert pampa on the west or the tropical plains on the
east.]
Further proof of recent and great uplift is afforded by the deeply
intrenched streams. After descending the long graded slopes one comes
upon the cliffed canyons with a feeling of consternation. The effect of
powerful erosion, incident upon uplift, is heightened by the ungraded
character of the river bed. Falls and rapids abound, the river profiles
suggest tumultuous descents, and much time will elapse before the river
beds have the regular and moderate gradients of the streams draining the
mature surface before uplift as shown in the profiles by the dotted
lines representing the restored valley floors of the older cycle. Since
the smooth-contoured landscape was formed great changes have taken
place. The streams have changed from completely graded to almost
completely ungraded profiles; in place of a subdued landscape we now
have upland slopes intersected by mile-deep canyons; the high-level
slopes could not have been formed under existing conditions, for they
are being dissected by the present streams.
[Illustration: THE YALE PERUVIAN EXPEDITION OF 1911
HIRAM BINGHAM, DIRECTOR
COTAHUASI QUADRANGLE
(_La Cumbre_)]
Since the slopes of the land in general undergo progressive changes in
the direction of flatter gradients during a given geographical cycle, it
follows that with the termination of one cycle and the beginning of
another, two sets of slopes will exist and that the gradients of the two
will be unlike. The result is a break in the descent of the slopes from
high to low levels to which the name “topographic unconformity” is now
applied. It will be a prominent feature of the landscape if the higher,
older, and flatter gradients have but little declivity, and the
gradients of the lower younger slopes are very steep. In those places
where the relief of the first cycle was still great at the time of
uplift, the erosion forms of the second cycle may not be differentiated
from those of the first, since both are marked by steep gradients. In
the Central Andes the change in gradient between the higher and lower
slopes is generally well marked. It occurs at variable heights above
the valley floors, though rarely more than 3,000 feet above them. In the
more central tracts, far from the main streams and their associated
canyons, dissection in the present erosion cycle has not yet been
initiated, the mature slopes are still intact, and a topographic
unconformity has not yet been developed. The higher slopes are faced
with rock and topped with slowly moving waste. Ascent of the spur end is
by steep zigzag trails; once the top is gained the trail runs along the
gentler slopes without special difficulties.
It is worth noting at this point that the surface of erosion still older
than the mature slopes herewith described appears not to have been
developed along the seventy-third meridian of Peru, or if developed at
one time, fragments of it no longer remain. The last well-developed
remnant is southwest of Cuzco, Fig. 130. I have elsewhere described the
character and geographic distribution of this oldest recognizable
surface of the Central Andes.[41] Southern Peru and Bolivia and northern
Chile display its features in what seems an unmistakable manner. The
best locality yet found is in the Desaguadero Valley between Ancoaqui
and Concordia. There one may see thousands of feet of strongly inclined
sediments of varying resistance beveled by a well-developed surface of
erosion whose preserval is owing to a moderate rainfall and to location
in an interior basin.[42]
The highest surface of a region, if formed during a prolonged period of
erosion, becomes a surface of reference in the determination of the
character and amount of later crustal deformations, having somewhat the
same functions as a key bed in stratigraphic geology. Indeed, concrete
physiographic facts may be the _only_ basis for arguments as to both
epeirogenic and orogenic movements. The following considerations may
show in condensed form the relative value of physiographic evidence:
1. If movements in the earth’s crust are predominantly _downward_,
sedimentation may be carried on continuously, and a clear geologic
record may be made.
2. Even if crustal movements are alternately downward and upward,
satisfactory conclusions may be drawn from both (a) the nature of the
buried surfaces of erosion, and (b) the alternating character of the
sediments.
3. If, however, the deformative processes effect steady or intermittent
uplifts, there may be no sediments, at least within the limits of the
positive crustal units, and a geologic record must be derived not from
sedimentary deposits but from topographic forms. We speak of the _lost
intervals_ represented by stratigraphic breaks or unconformities and
commonly emphasize our ignorance concerning them. The longest, and, from
the human standpoint, the most important, break in the sedimentary
record is that of the present wherever degradation is the predominant
physiographic process. Unlike the others the _lost interval_ of the
present is not lost, if we may so put it, but is in our possession, and
may be definitely described as a concrete thing. It is the physiography
of today.
Even where long-buried surfaces of erosion are exposed to view, as in
northern Wisconsin, where the Pre-Cambrian paleo-plain projects from
beneath the Paleozoic sediments, or, as in New Jersey and southeastern
Pennsylvania, where the surface developed on the crystalline rocks
became by depression the floor of the Triassic and by more recent uplift
and erosion has been exposed to view,--even in such cases the exposures
are of small extent and give us at best but meager records. In short,
many of the breaks in the geologic record are of such long duration as
to make imperative the use of physiographic principles and methods. The
great Appalachian System of eastern North America has been a land area
practically since the end of the Paleozoic. In the Central Andes the
“lost interval,” from the standpoint of the sedimentary, record, dates
from the close of the Cretaceous, except in a few local intermont basins
partially filled with Tertiary or Pleistocene deposits. Physiographic
interpretations, therefore, serve the double purpose of supplying a part
of the geologic record while at the same time forming a basis for the
scientific study of the surface distribution of living forms.
The geologic dates of origin of the principal topographic forms of the
Central Andes may be determined with a fair degree of accuracy. Geologic
studies in Peru and Bolivia have emphasized the wide distribution of the
Cretaceous formations. They consist principally of thick limestones
above and sandstones and conglomerates below, and thus represent
extensive marine submergence of the earth’s crust in the Cretaceous
where now there are very lofty mountains. The Cretaceous deposits are
everywhere strongly deformed or uplifted to a great height, and all have
been deeply eroded. They were involved, together with other and much
older sediments, in the erosion cycle which resulted in the development
of the widely extended series of mature slopes already described. From
low scattered island elevations projecting above sea level, as in the
Cretaceous period, the Andes were transformed by compression and uplift
to a rugged mountain belt subjected to deep and powerful erosion. The
products of erosion were in part swept into the adjacent seas, in part
accumulated on the floors of intermont basins, as in the great interior
basins of Titicaca and Poopó.
Since the early Tertiary strata are themselves deformed from once simple
and approximately horizontal structures and subjected to moderate
tilting and faulting, it follows that mountain-making movements again
affected the region during later Tertiary. They did not, however,
produce extreme effects. They did stimulate erosion and bring about a
reorganization of all the slopes with respect to the new levels.
This agrees closely with a second line of evidence which rests upon an
independent basis. The alluvial fill which lies upon all the canyon and
valley floors is of glacial origin, as shown by its interlocking
relations with morainal deposits at the valley heads. It is now in
process of dissection and since its deposition in the Pleistocene had
been eroded on the average about 200 feet. Clearly, to form a 3,000-foot
canyon in hard rock requires much more time than to deposit and again
partially to excavate an alluvial fill several hundred feet deep.
Moreover, the glacial material is coarse throughout, and was built up
rapidly and dissected rapidly. In most cases, furthermore, coarse
material at the bottom of the glacial series rests directly upon the
rock of a narrow and ungraded valley floor. From these and allied facts
it is concluded that there is no long time interval represented by the
transitions from degrading to aggrading processes and back again. The
early Pleistocene, therefore, seems quite too short a period in which to
produce the bold forms and effect the deep erosion which marks the
period between the close of the mature cycle and the beginnings of
deposition in the Pleistocene.
The alternative conclusion is that the greater part of the canyon
cutting was effected in the late Tertiary, and that it continued into
the early Pleistocene until further erosion was halted by changed
climatic conditions and the augmented delivery of land waste to all the
streams. The final development of the well-graded high-level slopes is,
therefore, closely confined to a small portion of the Tertiary. The
closest estimate which the facts support appears to be Miocene or early
Pliocene. It is clear, however, that only the culmination of the period
can be definitely assigned. Erosion was in full progress at the close of
the Cretaceous and by middle Tertiary had effected vast changes in the
landscape. The Tertiary strata are marked by coarse basal deposit and by
thin and very fine top deposits. Though their deformed condition
indicates a period of crustal disturbance, the Tertiary beds give no
indication of wholesale transformations. They indicate chiefly tilting
and moderate and normal faulting. The previously developed effects of
erosion were, therefore, not radically modified. The surface was thus in
large measure prepared by erosion in the early Tertiary for its final
condition of maturity reached during the early Pliocene.
It seems appropriate, in concluding this chapter, to summarize in its
main outlines the physiography of southern Peru, partly to condense the
extended discussion of the preceding paragraphs, and partly to supply a
background for the three chapters that follow. The outstanding features
are broad plateau areas separated by well-defined “Cordilleras.” The
plateau divisions are not everywhere of the same origin. Those southwest
of Cuzco (Fig. 130), and in the Anta Basin (Fig. 124), northwest of
Cuzco, are due to prolonged erosion and may be defined as peneplane
surfaces uplifted to a great height. They are now bordered on the one
hand by deep valleys and troughs and basins of erosion and deformation;
and, on the other hand, by residual elevations that owe their present
topography to glacial erosion superimposed upon the normal erosion of
the peneplane cycle. The residuals form true mountain chains like the
Cordillera Vilcanota and Cordillera Vilcapampa; the depressions due to
erosion or deformation or both are either basins like those of Anta and
Cuzco or valleys of the canyon type like the Urubamba canyon; the
plateaus are broad rolling surfaces, the punas of the Peruvian Andes.
There are two other types of plateaus. The one represents a mature stage
in the erosion cycle instead of an ultimate stage; the other is volcanic
in origin. The former is best developed about Antabamba (Figs. 122 and
123), where again deep canyons and residual ranges form the borders of
the plateau remnants. The latter is well developed above Cotahuasi and
in its simplest form is represented in Fig. 133. Its surface is the top
of a vast accumulation of lavas in places over a mile thick. While rough
in detail it is astonishingly smooth in a broad view (Fig. 29). Above it
rise two types of elevations: first, isolated volcanic cones of great
extent surrounded by huge lava flows of considerable relief; and second,
discontinuous lines of peaks where volcanic cones of less extent are
crowded closely together. The former type is displayed on the Coropuna
Quadrangle, the latter on the Cotahuasi and La Cumbre Quadrangles.
So high is the elevation of the lava plateau, so porous its soil, so dry
the climate, that a few through-flowing streams gather the drainage of a
vast territory and, as in the Grand Canyon country of our West, they
have at long intervals cut profound canyons. The Arma has cut a deep
gorge at Salamanca; the Cotahuasi runs in a canyon in places 7,000 feet
deep; the Majes heads at the edge of the volcanic field in a steep
amphitheatre of majestic proportions.
Finally, we have the plateaus of the coastal zone. These are plains with
surfaces several thousand feet in elevation separated by gorges several
thousand feet deep. The Pampa de Sihuas is an illustration. The
post-maturely dissected Coast Range separates it from the sea. The
pampas are in general an aggradational product formed in a past age
before uplift initiated the present canyon cycle of erosion. Other
plateaus of the coastal zone are erosion surfaces. The Tablazo de Ica
appears to be of this type. That at Arica, Chile, near the southern
boundary of Peru, is demonstrably of this type with a border on which
marine planation has in places given rise to a broad terrace
effect.[43]
CHAPTER XII
THE WESTERN ANDES: THE MARITIME CORDILLERA OR CORDILLERA OCCIDENTAL
The Western or Maritime Cordillera of Peru forms part of the great
volcanic field of South America which extends from Argentina to Ecuador.
On the walls of the Cotahuasi Canyon (Fig. 131), there are exposed over
one hundred separate lava flows piled 7,000 feet deep. They overflowed a
mountainous relief, completely burying a limestone range from 2,000 to
4,000 feet high. Finally, upon the surface of the lava plateau new
mountains were formed, a belt of volcanoes 5,000 feet (1,520 m.) high
and from 15,000 to 20,000 feet (4,570 to 6,100 m.) above the sea. There
were vast mud flows, great showers of lapilli, dust, and ashes, and with
these violent disturbances also came many changes in the drainage. Sixty
miles northeast of Cotahuasi the outlet of an unnamed deep valley was
blocked, a lake was formed, and several hundred feet of sediments were
deposited. They are now wasting rapidly, for they lie in the zone of
alternate freezing and thawing, a thousand feet and more below the
snowline. Some of their bad-land forms look like the solid bastions of
an ancient fortress, while others have the delicate beauty of a Japanese
temple.
Not all the striking effects of vulcanism belong to the remote geologic
past. A day’s journey northeast of Huaynacotas are a group of lakes only
recently hemmed in by flows from the small craters thereabouts. The
fires in some volcanic craters of the Peruvian Andes are still active,
and there is no assurance that devastating flows may not again inundate
the valleys. In the great Pacific zone or girdle of volcanoes the
earth’s crust is yet so unstable that earthquakes occur every year, and
at intervals of a few years they have destructive force. Cotahuasi was
greatly damaged in 1912; Abancay is shaken every few years; and the
violent earthquakes of Cuzco and Arequipa are historic.
On the eastern margin of the volcanic country the flows thin out and
terminate on the summit of a limestone (Cretaceous) plateau. On the
western margin they descend steeply to the narrow west-coast desert. The
greater part of the lava dips beneath the desert deposits; there are a
few intercalated flows in the deposits themselves, and the youngest
flows--limited in number--have extended down over the inner edge of the
desert.
The immediate coast of southern Peru is not volcanic. It is composed of
a very hard and ancient granite-gneiss which forms a narrow coastal
range (Fig. 171). It has been subjected to very long and continued
erosion and now exhibits mature erosion forms of great uniformity of
profile and declivity.
From the outcrops of older rocks beneath the lavas it is possible to
restore in a measure the pre-volcanic topography of the Maritime
Cordillera, In its present altitude it ranges from several thousand to
15,000 feet above sea level. The unburied topography has been smoothed
out; the buried topography is rough (Figs. 29 and 166). The contact
lines between lavas and buried surfaces in the deep Majes and Cotahuasi
valleys are in places excessively serrate. From this, it seems safe to
conclude that the period of vulcanism was so prolonged that great
changes in the unburied relief were effected by the agents of erosion.
Thus, while the dominant process of volcanic upbuilding smoothed the
former rough topography of the Maritime Cordillera, erosion likewise
measurably smoothed the present high extra-volcanic relief in the
central and eastern sections. The effect has been to develop a broad and
sufficiently smooth aspect to the summit topography of the entire Andes
to give them a plateau character. Afterward the whole mountain region
was uplifted about a mile above its former level so that at present it
is also continuously lofty.
The zone of most intense volcanic action does not coincide with the
highest part of the pre-volcanic topography. If the pre-volcanic relief
were even in a very general way like that which would be exhibited if
the lavas were now removed, we should have to say that the chief
volcanic outbursts took place on the western flank of an old and deeply
dissected limestone range.
[Illustration: FIG. 129--Composition of slopes at Puquiura, Vilcabamba
Valley, elevation 9,000 feet (2,740 m.). The second prominent spur
entering the valley on the left has a flattish top unrelated to the rock
structure. Like the spurs on the right its blunt end and flat top
indicate an earlier erosion cycle at a lower elevation.]
[Illustration: FIG. 130--Inclined Paleozoic strata truncated by an
undulating surface of erosion at 15,000 feet, southwest of Cuzco.]
[Illustration: FIG. 131--Terraced valley slopes at Huaynacotas,
Cotahuasi Valley, at 11,500 feet (3,500 m.). Solimana is in the
background. On the floor of the Cotahuasi Canyon fruit trees grow. At
Huaynacotas corn and potatoes are the chief products. The section is
composed almost entirely of lava. There are over a hundred major flows
aggregating 5,000 to 7,000 feet thick.]
The volume of the lavas is enormous. They are a mile and a half thick,
nearly a hundred miles wide, and of indefinite extent north and south.
Their addition to the Andes, therefore, _has greatly broadened the zone
of lofty mountains_. Their passes are from 2,000 to 3,000 feet higher
than the passes of the eastern Andes. They have a much smaller number of
valleys sufficiently deep to enjoy a mild climate. Their soil is far
more porous and dry. Their vegetation is more scanty. They more than
double the difficulties of transportation. And, finally, their all but
unpopulated loftier expanses are a great vacant barrier between farms in
the warm valleys of eastern Peru and the ports on the west coast.
The upbuilding process was not, of course, continuous. There were at
times intervals of quiet, and some of them were long enough to enable
streams to become established. Buried valleys may be observed in a
number of places on the canyon walls, where subsequently lava flows
displaced the streams and initiated new drainage systems. In these quiet
intervals the weathering agents attacked the rock surfaces and formed
soil. There were at least three or four such prolonged periods of
weathering and erosion wherein a land surface was exposed for many
thousands of years, stream systems organized, and a cultivable soil
formed. No evidence has been found, however, that man was there to
cultivate the soil.
The older valleys cut in the quiet period are mere pygmies beside the
giant canyons of today. The present is the time of dominant erosion. The
forces of vulcanism are at last relatively quiet. Recent flows have
occurred, but they are limited in extent and in effects. They alter only
the minor details of topography and drainage. Were it not for the oases
set in the now deep-cut canyon floors, the lava plateau of the Maritime
Cordillera would probably be the greatest single tract of unoccupied
volcanic country in the world.
The lava plateau has been dissected to a variable degree. Its high
eastern margin is almost in its original condition. Its western margin
is only a hundred miles from the sea, so that the streams have steep
gradients. In addition, it is lofty enough to have a moderate rainfall.
It is, therefore, deeply and generally dissected. Within the borders of
the plateau the degree of dissection depends chiefly upon position with
respect to the large streams. These were in turn located in an
accidental manner. The repeated upbuilding of the surface by the
extensive outflow of liquid rock obliterated all traces of the earlier
drainage. In the Cotahuasi Canyon the existing stream, working down
through a mile of lavas, at last uncovered and cut straight across a
mountain spur 2,000 feet high. Its course is at right angles to that
pursued by the stream that once drained the spur. It is noteworthy that
the Cotahuasi and adjacent streams take northerly courses and join
Atlantic rivers. The older drainage was directly west to the Pacific.
Thus, vulcanism not only broadened the Andes and increased their height,
but also moved the continental divide still nearer the west coast.
The glacial features of the western or Maritime Cordillera are of small
extent, partly because vulcanism has added a considerable amount of
material in post-glacial time, partly because the climate is so
exceedingly dry that the snowline lies near the top of the country. The
slopes of the volcanic cones are for the most part deeply recessed on
the southern or shady sides. Above 17,500 feet (5,330 m.) the process of
snow and ice excavation still continues, but the tracts that exceed this
elevation are confined to the loftiest peaks or their immediate
neighborhood. There is a distinct difference between the glacial forms
of the eastern or moister and the western or dryer flanks of this
Cordillera. Only peaks like Coropuna and Solimana near the western
border now bear or ever bore snowfields and glaciers. By contrast the
eastern aspect is heavily glaciated. On La Cumbre Quadrangle, there is a
huge glacial trough at 16,000 feet (4,876 m.), and this extends with
ramifications up into the snowfields that formerly included the highest
country. Prolonged glacial erosion produced a full set of topographic
forms characteristic of the work of Alpine glaciers. Thus, each of the
main mountain chains that make up the Andean system has, like the system
as a whole, a relatively more-dry and a relatively less-dry aspect. The
snowline is, therefore, canted from west to east on each chain as well
as on the system. However, this effect is combined with a solar effect
in an unequal way. In the driest places the solar factor is the more
efficient and the snowline is there canted from north to south.
CHAPTER XIII
THE EASTERN ANDES: THE CORDILLERA VILCAPAMPA
The culminating range of the eastern Andes is the so-called Cordillera
Vilcapampa. Its numerous, sharp, snow-covered peaks are visible in every
summit view from the central portion of the Andean system almost to the
western border of the Amazon basin. Though the range forms a water
parting nearly five hundred miles long, it is crossed in several places
by large streams that flow through deep canyons bordered by precipitous
cliffs. The Urubamba between Torontoy and Colpani is the finest
illustration. For height and ruggedness the Vilcapampa mountains are
among the most noteworthy in Peru. Furthermore, they display glacial
features on a scale unequaled elsewhere in South America north of the
ice fields of Patagonia.
GLACIERS AND GLACIAL FORMS
One of the most impressive sights in South America is a tropical forest
growing upon a glacial moraine. In many places in eastern Bolivia and
Peru the glaciers of the Ice Age were from 5 to 10 miles long--almost
the size of the Mer de Glace or the famous Rhone glacier. In the Juntas
Valley in eastern Bolivia the tree line is at 10,000 feet (3,050 m.),
but the terminal moraines lie several thousand feet lower. In eastern
Peru the glaciers in many places extended down nearly to the tree line
and in a few places well below it. In the Cordillera Vilcapampa vast
snowfields and glacier systems were spread out over a summit area as
broad as the Southern Appalachians. The snowfields have since shrunk to
the higher mountain recesses; the glaciers have retreated for the most
part to the valley heads or the cirque floors; and the lower limit of
perpetual snow has been raised to 15,500 feet.
[Illustration: FIG. 132--Recessed volcanoes in the right background and
eroded tuffs, ash beds, and lava flows on the left. Maritime Cordillera
above Cotahuasi.]
[Illustration: FIG. 133--The summit of the great lava plateau above
Cotahuasi on the trail to Antabamba. The lavas are a mile and a half in
thickness. The elevation is 16,000 feet. Hence the volcanoes in the
background, 17,000 feet above sea level, are mere hills on the surface
of the lofty plateau.]
[Illustration: FIG. 134--Southwestern aspect of the Cordillera
Vilcapampa between Anta and Urubamba from Lake Huaipo. Rugged summit
topography in the background, graded post-mature slopes in the middle
distance, and solution lake in limestone in the foreground.]
[Illustration: FIG. 135--Summit view, Cordillera Vilcapampa. There are
fifteen glaciers represented in this photograph. The camera stands on
the summit of a minor divide in the zone of nivation.]
These features are surprising because neither Whymper[44] nor Wolf[45]
mentions the former greater extent of the ice on the volcanoes of
Ecuador, only ten or twelve degrees farther north. Moreover, Reiss[46]
denies that the hypothesis of universal climatic change is supported by
the facts of a limited glaciation in the High Andes of Ecuador; and J.
W. Gregory[47] completely overlooks published proof of the existence of
former more extensive glaciers elsewhere in the Andes:
“... the absence not only of any traces of former more extensive
glaciation from the tropics, as in the Andes and Kilimandjaro, but also
from the Cape.” He says further: “In spite of the extensive glaciers now
in existence on the higher peaks of the Andes, there is practically no
evidence of their former greater extension.”(!)
Whymper spent most of his time in exploring recent volcanoes or those
recently in eruption, hence did not have the most favorable
opportunities for gathering significant data. Reiss was carried off his
feet by the attractiveness of the hypothesis[48] relating to the effect
of glacial denudation on the elevation of the snowline. Gregory appeared
not to have recognized the work of Hettner on the Cordillera of Bogotá
and of Sievers[49] and Acosta on the Sierra Nevada de Santa Marta in
northern Colombia.
The importance of the glacial features of the Cordillera Vilcapampa
developed on a great scale in very low latitudes in the southern
hemisphere is twofold: first, it bears on the still unsettled problem of
the universality of a colder climate in the Pleistocene, and, second, it
supplies additional data on the relative depression of the snowline in
glacial times in the tropics. Snow-clad mountains near the equator are
really quite rare. Mount Kenia rising from a great jungle on the
equator, Kilimandjaro with its two peaks, Kibo and Mawenzi, two hundred
miles farther south, and Ingomwimbi in the Ruwenzori group thirty miles
north of the equator, are the chief African examples. A few mountains
from the East Indies, such as Kinibalu in Borneo, latitude 6° north,
have been found glaciated, though now without a snow cover. In higher
latitudes evidences of an earlier extensive glaciation have been
gathered chiefly from South America, whose extension 13° north and 56°
south of the equator, combined with the great height of its dominating
Cordillera, give it unrivaled distinction in the study of mountain
glaciation in the tropics.
Furthermore, mountain summits in tropical lands are delicate climatic
registers. In this respect they compare favorably with the inclosed
basins of arid regions, where changes in climate are clearly recorded in
shoreline phenomena of a familiar kind. Lofty mountains in the tropics
are in a sense inverted basins, the lower snowline of the past is like
the higher shoreline of an interior basin; the terminal moraines and the
alluvial fans in front of them are like the alluvial fans above the
highest strandline; the present snow cover is restricted to mountain
summits of small areal extent, just as the present water bodies are
restricted to the lowest portions of the interior basin; and successive
retreatal stages are marked by terminal moraines in the one case as they
are marked in the other by flights of terraces and beach ridges.
I made only a rapid reconnaissance across the Cordillera Vilcapampa in
the winter season, and cannot pretend from my limited observations to
solve many of the problems of the field. The data are incorporated
chiefly in the chapter on Glacial Features. In this place it is proposed
to describe only the more prominent glacial features, leaving to later
expeditions the detailed descriptions upon which the solution of some of
the larger problems must depend.
At Choquetira three prominent stages in the retreat of the ice are
recorded. The lowermost stage is represented by the great fill of
morainic and outwash material at the junction of the Choquetira, and an
unnamed valley farther south at an elevation of 11,500 feet (3,500 m.).
A mile below Choquetira a second moraine appears, elevation 12,000 feet
(3,658 m.), and immediately above the village a third at 12,800 (3,900
m.). The lowermost moraine is well dissected, the second is ravined and
broken but topographically distinct, the third is sharp-crested and
regular. A fourth though minor stage is represented by the moraine at
the snout of the living glacier and still less important phases are
represented in some valleys--possibly the record of post-glacial changes
of climate. Each main moraine is marked by an important amount of
outwash, the first and third moraines being associated with the greatest
masses. The material in the moraines represents only a part of that
removed to form the successive steps in the valley profile. The
lowermost one has an enormous volume, since it is the oldest and was
built at a time when the valley was full of waste. It is fronted by a
deep fill, over the dissected edge of which one may descend 800 feet in
half an hour. It is chiefly alluvial in character, whereas the next
higher one is composed chiefly of bowlders and is fronted by a
pronounced bowlder train, which includes a remarkable perched bowlder of
huge size. Once the valley became cleaned out the ice would derive its
material chiefly by the slower process of plucking and abrasion, hence
would build much smaller moraines during later recessional stages, even
though the stages were of equivalent length.
[Illustration: FIG. 136--Glacial sculpture on the southwestern flank of
the Cordillera Vilcapampa. Flat-floored valleys and looped terminal
moraines below and glacial steps and hanging valleys are characteristic.
The present snowfields and glaciers are shown by dotted contours.]
There is a marked difference in the degree of dissection of the
moraines. The lowermost and oldest is so thoroughly dissected as to
exhibit but little of its original surface. The second has been greatly
modified, but still possesses a ridge-like quality and marks the
beginning of a noteworthy flattening of the valley gradient. The third
is as sharp-crested as a roof, and yet was built so long ago that the
flat valley floor behind it has been modified by the meandering stream.
From this point the glacier retreated up-valley several miles
(estimated) without leaving more than the thinnest veneer on the valley
floor. The retreat must, therefore, have been rapid and without even
temporary halts until the glacier reached a position near that occupied
today. Both the present ice tongues and snowfields and those of a past
age are emphasized by the presence of a patch of scrub and woodland that
extends on the north side of the valley from near the snowline down over
the glacial forms to the lower valley levels.
The retreatal stages sketched above would call for no special comment if
they were encountered in mountains in northern latitudes. They would be
recognized at once as evidence of successive periodic retreats of the
ice, due to successive changes in temperature. To understand their
importance when encountered in very low latitudes it is necessary to
turn aside for a moment and consider two rival hypotheses of glacial
retreat. First we have the hypothesis of periodic retreat, so generally
applied to terminal moraines and associated outwash in glaciated
mountain valleys. This implies also an advance of the ice from a higher
position, the whole taking place as a result of a climatic change from
warmer to colder and back again to warmer.
[Illustration: FIG. 137--Looking up a spurless flat-floored glacial
trough near the Chucuito pass in the Cordillera Vilcapampa from 14,200
feet (4,330 m.). Note the looped terminal and lateral moraines on the
steep valley wall on the left. A stone fence from wall to wall serves to
inclose the flock of the mountain shepherd.]
[Illustration: FIG. 138--Terminal moraine in the glaciated Choquetira
Valley below Choquetira. The people who live here have an abundance of
stones for building corrals and stone houses. The upper edge of the
timber belt (cold timber line) is visible beyond the houses. Elevation
12,100 feet (3,690 m.).]
But evidences of more extensive mountain glaciation in the past do not
in themselves prove a change in climate over the whole earth. In an
epoch of fixed climate a glacier system may so deeply and thoroughly
erode a mountain mass, that the former glaciers may either diminish in
size or disappear altogether. As the work of excavation proceeds, the
catchment basins are sunk to, and at last below, the snowline; broad
tributary spurs whose snows nourish the glaciers, may be reduced to
narrow or skeleton ridges with little snow to contribute to the valleys
on either hand; the glaciers retreat and at last disappear. There
would be evidences of glaciation all about the ruins of the former
loftier mountain, but there would be no living glaciers. And yet the
climate might remain the same throughout.
It is this “topographic” hypothesis that Reiss and Stübel accept for the
Ecuadorean volcanoes. Moreover, the volcanoes of Ecuador are practically
on the equator--a very critical situation when we wish to use the facts
they exhibit in the solution of such large problems as the
contemporaneous glaciation of the two hemispheres, or the periodic
advance and retreat of the ice over the whole earth. This is not the
place to scrutinize either their facts or their hypothesis, but I am
under obligations to state very emphatically that the glacial features
of the Cordillera Vilcapampa require the climatic and not the
topographic hypothesis. Let us see why.
The differences in degree of dissection and the flattening gradient
up-valley that we noted in a preceding paragraph leave no doubt that
each moraine of the bordering valleys in the Vilcapampa region,
represents a prolonged period of stability in the conditions of
topography as well as of temperature and precipitation. If change in
topographic conditions is invoked to explain retreat from one position
to the other there is left no explanation of the periodicity of retreat
which has just been established. If a period of cold is inaugurated and
glaciers advance to an ultimate position, they can retreat only through
change of climate effected either by general causes or by topographic
development to the point where the snowfields become restricted in size.
In the case of climatic change the ice changes are periodic. In the case
of retreat due to topographic change there should be a steady or
non-periodic falling back of the ice front as the catchment basins
decrease in elevation and the snow-gathering ridges tributary to them
are reduced in height.
Further, the matterhorns of the Cordillera Vilcapampa are not bare but
snow-covered, vigorous glaciers several miles in length and large
snowfields still survive and the divides are not arêtes but broad
ridges. In addition, the last two moraines, composed of very loose
material, are well preserved. They indicate clearly that the time since
their formation has witnessed no wholesale topographic change. If (1) no
important topographic changes have taken place, and (2) a vigorous
glacier lay for a long period back of a given moraine, and (3) _suddenly
retreated several miles and again became stable_, we are left without
confidence in the application of the topographic hypothesis to the
glacial features of the Vilcapampa region. Glacial retreat may be
suddenly begun in the case of a late stage of topographic development,
but it should be an orderly retreat marked by a large number of small
moraines, or at least a plentiful strewing of the valley floor with
débris.
[Illustration: FIG. 139--Glacial features on the eastern slopes of the
Cordillera Vilcapampa.]
The number of moraines in the various glaciated valleys of the
Cordillera Vilcapampa differ, owing to differences in elevation and to
the variable size of the catchment basins. All valleys, however, display
the same sudden change from moraine to moraine and the same
characteristics of gradient. In all of them the lowermost moraine is
always more deeply eroded than the higher moraines, in all of them
glacial erosion was sufficiently prolonged greatly to modify the valley
walls, scour out lake basins, or broad flat valley floors, develop
cirques, arêtes, and pinnacled ridges in limited number. In some,
glaciation was carried to the point where only skeleton divides
remained, in most places broad massive ridges or mountain knots persist.
In spite of all these differences successive moraines were formed,
separated by long stretches either thinly covered with till or exposing
bare rock.
In examining this group of features it is important to recognize the
essential fact that though the number of moraines varies from valley to
valley, the differences in character between the moraines at low and at
high elevations in a single valley are constant. It is also clear that
everywhere the ice retreated and advanced periodically, no matter with
what topographic features it was associated, whether those of maturity
or of youth in the glacial cycle. We, therefore, conclude that
topographic changes had no significant part to play in the glacial
variations in the Cordillera Vilcapampa.
The country west of the Cordillera Vilcapampa had been reduced to early
topographic maturity before the Ice Age, and then uplifted with only
moderate erosion of the masses of the interfluves. That on the east had
passed through the same sequence of events, but erosion had been carried
much farther. The reason for this is found in a strong climatic
contrast. The eastern is the windward aspect and receives much more rain
than the western. Therefore, it has more streams and more rapid
dissection. The result was that the eastern slopes were cut to pieces
rapidly after the last great regional uplift; the broad interfluves were
narrowed to ridges. The region eastward from the crest of the Cordillera
to the Pongo de Mainique looks very much like the western half of the
Cascade Mountains in Oregon--the summit tracts of moderate declivity are
almost all consumed.
The effect of these climatic and topographic contrasts is manifested in
strong contrasts in the position and character of the glacial forms on
the opposite slopes of the range. At Pampaconas on the east the
lowermost terminal moraine is at least a thousand feet below timber
line. Between Vilcabamba pueblo and Puquiura the terminal moraine lies
at 11,200 feet (3,414 m.). By contrast the largest Pleistocene glacier
on the western slope, nearly twelve miles long, and the largest along
the traverse, ended several miles below Choquetira at 11,500 feet (3,504
m.) elevation, or just at the timber line. Thus, the steeper descents of
the eastern side of the range appear to have carried short glaciers to
levels far lower than those attained by the glaciers of the western
slope.
It seems at first strange that the largest glaciers were west of the
divide between the Urubamba and the Apurimac, that is, on the relatively
dry side of the range. The reason lies in a striking combination of
topographic and climatic conditions. Snow is a mobile form of
precipitation that is shifted about by the wind like a sand dune in the
desert. It is not required, like water, to begin a downhill movement as
soon as it strikes the earth. Thus, it is a noteworthy fact that snow
drifting across the divides may ultimately cause the largest snowfields
to lie where the least snow actually falls. This is illustrated in the
Bighorns of Wyoming and others of our western ranges. It is, however,
not the wet snow near the snowline, but chiefly the dry snow of higher
altitudes that is affected. What is now the dry or leeward side of the
Cordillera appears in glacial times to have actually received more snow
than the wet windward side.
[Illustration: FIG. 140--Glacial sculpture in the heart of the
Cordillera Vilcapampa. In places the topography has so high a relief
that the glaciers seem almost to overhang the valleys. See Figs. 96 and
179 for photographs.]
The topography conspired to increase this contrast. In place of many
streams, direct descents, a dispersion of snow in many valleys, as on
the east, the western slopes had indirect descents, gentler valley
profiles, and that higher degree of concentration of drainage which
naturally goes with topographic maturity. For example, there is nothing
in the east to compare with the big spurless valley near the pass above
Arma. The side walls were so extensively trimmed that the valley was
turned into a trough. The floor was smoothed and deepened and all the
tributary glaciers were either left high up on the bordering slopes or
entered the main valley with very steep profiles; their lateral and
terminal moraines now hang in festoons on the steep side walls.
Moreover, the range crest is trimmed from the west so that the serrate
skyline is a feature rarely seen from eastern viewpoints. This may not
hold true for more than a small part of the Cordillera. It was probably
emphasized here less by the contrasts already noted than by the geologic
structure. The eastward-flowing glaciers descended over dip slopes on
highly inclined sandstones, as at Pampaconas. Those flowing westward
worked either in a jointed granite or on the outcropping _edges_ of the
sandstones, where the quarrying process known as glacial plucking
permitted the development of excessively steep slopes.
There are few glacial steps in the eastern valleys. The western valleys
have a marvelous display of this striking glacial feature. The
accompanying hachure maps show them so well that little description is
needed. They are from 50 to 200 feet high. Each one has a lake at its
foot into which the divided stream trickles over charming waterfalls.
All of them are clearly associated with a change in the volume of the
glacier that carved the valley. Wherever a tributary glacier entered, or
the side slopes increased notably in area, a step was formed. By retreat
some of them became divided, for the process once begun would push the
step far up valley after the manner of an extinguishing waterfall.
The retreat of the steps, the abrasion of the rock, and the sapping of
the cirques at the valley heads excavated the upper valleys so deeply
that they are nearly all, as W. D. Johnson has put it, “down at the
heel.” Thus, above Arma, one plunges suddenly from the smooth, grassy
glades of the strongly glaciated valley head down over the outer slopes
of the lowermost terminal moraine to the steep lower valley. Above the
moraine are fine pastures, in the steep valley below are thickets and
rocky defiles. There are long quiet reaches in the streams of the
glaciated valley heads besides pretty lakes and marshes. Below, the
stream is swift, almost torrential. Arma itself is built upon alluvial
deposits of glacial origin. A mile farther down the valley is
constricted and steep-walled--really a canyon.
Though the glaciers have retreated to the summit region, they are by no
means nearing extinction. The clear blue ice of the glacier descending
from Mt. Soiroccocha in the Arma Valley seems almost to hang over the
precipitous valley border. In curious contrast to its suggestion of cold
and storm is the patch of dark green woodland which extends right up to
its border. An earthquake might easily cause the glacier to invade the
woodland. Some of the glaciers between Choquetira and Arma rest on
terminal moraines whose distal faces are from 200 to 300 feet high. The
ice descending southeasterly from Panta Mt. is a good illustration.
Earlier positions of the ice front are marked by equally large moraines.
The one nearest that engaged by the living glacier confines a large lake
that discharges through a gap in the moraine and over a waterfall to the
marshy floor of the valley.
Retreat has gone so far, however, that there are only a few large
glacier systems. Most of the tributaries have withdrawn toward their
snowfields. In place of the twenty distinct glaciers now lying between
the pass and the terminal moraine below Choquetira, there was in glacial
times one great glacier with twenty minor tributaries. The cirques now
partly filled with damp snow must then have been overflowing with dry
snow above and ice below. Some of the glaciers were over a thousand feet
thick; a few were nearly two thousand feet thick, and the cirques that
fed them held snow and ice at least a half mile deep. Such a remarkably
complete set of glacial features only 700 miles from the equator is
striking evidence of the moist climate on the windward eastern part of
the great Andean Cordillera, of the universal change in climate in the
glacial period, and of the powerful dominating effects of ice erosion in
this region of unsurpassed Alpine relief.
THE VILCAPAMPA BATHOLITH AND ITS TOPOGRAPHIC EFFECTS
[Illustration: FIG. 141--Composite geologic section on the northeastern
border of the Cordillera Vilcapampa, in the vicinity of Pampaconas, to
show the deformative effects of the granite intrusion. There is a
limited amount of limestone near the border of the Cordillera. Both
limestone and sandstone are Carboniferous. See Appendix B. See also
Figs. 142 and 146. The section is about 15 miles long.]
The main axis of the Cordillera Vilcapampa consists of granite in the
form of a batholith between crystalline schists on the one hand
(southwest), and Carboniferous limestones and sandstones and Silurian
shales and slates on the other (northeast). It is not a domal uplift in
the region in which it was observed in 1911, but an axial intrusion, in
places restricted to a narrow belt not more than a score of miles
across. As we should expect from the variable nature of the invaded
material, the granite belt is not uniform in width nor in the character
of its marginal features. In places the intrusion has produced
strikingly little alteration of the country rock; in other localities
the granite has been injected into the original material in so intimate
a manner as almost completely to alter it, and to give rise to a very
broad zone of highly metamorphosed rock. Furthermore, branches were
developed so that here and there tributary belts of granite extend from
the main mass to a distance of many miles. Outlying batholiths occur
whose common petrographic character and similar manner of occurrence
leave little doubt that they are related abyssally to a common plutonic
mass.
The Vilcapampa batholith has two highly contrasted borders, whether we
consider the degree of metamorphism of the country rock, the definition
of the border, or the resulting topographic forms. On the northeastern
ridge at Colpani the contact is so sharp that the outstretched arms in
some places embrace typical granite on the one hand and almost
unaltered shales and slates on the other. Inclusions or xenoliths of
shale are common, however, ten and fifteen miles distant, though they
are prominent features in a belt only a few miles wide. The lack of more
intense contact effects is a little remarkable in view of the altered
character of the inclusions, all of which are crystalline in contrast to
the fissile shales from which they are chiefly derived. Inclusions
within a few inches of the border fall into a separate class, since they
show in general but trifling alteration and preserve their original
cleavage plains. It appears that the depth of the intrusion must have
been relatively slight or the intrusion sudden, or both shallow and
sudden, conditions which produce a narrow zone of metamorphosed material
and a sharp contact.
[Illustration: FIG. 142--The deformative effects of the Vilcapampa
intrusion on the northeastern border of the Cordillera. The deformed
strata are heavy-bedded sandstones and shales and the igneous rocks are
chiefly granites with bordering porphyries. Looking northwest near
Puquiura. For conditions near Pampaconas, looking in the opposite
direction, see Fig. 141. For conditions on the other side of the
Cordillera, see Fig. 146.]
The relation between shale and granite at Colpani is shown in Fig. 143.
Projections of granite extend several feet into the shale and slate and
generally end in blunt barbs or knobs. In a few places there is an
intimate mixture of irregular slivers and blocks of crystallized
sediments in a granitic groundmass, with sharp lines of demarcation
between igneous and included material. The contact is vertical for at
least several miles. It is probable that other localities on the contact
exhibit much greater modification and invasion of the weak shales and
slates, but at Colpani the phenomena are both simple and restricted in
development.
[Illustration: FIG. 143--Relation of granite intrusion to schist on the
northeastern border of the Vilcapampa batholith near the bridge of
Colpani, lower end of the granite Canyon of Torontoy. The sections are
from 15 to 25 feet high and represent conditions at different levels
along the well-defined contact.]
The highly mineralized character of the bordering sedimentary strata,
and the presence of numbers of complementary dikes, nearly identical in
character to those in the parent granite now exposed by erosion over a
broad belt roughly parallel to the contact, supplies a basis for the
inference that the granite may underlie the former at a slight depth, or
may have had far greater metamorphic effects upon its sedimentary roof
than the intruded granite has had upon its sedimentary rim.
The physiographic features of the contact belt are of special interest.
No available physiographic interpretation of the topography of a
batholith includes a discussion of those topographic and drainage
features that are related to the lithologic character of the intruded
rock, the manner of its intrusion, or the depth of erosion since
intrusion. Yet each one of these factors has a distinct topographic
effect. We shall, therefore, turn aside for a moment from the detailed
discussion of the Vilcapampa region to an examination of several
physiographic principles and then return to the main theme for
applications.
It is recognized that igneous intrusions are of many varieties and that
even batholithic invasions may take place in rather widely different
ways. Highly heated magmas deeply buried beneath the earth’s surface
produce maximum contact effects, those nearer the surface may force the
strata apart without extreme lithologic alterations of the displaced
beds, while through the stoping process a sedimentary cover may be
largely absorbed and the magmas may even break forth at the surface as
in ordinary vulcanism. If the sedimentary beds have great vertical
variation in resistance, in attitude, and in composition, there may be
afforded an opportunity for the display of quite different effects at
different levels along a given contact, so that a great variety of
physical conditions will be passed by the descending levels of erosion.
At one place erosion may have exposed only the summit of the batholith,
at another the associated dikes and sheets and ramifying branches may be
exposed as in the zone of fracture, at a third point the original zone
of flowage may be reached with characteristic marginal schistosity,
while at still greater depths there may be uncovered a highly
metamorphosed rim of resistant sedimentary rock.
The mere enumeration of these variable structural features is sufficient
to show how variable we should expect the associated land forms to be.
Were the forms of small extent, or had they but slight distinction upon
comparison with other erosional effects, they would be of little
concern. They are, on the contrary, very extensively developed; they
affect large numbers of lofty mountain ranges besides still larger areas
of old land masses subjected to extensive and deep erosion, thus laying
bare many batholiths long concealed by a thick sedimentary roof.
The differences between intruded and country rock dependent upon these
diversified conditions of occurrence are increased or diminished
according to the history of the region after batholithic invasion takes
place. Regional metamorphism may subsequently induce new structures or
minimize the effects of the old. Joint systems may be developed, the
planes widely spaced in one group of rocks giving rise to monolithic
masses very resistant to the agents of weathering, while those of an
adjacent group may be so closely spaced as greatly to hasten the rate of
denudation. There may be developed so great a degree of schistosity in
one rock as to give rise (with vigorous erosion) to a serrate
topography; on the other hand the forms developed on the rocks of a
batholith may be massive and coarse-textured.
To these diversifying conditions may be added many others involving a
large part of the field of dynamic geology. It will perhaps suffice to
mention two others: the stage of erosion and the special features
related to climate. If a given intrusion has been accompanied by an
important amount of uplift or marginal compression, vigorous erosion may
follow, whereupon a chance will be offered for the development of the
greatest contrast in the degree of boldness of topographic forms
developed upon rocks of unequal resistance. Ultimately these contrasts
will diminish in intensity, as in the case of all regional differences
of relief, with progress toward the end of the normal cycle of erosion.
If peneplanation ensue, only feeble topographic differences may mark
the line of contact which was once a prominent topographic feature. With
reference to the effects of climate it may be said simply that a granite
core of batholithic origin may extend above the snowline or above timber
line or into the timbered belt, whereas the invaded rock may occur
largely below these levels with obvious differences in both the rate and
the kind of erosion affecting the intruded mass.
[Illustration: FIG. 144--Cliffed canyon wall in the Urubamba Valley
between Huadquiña and Torontoy. There is a descent of nearly 2,000 feet
shown in the photograph and it is developed almost wholly along
successive joint planes.]
[Illustration: FIG. 145--Another aspect of the canyon wall of Fig. 144.
The almost sheer descents are in contrast with the cliff and platform
type of topography characteristic of the Grand Canyon of Colorado.]
If we apply the foregoing considerations to the Cordillera Vilcapampa,
we shall find some striking illustrations of the principles involved.
The invasion of the granite was accompanied by moderate absorption of
the displaced rock, and more especially by the marginal pushing aside of
the sedimentary rim. The immediate effect must have been to give both
intruded rock and country rock greater height and marked ruggedness.
There followed a period of regional compression and torsion, and the
development of widespread joint systems with strikingly regular
features. In the Silurian shales and slates these joints are closely
spaced; in the granites they are in many places twenty to thirty feet
apart. The shales, therefore, offer many more points of attack and have
weathered down into a smooth-contoured topography boldly overlooked
along the contact by walls and peaks of granite. _In some cases a canyon
wall a mile high is developed entirely on two or three joint planes
inclined at an angle no greater than 15°._ The effect in the granite is
to give a marked boldness of relief, nowhere more strikingly exhibited
than at Huadquiña, below Colpani, where the foot-hill slopes developed
on shales and slates suddenly become moderate. The river flows from a
steep and all but uninhabited canyon into a broad valley whose slopes
are dotted with the terraced _chacras_, or farms, of the mountain
Indians.
The Torontoy granite is also homogeneous while the shales and slates
together with their more arenaceous associates occur in alternating
belts, a diversity which increases the points of attack and the
complexity of the forms. Tending toward the same result is the greater
hardness of the granite. The tendency of the granite to develop bold
forms is accelerated in lofty valleys disposed about snow-clad peaks,
where glaciers of great size once existed, and where small glaciers
still linger. The plucking action of ice has an excellent chance for
expression, since the granite may be quarried cleanly without the
production of a large amount of spoil which would load the ice and
diminish the intensity of its plucking action.
As a whole the Central Andes passed through a cycle of erosion in late
Tertiary time which was interrupted by uplift after the general surface
had been reduced to a condition of topographic maturity. Upon the
granites mature slopes are not developed except under special conditions
(1) of elevation as in the small batholith above Chuquibambilla, and (2)
where the granite is itself bordered by resistant schists which have
upheld the surface over a broad transitional belt. Elsewhere the granite
is marked by exceedingly rugged forms: deep steep-walled canyons,
precipitous cirques, matterhorns, and bold and extended escarpments of
erosion. In the shale belt the trails run from valley to valley in every
direction without special difficulties, but in the granite they follow
the rivers closely or cross the axis of the range by carefully selected
routes which generally reach the limit of perpetual snow. Added interest
attaches to these bold topographic forms because of the ruins now found
along the canyon walls, as at Torontoy, or high up on the summit of a
precipitous spur, as at Machu Picchu near the bridge of San Miguel.
The Vilcapampa batholith is bordered on the southwest by a series of
ancient schists with which the granite sustains quite different
relations. No sharp dividing line is visible, the granite extending
along the planes of foliation for such long distances as in places to
appear almost interbedded with the schists. The relation is all the more
striking in view of the trifling intrusions effected in the case of the
seemingly much weaker shales on the opposite contact. Nor is the
metamorphism of the invaded rock limited to simple intrusion. For
several miles beyond the zone of intenser effects the schists have been
enriched with quartz to such an extent that their original darker color
has been changed to light gray or dull white. At a distance they may
even appear as homogeneous and light-colored as the granite. At distant
points the schists assume a darker hue and take on the characters of a
rather typical mica schist.
It is probable that the Vilcapampa intrusion is one of a family of
batholiths which further study may show to extend over a much larger
territory. The trail west of Abancay was followed quite closely and
accidentally crosses two small batholiths of peculiar interest. Their
limits were not closely followed out, but were accurately determined at
a number of points and the remaining portion of the contact inferred
from the topography. In the case of the larger area there may indeed be
a connection westward with a larger mass which probably constitutes the
ranges distant some five to ten miles from the line of traverse.
[Illustration: FIG. 146--Deformative effects on limestone strata of the
granite intrusion on the southwestern border of the Vilcapampa batholith
above Chuquibambilla. Fig. 147 is on the same border of the batholith
several miles farther northwest. The granite mass on the right is a
small outlier of the main batholith looking south. The limestone is
Cretaceous. See Appendix C for locations.]
These smaller intrusions are remarkable in that they appear to have been
attended by little alteration of either invading or invaded rock, though
the granites were observed to become distinctly more acid in the contact
zone. Space was made for them by displacing the sedimentary cover and by
a marked shortening of the sedimentary rim through such structures as
overthrust faults and folds. The contact is observable in a highly
metamorphosed belt about twenty feet wide, and for several hundred feet
more the granite has absorbed the limestone in small amounts with the
production of new minerals and the development of a distinctly lighter
color. The deformative effects of the batholithic invasion are shown in
their gross details in Figs. 141, 142, and 146; the finer details of
structure are represented in Fig. 147, which is drawn from a measured
outcrop above Chuquibambilla.
It will be seen that we have here more than a mere crinkling, such as
the mica schists of the Cordillera Vilcapampa display. The diversified
sedimentary series is folded and faulted on a large scale with broad
structural undulations visible for miles along the abrupt valley walls.
Here and there, however, the strata become weaker generally through the
thinning of the beds and the more rapid alternation of hard and soft
layers, and for short distances they have absorbed notable amounts of
the stresses induced by the igneous intrusions. In such places not only
the structure but the composition of the rock shows the effects of the
intrusion. Certain shales in the section are carbonaceous and in all
observed cases the organic matter has been transformed to anthracite, a
condition generally associated with a certain amount of minute mashing
and a cementation of both limestone and sandstone.
[Illustration: FIG. 147--Overthrust folds in detail on the southwestern
border of the Vilcapampa batholith near Chuquibambilla. The section is
fifteen feet high. Elevation, 13,100 feet (4,000 m.). For comparison
with the structural effects of the Vilcapampa intrusion on the northeast
see Fig. 142.]
The granite becomes notably darker on approach to the northeastern
contact near Colpani; the proportion of ferro-magnesian minerals in some
cases is so large as to give a distinctly black color in sharp contrast
to the nearly white granite typical of the central portion of the mass.
Large masses of shale foundered in the invading magma, and upon fusion
gave rise to huge black masses impregnated with quartz and in places
smeared or injected with granite magma. Everywhere the granite is marked
by numbers of black masses which appear at first sight to be
aggregations of dark minerals normal to the granite and due to
differentiation processes at the time of crystallization. It is,
however, noteworthy that these increase rapidly in number on approach to
the contact, until in the last half-mile they appear to grade into the
shale inclusions. It may, therefore, be doubted that they are
aggregations. From their universal distribution, their uniform
character, and their marked increase in numbers on approach to lateral
contacts, it may reasonably be inferred that they represent foundered
masses of country rock. Those distant from present contacts are in
almost all cases from a few inches to a foot in diameter, while on
approach to lateral contacts they are in places ten to twenty feet in
width, as if the smaller areas represented the last remnants of large
inclusions engulfed in the magma near the upper or roof contact. They
are so thoroughly injected with silica and also with typical granite
magma as to make their reference to the country rock less secure on
petrographical than on purely distributional grounds.
A parallel line of evidence relates to the distribution of complementary
dikes throughout the granite. In the main mass of the batholith the
dikes are rather evenly distributed as to kind with a slight
preponderance of the dark-colored group. Near the contact, however,
aplitic dikes cease altogether and great numbers of melanocratic dikes
appear. It may be inferred that we have in this pronounced condition
suggestions of strong influence upon the final processes of invasion and
cooling of the granite magma, on the part of the country rock detached
and absorbed by the invading mass. It might be supposed that the
indicated change in the character of the complementary dikes could be
ascribed to possible differentiation of the granite magma whereby a
darker facies would be developed toward the Colpani contact. It has,
however, been pointed out already that the darkening of the granite in
this direction is intimately related to a marked increase in the number
of inclusions, leaving little doubt that the thorough digestion of the
smaller masses of detached shales is responsible for the marked increase
in the number and variety of the ferro-magnesian and special contact
minerals.
Upon the southwestern border of the batholith the number of aplitic
dikes greatly increases. They form prominent features, not only of the
granite, but also of the schists, adding greatly to the strong contrast
between the schist of the border zone and that outside the zone of
metamorphism. In places in the border schists, these are so numerous
that one may count up to twenty in a single view, and they range in size
from a few inches to ten or fifteen feet. The greater fissility of the
schists as contrasted with the shales on the opposite or eastern margin
of the batholith caused them to be relatively much more passive in
relation to the granite magma. They were not so much torn off and
incorporated in the magma, as they were thoroughly injected and
metamorphosed. Added to this is the fact that they are petrographically
more closely allied to the granite than are the shales upon the
northeastern contact.
CHAPTER XIV
THE COASTAL TERRACES
Along the entire coast of Peru are upraised and dissected terraces of
marine origin. They extend from sea level to 1,500 feet above it, and
are best displayed north of Mollendo and in the desert south of Payta.
The following discussion relates to that portion of the coast between
Mollendo and Camaná.
At the time of the development of the coastal terraces the land was in a
state of temporary equilibrium, for the terraces were cut to a mature
stage as indicated by the following facts: (1) the terraces have great
width--from one to five and more miles; (2) their inner border is
straight, or, where curves exist, they are broad and regular; (3) the
terrace tops are planed off smoothly so that they now have an even
gradient and an almost total absence of rock stacks or unreduced spurs;
(4) the mature slopes of the Coast Range, strikingly uniform in gradient
and stage of development (Fig. 148), are perfectly organized with
respect to the inner edge of the terrace. They descend gradually to the
terrace margin, showing that they were graded with respect to sea level
when the sea stood at the inner edge of the highest terrace.
From the composition and even distribution of the thick-bedded Tertiary
deposits of the desert east of the Coast Range, it is concluded that the
precipitation of Tertiary time was greater than that of today (see p.
261). Therefore, if the present major streams reach the sea, it may also
be concluded that those of an earlier period reached the sea, provided
the topography indicates the perfect adjustment of streams to structure.
Lacustrine sediments are absent throughout the Tertiary section. Such
through-flowing streams, discharging on a stable coast, would also have
mature valleys as a consequence of long uninterrupted erosion at a fixed
level. The Majes river must have cut through the Coast Range at Camaná
then as now. Likewise the Vitor at Quilca must have cut straight across
the Coast Range. An examination of the surface leading down from the
Coast Range to the upper edge of these valleys fully confirms this
deduction. Flowing and well-graded slopes descend to the brink of the
inner valley in each case, where they give way to the gorge walls that
continue the descent to the valley floor.
Confirmatory evidence is found in the wide Majes Valley at Cantas and
Aplao. (See the Aplao Quadrangle for details.) Though the observer is
first impressed with the depth of the valley, its width is more
impressive still. It is also clear that two periods of erosion are
represented on its walls. Above Aplao the valley walls swing off to the
west in a great embayment quite inexplicable on structural grounds; in
fact the floor of the embayment is developed across the structure, which
is here more disordered than usual. The same is true below Cantas, as
seen from the trail, which drops over two scarps to get to the valley
floor. The upper, widely opened valley is correlated with the latter
part of the period in which were formed the mature terraces of the coast
and the mature slopes bordering the larger valleys where they cross the
Coast Range.
After its mature development the well-graded marine terrace was upraised
and dissected. The deepest and broadest incisions in it were made where
the largest streams crossed it. Shallower and narrower valleys were
formed where the smaller streams that headed in the Coast Range flowed
across it. Their depth and breadth was in general proportional to the
height of that part of the Coast Range in which their headwaters lay and
to the size of their catchment basins.
When the dissection of the terrace had progressed to the point where
about one-third of it had been destroyed, there came depression and the
deposition of Pliocene or early Pleistocene sands, gravels, and local
clay beds. Everywhere the valleys were partly or wholly filled and over
broad stretches, as in the vicinity of stream mouths and upon lower
portions of the terrace, extensive deposits were laid down. The largest
deposits lie several hours’ ride south of Camaná, where locally they
attain a thickness of several hundred feet. Their upper surface was well
graded and they show a prolonged period of deposition in which the
former coastal terrace was all but concealed.
[Illustration: FIG. 148--The Coast Range between Mollendo and Arequipa
at the end of June, 1911. There is practically no grass and only a few
dry shrubs. The fine network over the hill slopes is composed of
interlacing cattle tracks. The cattle roam over these hills after the
rains which come at long intervals. (See page 141 for description of the
rains and the transformations they effect. For example, in October,
1911, these hills were covered with grass.)]
[Illustration: FIG. 149--The great marine terrace at Mollendo. See Fig.
150 for profile.]
The uplift of the coast terrace and its subsequent dissection bring the
physical history down to the present. The uplift was not uniform; three
notches in the terrace show more faintly upon the granite-gneiss where
the buried rock terrace has been swept clean again, more strongly upon
the softer superimposed sands. They lie below the 700-foot contour and
are insignificant in appearance beside the slopes of the Coast Range or
the ragged bluff of the present coast.
The effect of the last uplift of the coast was to impel the Majes River
again to cut down its lower course nearly to sea level. The Pliocene
terrace deposits are here entirely removed over an area several leagues
wide. In their place an extensive delta and alluvial fan have been
formed. At first the river undoubtedly cut down to base level at its
mouth and deposited the cut material on the sea floor, now shoal, for a
considerable distance from shore. We should still find the river in that
position had other agents not intervened. But in the Pleistocene a great
quantity of waste was swept into the Majes Valley, whereupon aggradation
began; and in the middle and lower valley it has continued down to the
present.
[Illustration: FIG. 150--Profile of the coastal terraces at Mollendo. At
1, in a tributary gorge, fossiliferous clay occurs at 800 feet elevation
above the sea. At 2 is a characteristic change of profile marking a drop
from a higher to a lower terrace. On the extreme left is the highest
terrace, just under 1,500 feet (460 m.).]
[Illustration: FIGS. 151-154--These four diagrams represent the physical
history and the corresponding physiographic development of the coastal
region of Peru between Camaná and Mollendo. The sedimentary beds in the
background of the first diagram are hypothetical and are supposed to
correspond to the quartzites of the Majes Valley at Aplao.]
The effect has been not only the general aggradation of the valley
floor, but also the development of a combined delta and superimposed
alluvial fan at the valley mouth. The seaward extension of the delta has
been hastened by the gradation of the shore between the bounding
headlands, thus giving rise to marine marshes in which every particle of
contributed waste is firmly held. The plain of Camaná, therefore,
includes parts of each of the following: a delta, a superposed alluvial
fan, a salt-water marsh, a fresh-water marsh, a series of beaches, small
amounts of piedmont fringe at the foot of Pliocene deposits once trimmed
by the river and by waves, and extensive tracts of indefinite fill. (See
the Camaná Quadrangle for details.)
With the coastal conditions now before us it will be possible to attempt
a correlation between the erosion features and the deposits of the coast
and those of the interior. An understanding of the comparisons will be
facilitated by the use of diagrams, Figs. 151-154, and by a series of
concise summary statements. From the relations of the figure it appears
that:
1. The Tertiary deposits bordering the Majes Valley east of the Coast
Range were in process of deposition when the sea planed the coastal
terrace (Fig. 151).
2. A broad mature marine terrace without stacks or sharply alternating
spurs and reëntrants (though the rock is a very resistant granite) is
correlated with the mature grades of the Coast Range, with which they
are integrated and with the mature profiles of the main Cordillera.
3. Such a high degree of topographic organization requires the
dissection in the _late_ stages of the erosion cycle of at least the
inner or eastern border of the piedmont deposits of the desert, largely
accumulated during the _early_ stages of the cycle.
4. Since the graded slopes of the Coast Range on the one side descend to
a former shore whose elevation is now but 1,500 feet above sea level,
and since only ten to twenty miles inland on the other side of the
range, the same kind of slope extends beneath Tertiary deposits 4,000
feet above sea level, it appears that aggradation of the outer (or
western) part of the Tertiary deposits on the eastern border of the
Coast Range continued down to the end of the cycle of erosion, though
5. There must have been an outlet to the sea, since, as we have already
seen, the water supply of the Tertiary was greater than that of today
and the present streams reach the sea. Moreover, the mature upper slopes
and the steep lower slopes of the large valleys make a pronounced
topographic unconformity, showing two cycles of valley development.
6. Upon uplift of the coast and dissection of the marine terraces at the
foot of the Coast Range, the streams cut deep trenches on the floors of
their former valleys (Fig. 152) and removed (a) large portions of the
coast terrace, and (b) large portions of the Tertiary deposits east of
the Coast Range.
7. Depression of the coastal terrace and its partial burial meant the
drowning of the lower Majes Valley and its partial filling with marine
and later with terrestrial deposits. It also brought about the partial
filling by stream aggradation of the middle portion of the valley,
causing the valley fill to abut sharply against the steep valley walls.
(See Fig. 155.)
8. Uplift and dissection of both the terrace and its overlying sediments
would be accompanied by dissection of the former valley fill, provided
that the waste supply was not increased and that the uplift was regional
and approximately equal throughout--not a bowing up of the coast on the
one hand, or an excessive bowing up of the mountains on the other. But
the waste supply has not remained constant, and the uplift has been
greater in the Cordillera than on the coast. Let us proceed to the proof
of these two conclusions, since upon them depends the interpretation of
the later physical history of the coastal valleys.
[Illustration: FIG. 155--Steep walls in the Majes Valley below Cantas
and the abrupt termination against them of a deep alluvial fill.]
[Illustration: FIG. 156--Canyon of the Majes River through the Coast
Range north of Camaná. The rock is a granite-gneiss capped by rather
flat-lying sedimentaries.]
It is known that the Pleistocene was a time of augmented waste delivery.
At the head of the broadly opened Majes Valley there was deposited a
huge mass of extremely coarse waste several hundred feet deep and
several miles long. Forward from it, interstratified with its outer
margin, and continuing the same alluvial grade, is a still greater mass
of finer material which descends to lower levels. The fine material is
deposited on the floor of a valley cut into Tertiary strata, hence it
is younger than the Tertiary. It is now, and has been for some time
past, in process of dissection, hence it was not formed under present
conditions of climate and relief. It is confidently assigned to the
Pleistocene, since this is definitely known to have been a time of
greater precipitation and waste removal on the mountains, and deposition
on the plains and the floors of mountain valleys. Such a conclusion
appears, even on general grounds, to be but a shade less reliable than
if we were able to find in the upper Majes Valley, as in so many other
Andean valleys, similar alluvial deposits interlocked with glacial
moraines and valley trains.
In regard to the second consideration--the upbowing of the
Cordillera--it may be noted that the valley and slope profiles of the
main Cordillera shown on p. 191, when extended toward the margin of the
mountain belt, lie nearly a mile above the level of the sea on the west
and the Amazon plains on the east. The evidence of regional bowing thus
afforded is checked by the depths of the mountain valleys and the stream
profiles in them. The streams are now sunk from one to three thousand
feet below their former level. Even in the case of three thousand feet
of erosion the stream profiles are still ungraded, the streams
themselves are almost torrential, and from one thousand to three
thousand feet of vertical cutting must still be accomplished before the
profiles will be as gentle and regular as those of the preceding cycle
of erosion, in which were formed the mature slopes now lying high above
the valley floors.
Further evidence of bowing is afforded by the attitude of the Tertiary
strata themselves, more highly inclined in the case of the older
Tertiary, less highly inclined in the case of the younger Tertiary. It
is noteworthy that the gradient of the present valley floor is
distinctly less than that of the least highly inclined strata. This is
true even where aggradation is now just able to continue, as near the
nodal point of the valley, above Aplao, where cutting ceases and
aggradation begins. (See the Aplao Quadrangle for change of function on
the part of the stream a half mile above Cosos). Such a progressive
steepening of gradients in the direction of the oldest deposits, shows
very clearly a corresponding progression in the growth of the Andes at
intervals throughout the Tertiary.
Thus we have aggradation in the Tertiary at the foot of the growing
Andes; aggradation in the Pliocene or early Pleistocene on the floor of
a deep valley cut in earlier deposits; aggradation in the glacial epoch;
and aggradation now in progress. Basin deposits within the borders of
the Peruvian Andes are relatively rare. The profound erosion implied by
the development, first of a mature topography across this great
Cordillera, and second of many deep canyons, calls for deposition on an
equally great scale on the mountain borders. The deposits of the western
border are a mile thick, but they are confined to a narrow zone between
the Coast Range and the Cordillera. Whatever material is swept beyond
the immediate coast is deposited in deep ocean water, for the bottom
falls off rapidly. The deposits of the eastern border of the Andes are
carried far out over the Amazon lowland. Those of earlier geologic
periods were largely confined to the mountain border, where they are now
upturned to form the front range of the Andes. The Tertiary deposits of
the eastern border are less restricted, though they appear to have
gathered chiefly in a belt from fifty to one hundred miles wide.
The deposits of the western border were laid down by short streams
rising on a divide only 100 to 200 miles from the Pacific. Furthermore,
they drain the dry leeward slopes of the Andes. The deposits of the wet
eastern border were made by far larger streams that carry the waste of
nearly the whole Cordillera. Their shoaling effect upon the Amazon
depression must have been a large factor in its steady growth from an
inland sea to a river lowland.
CHAPTER XV
PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT
GENERAL FEATURES
In the preceding chapter we employed geologic facts in the determination
of the age of the principal topographic forms. These facts require
further discussion in connection with their closest physiographic allies
if we wish to show how the topography of today originated. There are
many topographic details that have a fundamental relation to structure;
indeed, without a somewhat detailed knowledge of geology only the
broader and more general features of the landscape can be interpreted.
In this chapter we shall therefore refer not to the scenic features as
in a purely topographic description, but to the rock structure and the
fossils. A complete and technical geologic discussion is not desirable,
first, because it should be based upon much more detailed geologic field
work, and second because after all our main purpose is not to discuss
the geologic features _per se_, but the physiographic background which
the geologic facts afford. I make this preliminary observation partly to
indicate the point of view and partly to emphasize the necessity, in a
broad, geographic study, for the reconstruction of the landscapes of the
past.
The two dominating ranges of the Peruvian Andes, called the Maritime
Cordillera and the Cordillera Vilcapampa, are composed of igneous
rock--the one volcanic lava, the other intrusive granite. The chief rock
belts of the Andes of southern Peru are shown in Fig. 157. The Maritime
Cordillera is bordered on the west by Tertiary strata that rest
unconformably upon Palaeozoic quartzites. It is bordered on the east by
Cretaceous limestones that grade downward into sandstones, shales, and
basal conglomerates. At some places the Cretaceous deposits rest upon
old schists, at others upon Carboniferous limestones and related
strata, upon small granite intrusives and upon old and greatly altered
volcanic rock.
The Cordillera Vilcapampa has an axis of granitic rock which was thrust
upward through schists that now border it on the west and slates that
now border it on the east. The slate series forms a broad belt which
terminates near the eastern border of the Andes, where the mountains
break down abruptly to the river plains of the Amazon Basin. The
immediate border on the east is formed of vertical Carboniferous
limestones. The narrow foothill belt is composed of Tertiary sandstones
that grade into loose sands and conglomerates. The inclined Tertiary
strata were leveled by erosion and in part overlain by coarse and now
dissected river gravels, probably of Pleistocene age. Well east of the
main border are low ranges that have never been described. They could
not be reached by the present expedition on account of lack of time. On
the extreme western border of that portion of the Peruvian Andes herein
described, there is a second distinct border chain, the Coast Range. It
is composed of granite and once had considerable relief, but erosion has
reduced its former bold forms to gentle slopes and graded profiles.
The continued and extreme growth of the Andes in later geologic periods
has greatly favored structural and physiographic studies. Successive
uplifts have raised earlier deposits once buried on the mountain flanks
and erosion has opened canyons on whose walls and floors are the clearly
exposed records of the past. In addition there have been igneous
intrusions of great extent that have thrust aside and upturned the
invaded strata exposing still further the internal structures of the
mountains. From sections thus revealed it is possible to outline the
chief events in the history of the Peruvian Andes, though the outline is
still necessarily broad and general because based on rapid
reconnaissance. However, it shows clearly that the landscape of the
present represents but a temporary stage in the evolution of a great
mountain belt. At the dawn of geologic history there were chains of
mountains where the Andes now stand. They were swept away and even their
roots deeply submerged under invading seas. Repeated uplifts of the
earth’s crust reformed the ancient chains or created new ones out of the
rock waste derived from them. Each new set of forms, therefore, exhibits
some features transmitted from the past. Indeed, the landscape of today
is like the human race--inheriting much of its character from past
generations. For this reason the philosophical study of topographic
forms requires at least a broad knowledge of related geologic
structures.
[Illustration: FIG. 157--Outline sketch showing the principal rock belts
of Peru along the seventy-third meridian. They are: _1_, Pleistocene and
Recent gravels and sands, the former partly indurated and slightly
deformed, with the degree of deformation increasing toward the mountain
border (south). _2_, Tertiary sandstones, inclined from 15° to 30°
toward the north and unconformably overlain by Pleistocene gravels. _3_,
fossil-bearing Carboniferous limestones with vertical dip. _4_,
non-fossiliferous slates, shales, and slaty schists (Silurian) with
great variation in degree of induration and in type of structure. South
of the parallel of 13° is a belt of Carboniferous limestones and
sandstones bordering (_5_), the granite axis of the Cordillera
Vilcapampa. For its structural relations to the Cordillera see Figs. 141
and 142. _6_, old and greatly disturbed volcanic agglomerates, tuffs and
porphyries, and quartzitic schists and granite-gneiss. _7_, principally
Carboniferous limestones north of the axis of the Central Ranges and
Cretaceous limestones south of it. Local granite batholiths in the axis
of the Central Ranges. _8_, quartzites and slates predominating with
thin limestones locally. South of 8 is a belt of shale, sandstone, and
limestone with a basement quartzite appearing on the valley floors. _9_,
a portion of the great volcanic field of the Central Andes and
characteristically developed in the Western or Maritime Cordillera,
throughout northern Chile, western Bolivia, and Peru. At Cotahuasi (see
also Fig. 20) Cretaceous limestones appear beneath the lavas. _10_,
Tertiary sandstones of the coastal desert with a basement of old
volcanics and quartzites appearing on the valley walls. The valley floor
is aggraded with Pleistocene and Recent alluvium. _11_, granite-gneiss
of the Coast Range. _12_, late Tertiary or Pleistocene sands and gravels
deposited on broad coastal terraces. For rock structure and character
see the other figures in this chapter. For a brief designation of index
fossils and related forms see Appendix B. For the names of the drainage
lines and the locations of the principal towns see Figs. 20 and 204.]
SCHISTS AND SILURIAN SLATES[50]
The oldest series of rocks along the seventy-third meridian of Peru
extends eastward from the Vilcapampa batholith nearly to the border of
the Cordillera, Fig. 157. It consists of (1) a great mass of slates and
shales with remarkable uniformity of composition and structure over
great areas, and (2) older schists and siliceous members in restricted
belts. They are everywhere thoroughly jointed; near the batholith they
are also mineralized and altered from their original condition; in a few
places they have been intruded with dikes and other form of igneous
rock.
The slates and shales underlie known Carboniferous strata on their
eastern border and appear to be a physical continuation of the
fossiliferous slates of Bolivia; hence they are provisionally referred
to the Silurian, though they may possibly be Devonian. Certainly the
known Devonian exceeds in extent the known Silurian in the Central Andes
but its lithological character is generally quite unlike the character
of the slates here referred to the Silurian. The schists are of great
but unknown age. They are unconformably overlain by known Carboniferous
at Puquiura in the Vilcapampa Valley (Fig. 158), and near Chuquibambilla
on the opposite side of the Cordillera Vilcapampa. The deeply weathered
fissile mica schists east of Pasaje (see Appendix C for all locations)
are also unconformably overlain by conglomerate and sandstone of
Carboniferous age. While the schists vary considerably in lithological
appearance and also in structure, they are everywhere the lowest rocks
in the series and may with confidence be referred to the early
Palaeozoic, while some of them may date from the Proteriozoic.
[Illustration: FIG. 158--Geologic sketch map of the lower Urubamba
Valley. A single traverse was made along the valley, hence the
boundaries are not accurate in detail. They were sketched in along a few
lateral traverses and also inferred from the topography. The country
rock is schist and the granite intruded in it is an arm of the main
granite mass that constitutes the axis of the Cordillera Vilcapampa. The
structure and to some degree the extent of the sandstone on the left are
represented in Figs. 141 and 142.]
The Silurian beds are composed of shale, sandstone, shaly sandstone,
limestone, and slate with some slaty schist, among which the shales are
predominent and the limestones least important. Near their contact with
the granite the slate series is composed of alternating beds of
sandstone and shale arranged in beds from one to three feet thick. At
Santa Ana they become more fissile and slaty in character and in several
places are quarried and used for roofing. At Rosalina they consist of
almost uniform beds of shale so soft and so minutely and thoroughly
jointed as to weather easily. Under prolonged erosion they have,
therefore, given rise to a well-rounded and soft-featured landscape.
Farther down the Urubamba Valley they again take on the character of
alternating beds of sandstone and shale from a few feet to fifteen and
more feet thick. In places the metamorphism of the series has been
carried further--the shales have become slates and the sandstones have
been altered to extremely resistant quartzites. The result is again
clearly shown in the topography of the valley wall which becomes bold,
inclosing the river in narrow “pongos” or canyons filled with huge
bowlders and dangerous rapids. The hills become mountains, ledges
appear, and even the heavy forest cover fails to smooth out the natural
ruggedness of the landscape.
It is only upon their eastern border that the Silurian series includes
calcareous beds, and all of these lie within a few thousand yards of the
contact with the Carboniferous limestones and shales. At first they are
thin paper-like layers; nearer the top they are a few inches wide and
finally attain a thickness of ten or twelve feet. The available
limestone outcrops were rigorously examined for fossils but none were
found, although they are lavishly distributed throughout the younger
Carboniferous beds just above them. It is also remarkable that though
the Silurian age of these beds is reasonably inferred they are not
separated from the Carboniferous by an unconformity, at least we could
find none in this locality. The later beds disconformably overlie the
earlier beds, although the sharp differences in lithology and fossils
make it easy to locate the line of separation. The limestone beds of the
Silurian series are extremely compact and unfossiliferous. At least in
this region those of Carboniferous age are friable and the fossils
varied and abundant. The Silurian beds are everywhere strongly inclined
and throughout the eastern half or third of their outcrop in the
Urubamba Valley they are nearly vertical.
In view of the enormous thickness of the repeated layers of shale and
sandstone this series is of great interest. Added importance attaches to
their occurrence in a long belt from the eastern edge of the Bolivian
highlands northward through Peru and possibly farther. From the fact
that their disturbance has been on broad lines over wide areas with
extreme metamorphism, they are to be separated from the older
mica-schists and the crumpled chlorite schists of Puquiura and Pasaje.
Further reasons for this distinction lie in their lithologic difference
and, to a more important degree, in the strong unconformity between the
Carboniferous and the schists in contrast to the disconformable
relations shown between the Carboniferous and Silurian fifty miles away
at Pongo de Mainique. The mashing and crumpling that the schists have
experienced at Puquiura is so intense, that were they a part of the
Silurian series the latter should exhibit at least a slight unconformity
in relation to the Carboniferous limestones deposited upon them.
If our interpretation of the relation of the schists to the slates and
shales be correct, we should have a mountain-making period introduced in
pre-Silurian time, affecting the accumulated sediments and bringing
about their metamorphism and crumpling on a large scale. From the
mountains and uplands thus created on the schists, sediments were washed
into adjacent waters and accumulated as even-bedded and extensive sheets
of sands and muds (the present slates, shales, quartzites, etc.).
Nowhere do the sediments of the slate series show a conglomeratic phase;
they are remarkably well-sorted and consist of material disposed with
great regularity. Though they are coarsest at the bottom the lower beds
do not show cross-bedding, ripple marking, or other signs of
shallow-water conditions. Toward the upper part of the series these
features, especially the ripple-marking, make their appearance. During
the deposition of the last third of the series, and again just before
the deposition of the limestone, the beds took on a predominantly
arenaceous character associated with ripple marks and cross-bedding
characteristic of shallow-water deposits.
In the persistence of arenaceous sediments throughout the series and the
distribution of the ripple marks through the upper third of the beds, we
have a clear indication that the degree of shallowness was sufficient to
bring the bottom on which the sediments accumulated into the zone of
current action and possibly wave action. It is also worth considering
whether the currents involved were not of similar origin to those now a
part of the great counter-clockwise movements in the southern seas. If
so, their action would be peculiarly effective in the wide distribution
of the sediment derived from a land mass on the eastern edge of a
continental coast, since they would spread out the material to a greater
and greater degree as they flowed into more southerly latitudes. Among
geologic agents a broad ocean current of relatively uniform flow would
produce the most uniform effects throughout a geologic period, in which
many thousand feet of clastic sediments were being accumulated. A
powerful ocean current would also work on flats (in contrast to the
gradient required by near-shore processes), and at the same time be of
such deep and steady flow as to result in neither ripple marks nor
cross-bedding.
The increasing volume of shallow-water sediments of uniform character
near the end of the Silurian, indicates great crustal stability at a
level which brought about neither a marked gain nor loss of material to
the region. At any rate we have here no Devonian sediments, a
characteristic shared by almost all the great sedimentary formations of
Peru. At the beginning of the Carboniferous the water deepened, and
great heavy-bedded limestones appear with only thin shale partings
through a vertical distance of several hundreds of feet. The enormous
volume of Silurian sediments indicates the deep and prolonged erosion of
the land masses then existing, a conclusion further supported (1) by the
extensive development of the Silurian throughout Bolivia as well as
Peru, (2) by the entire absence of coarse material whether at the top or
bottom of the section, and (3) by the very limited extent of older rock
now exposed even after repeated and irregular uplift and deep
dissection. Indeed, from the latter very striking fact, it may be
reasonably argued that in a general way the relief of the country was
reduced to sea level at the close of the Silurian. Over the perfected
grades of that time there would then be afforded an opportunity for the
effective transportation of waste to the extreme limits of the land.
Further evidence of the great reduction of surface during the Silurian
and Devonian is supplied by the extensive development of the
Carboniferous strata. Their outcrops are now scattered across the higher
portions of the Andean Cordillera and are prevailingly calcareous in
their upper portions. Upon the eastern border of the Silurian they
indicate marine conditions from the opening of the period, but at Pasaje
in the Apurimac Valley they are marked by heavy beds of basal
conglomerate and sandstone, and an abundance of ripple marking and other
features associated with shallow-water and possibly near-shore
conditions.
CARBONIFEROUS
Carboniferous strata are distributed along the seventy-third meridian
and rival in extent the volcanic material that forms the western border
of the Andes. They range in character from basal conglomerates,
sandstones, and shales of limited development, to enormous beds of
extremely resistant blue limestone, in general well supplied with
fossils. On the eastern border of the Andes they are abruptly terminated
by a great fault, the continuation northward of the marginal fault
recognized in eastern Bolivia by Minchin[51] and farther north by the
writer.[52] Coarse red sandstones with conglomeratic phase abut sharply
and with moderate inclination against almost vertical sandstones and
limestones of Carboniferous age. The break between the vertical
limestones and the gently inclined sandstones is marked by a prominent
scarp nearly four thousand feet high (Fig. 159), and the limestone
itself forms a high ridge through which the Urubamba has cut a narrow
gateway, the celebrated Pongo de Mainique.
[Illustration: FIG. 159--Topographic and structural section at the
northeastern border of the Peruvian Andes. The slates are probably
Silurian, the fossiliferous limestones are known Carboniferous, and the
sandstones are Tertiary grading up to Pleistocene.]
At Pasaje, on the western side of the Apurimac, the Carboniferous again
appears resting upon the old schists described on p. 236. It is steeply
upturned, in places vertical, is highly conglomeratic, and in a belt a
half-mile wide it forms true badlands topography. It is succeeded by
evenly bedded sandstones of fine and coarse composition in alternate
beds, then follow shales and sandstones and finally the enormous beds of
limestone that characterize the series. The structure is on the whole
relatively simple in this region, the character and attitude of the beds
indicating their accumulation in a nearly horizontal position. Since the
basal conglomerate contains only pebbles and stones derived from the
subjacent schists and does not contain granites like those in the
Cordillera Vilcapampa batholith on the east it is concluded that the
batholithic invasion was accompanied by the compression and tilting of
the Carboniferous beds and that the batholith itself is
post-Carboniferous. From the ridge summits above Huascatay and in the
deep valleys thereabouts the Carboniferous strata may be seen to extend
far toward the west, and also to have great extent north and south.
Because of their dissected, bare, and, therefore, well-exposed condition
they present exceptional opportunities for the study of Carboniferous
geology in central Peru.
[Illustration: FIG. 160--The deformative effects of the granite
intrusion of the Cordillera Vilcapampa are here shown as transmitted
through ancient schists to the overlying conglomerates, sandstones, and
limestones of Carboniferous age, in the Apurimac Valley at Pasaje.]
Carboniferous strata again appear at Puquiura, Vilcapampa, and
Pampaconas. They are sharply upturned against the Vilcapampa batholith
and associated volcanic material, chiefly basalt, porphyry, and various
tuffs and related breccias. The Carboniferous beds are here more
arenaceous, consisting chiefly of alternating beds of sandstone and
shale. The lowermost beds, as at Pongo de Mainique, are dominantly
marine, fossiliferous limestone beds having a thickness estimated to be
over two miles.
From Huascatay westward and southward the Carboniferous is in part
displaced by secondary batholiths of granite, in part cut off or crowded
aside by igneous intrusions of later date, and in still larger part
buried under great masses of Tertiary volcanic material. Nevertheless,
it remains the dominating rock type over the whole stretch of country
from Huascatay to Huancarama. In the northwestern part of the Abancay
sheet its effect on the landscape may be observed in the knife-like
ridge extending from west to east just above Huambo. Above
Chuquibambilla it again outcrops, resting upon a thick resistant
quartzite of unknown age, Fig. 162. It is strongly developed about
Huadquirca and Antabamba and, still associated with a quartzite floor,
it finally disappears under the lavas of the great volcanic field on the
western border of the Andes. Figs. 141 and 142 show its relation to the
invading granite batholiths and Fig. 162 shows further structural
features as developed about Antabamba where the great volcanic field of
the Maritime Cordillera begins.
[Illustration: FIG. 161--Types of deformation north of Lambrama near
Sotospampa. A dark basaltic rock has invaded both granite-gneiss and
slate. Sills and dikes occur in great numbers. The topographic
depression in the profile is the Lambrama Valley. See the Lambrama
Quadrangle.]
Both the enormous thickness of the Carboniferous limestone series and
the absence of clastic members over great areas in the upper portion of
the series prove the widespread extent of the Carboniferous seas and
their former occurrence in large interlimestone tracts from which they
have since been eroded. At Puquiura they extend far over the schist, in
fact almost completely conceal it; at Pasaje they formerly covered the
mica-schists extensively, their erosion in both cases being conditioned
by the pronounced uplift and marginal deformation which accompanied the
development of the Vilcapampa batholith.
[Illustration: FIG. 162--Sketch sections at Antabamba to show (a)
deformed limestones on the upper edge of the geologic map, Fig. 163 A;
and (b) the structural relations of limestone and quartzite. See also
Fig. 163.]
The degree of deformation of the Carboniferous sediments varies between
simple uplift through moderate folding and complex disturbances
resulting in nearly vertical attitudes. The simplest structures are
represented at Pasaje, where the uplift of the intruded schists,
marginal to the Vilcapampa batholith, has produced an enormous
monoclinal fold exposing the entire section from basal conglomerates and
sandstones to the thickest limestone. Above Chuquibambilla the
limestones have been uplifted and very gently folded by the invasion of
granite associated with the main batholith and several satellitic
batholiths of limited extent. A higher degree of complexity is shown at
Pampaconas (Fig. 141), where the main monoclinal fold is traversed
almost at right angles by secondary folds of great amplitude. The
limestones are there carried to the limit of the winter snows almost at
the summit of the Cordillera. The crest of each secondary anticline
rises to form a group of conspicuous peaks and tabular ridges. Higher in
the section, as at Puquiura, the sandstones are thrown into a series of
huge anticlines and synclines, apparently by the marginal compression
brought about at the time of the intrusion of the granite core of the
range. At Pongo de Mainique the whole of the visible Carboniferous is
practically vertical, and is cut off by a great fault marking the abrupt
eastern border of the Cordillera.
[Illustration: FIG. 163--Geologic sketch section to show the relation of
the volcanic flows of Fig. 164 to the sandstones and quartzites
beneath.]
It is noteworthy that the farther east the Carboniferous extends the
more dominantly marine it becomes, though marine beds of great thickness
constitute a large part of the series in whatever location. From
Huascatay westward the limestones become more and more argillaceous, and
finally give way altogether to an enormous thickness of shales,
sandstones, and thin conglomerates. These were observed to extend with
strong inclination westward out of the region studied and into and under
the volcanoes crowning the western border of the Cordillera. Along the
line of traverse opportunity was not afforded for further study of this
aspect of the series, since our route led generally along the strike
rather than along the dip of the beds. It is interesting to note,
however, that these observations as to the increasing amounts of clastic
material in a westward direction were afterwards confirmed by Señor José
Bravo, the Director of the Bureau of Mines at Lima, who had found
Carboniferous land plants in shales at Pacasmayo, the only fossils of
their kind found in Peru. Formerly it had been supposed that non-marine
Carboniferous was not represented in Peru. From the varied nature of the
flora, the great thickness of the shales in which the specimens were
collected, and the fact that the dominantly marine Carboniferous
elsewhere in Peru is of great extent, it is concluded that the land upon
which the plants grew had a considerable area and probably extended far
west of the present coast line. Since its emergence it has passed
through several orogenic movements. These have resulted in the uplift of
the marine portion of the Carboniferous, while the terrestrial deposits
seem to have all but disappeared in the down-sunken blocks of the ocean
floor, west of the great fault developed along the margin of the
Cordillera. The following figures are graphic representations of this
hypothesis.
[Illustration: FIG. 164--Geologic sketch map and section, Antabamba
region. The Antabamba River has cut through almost the entire series of
bedded strata].
[Illustration: FIG. 165--The upper diagram (A) represents the
hypothetical distribution of land and sea during the Carboniferous
Period, as inferred from the present distribution and character of
Carboniferous limestones and slates. The lower diagram (B) represents
the present relief. The dotted line at the left of the two diagrams
connects identical points. The fragmentation of the former continental
border is believed to have left only a small portion of a former coastal
chain and to have been contemporaneous with the development of ocean
abysses near the present shore.]
The wide distribution of the Carboniferous sediments and especially the
limestones, together with the uniformity of the fossil faunas, makes it
certain that the sea extended entirely across the region now occupied
by the Andes. However, from the relation of the Carboniferous to the
basal schists, and the most conservative extension of the known
Carboniferous, it may be inferred that the Carboniferous sea did not
completely cover the entire area but was broken here and there by island
masses in the form of an elongated archipelago. The presence of land
plants in the Carboniferous of Pisco warrants the conclusion that a
second island mass, possibly an island chain parallel to the first,
extended along and west of the present shore.
CRETACEOUS
The Cretaceous formations are of very limited extent in the belt of
country under consideration, in spite of their generally wide
distribution in Peru. They are exposed distinctly only on the western
border of the Cordillera and in special relations. In the gorge of
Cotahuasi, over seven thousand feet deep, about two thousand feet of
Cretaceous limestones are exposed. The series includes only a very
resistant blue limestone and terminates abruptly along a well-marked and
highly irregular erosion surface covered by almost a mile of volcanic
material, chiefly lava flows. The character of the bottom of the section
is likewise unknown, since it lies apparently far below the present
level of erosion.
[Illustration: FIG. 166--Geologic sketch map and cross-section in the
Cotahuasi Canyon at Cotahuasi. With a slight gap this figure continues
Fig. 167 to the left. The section represents a spur of the main plateau
about 1,500 feet high in the center of the map.]
The Cretaceous limestones of the Cotahuasi Canyon are everywhere greatly
and irregularly disturbed. Typical conditions are represented in the
maps and sections, Figs. 166 and 167. They are penetrated and tilted by
igneous masses, apparently the feeders of the great lava sheets that
form the western summit of the Cordillera. From the restricted
development of the limestones along a western border zone it might be
inferred that they represent a very limited marine invasion. It is
certainly clear that great deformative movements were in progress from
at least late Palæozoic time since all the Palæozoic deposits are broken
abruptly down in this direction, and, except for such isolated
occurrences as the land Carboniferous at Pacasmayo, are not found
anywhere in the coastal region today. The Cretaceous is not only limited
within a relatively narrow shore zone, but also, like the Palæozoic, it
is broken down toward the west, not reappearing from beneath the
Tertiary cover of the desert region or upon the granite-gneisses that
form the foundation for all the known sedimentary strata of the
immediate coast.
[Illustration: FIG. 167--Geologic sketch map and cross-section in the
Cotahuasi Canyon at Taurisma, above Cotahuasi. The relations of
limestone and lava flows in the center of the map and on a spur top near
the canyon floor. Thousands of feet of lava extend upward from the flows
that cap the limestone.]
From these considerations I think we have a strong suggestion of the
geologic date assignable to the development of the great fault that is
the most strongly marked structural and physiographic feature of the
west coast of South America. Since the development of this fault is so
intimately related to the origin of the Pacific Ocean basin its study is
of special importance. The points of chief interest may be summarized as
follows:
(1) The character of the land Carboniferous implies a much greater
extent of the land than is now visible.
(2) The progressive coarsening of the Carboniferous deposits westward
and their land derivation, together with the great thickness of the
series, point to an elevated land mass in process of erosion west of
the series as a whole, that is west of the present coast.
(3) The restricted development of the Cretaceous seas upon the western
border of the Carboniferous, and the still more restricted development
of the Tertiary deposits between the mountains and the present coast,
point to increasing definition of the submarine scarp through the
Mesozoic and the Tertiary.
(4) The Tertiary deposits are all clearly derived from the present
mountains and have been washed seaward down slopes with geographic
relations approximately like those of the present.
(5) From the great width, deep dissection, and subsequent burial of the
Tertiary terraces of the coast, it is clear that the greater part of the
adjustment of the crust to which the bordering ocean basin is due was
accomplished at least by mid-Tertiary time.
[Illustration: FIG. 168--Composite structure section representing the
succession of rocks in the Urubamba Valley from Urubamba to Torontoy.]
Aside from the fossiliferous limestones of known Cretaceous age there
have been referred to the Cretaceous certain red sandstones and shales
marked, especially in the central portions of the Cordillera, by the
presence of large amounts of salt and gypsum. These beds were at first
considered Permian, but Steinmann has since found at Potosí related and
similar formations with Cretaceous fossils. In this connection it is
also necessary to add that the great red sandstone series forming the
eastern border of the Andes in Bolivia is of uncertain age and has
likewise been referred to the Cretaceous, though the matter of its age
has not yet been definitely determined. In 1913 I found it appearing in
northwestern Argentina in the Calchaquí Valley in a relation to the main
Andean mass, similar to that displayed farther north. It contains
fossils and its age was, therefore, readily determinable there.[53]
In the Peruvian field the red beds of questionable age were not examined
in sufficient detail to make possible a definite age determination. They
occur in a great and only moderately disturbed series in the Anta basin
north of Cuzco, but are there not fossiliferous. The northeastern side
of the hill back of Puqura (of the Anta basin: to be distinguished from
Puquiura in the Vilcabamba Valley) is composed largely of rocks of this
class. In a few places their calcareous members have been weathered out
in such a manner as to show karst topography. Where they occur on the
well-drained brow of a bluff the caves are used in place of houses by
Indian farmers. The large and strikingly beautiful Lake Huaipo, ten
miles north of Anta, and several smaller, neighboring lakes, appear to
have originated in solution depressions formed in these beds.
[Illustration: FIG. 169--The line of unconformity between the igneous
basement rocks (agglomerates at this point) and the quartzites and
sandstones of the Urubamba Valley, between the town of Urubamba and
Ollantaytambo.]
[Illustration: FIG. 170--The inclined lower and horizontal upper
sandstone on the southeastern wall of the Majes Valley at Hacienda
Cantas. The section is a half-mile high.]
The structural relation of the red sandstone series to the older rocks
is well displayed about half-way between Urubamba and Ollantaytambo in
the deep Urubamba Valley. The basal rocks are slaty schist and granite
succeeded by agglomerates and basalt porphyries upon whose eroded
surfaces (Fig. 169) are gray to yellow cross-bedded sandstones. Within a
few hundred feet of the unconformity gypsum deposits begin to appear and
increase in number to such an extent that the resulting soil is in
places rendered worthless. Copper-stained bands are also common near the
bottom of the series, but these are confined to the lower beds. Higher
up in the section, for example, just above the gorge between Urubamba
and Ollantaytambo, even-bedded sandstones occur whose most prominent
characteristic is the regular succession of coarse and fine sandstone
beds. Such alternations of character in sedimentary rocks are commonly
marked by alternating shales and sandstones, but in this locality shales
are practically absent. Toward the top of the section gypsum deposits
again appear first as beds and later, as in the case of the hill-slope
on the southern shore of Lake Huaipo, as veins and irregular masses of
gypsum. The top of the deformed Cretaceous (?) is eroded and again
covered unconformably by practically flat-lying Tertiary deposits.
TERTIARY
The Tertiary deposits of the region under discussion are limited to
three regions: (1) the extreme eastern border of the main Cordillera,
(2) intermontane basins, the largest and most important of which are (a)
the Cuzco basin and (b) the Titicaca-Poopó basin on the
Peruvian-Bolivian frontier, and (3) in the west-coast desert and in
places upon the huge terraces that form a striking feature of the
topography of the coast of Peru.
It has already been pointed out that the eastern border of the
Cordillera is marked by a fault of great but undetermined throw, whose
topographic importance may be estimated from the fact that even after
prolonged erosion it stands nearly four thousand feet high. Cross-bedded
and ripple-marked features and small lenses of conglomerate are common.
The beds now dip at an angle approximately 20° to 50° northward at the
base of the scarp, but have decreasing dip as they extend farther north
and east. It is noteworthy that the deposits become distinctly
conglomeratic as flatter dips are attained, and that there seems to have
been a steady accumulation of detrital material from the mountains for a
long period, since the deposits pass in unbroken succession from the
highly indurated and massive beds of the mountain base to loose
conglomerates that now weather down much like an ordinary gravel bank.
In a few places just below the mouth of the Ticumpinea, logs about six
inches in diameter were observed embeded in the deposits, but these
belong distinctly to the upper horizons.
The border deposits, though they vary in dip from nearly flat to 50°,
are everywhere somewhat inclined and now lie up to several hundred feet
above the level of the Urubamba River. Their upper surface is moderately
dissected, the degree of dissection being most pronounced where the dips
are steepest and the height greatest. In fact, the attitude of the
deposits and their progressive change in character point toward, if they
do not actually prove, the steady and progressive character of the beds
first deposited and their erosion and redeposition in beds now higher in
the series.
Upon the eroded upper surfaces of the inclined border deposits, gravel
beds have been laid which, from evidence discussed in a later paragraph,
are without doubt referable to the Pleistocene. These in turn are now
dissected. They do not extend to the highest summits of the deformed
beds but are confined, so far as observations have gone, to elevations
about one hundred feet above the river. From the evidence that the
overlying horizontal beds are Pleistocene, the thick, inclined beds are
referred to Tertiary age, though they are nowhere fossiliferous.
Observations along the Urubamba River were extended as far northward as
the mouth of the Timpia, one of the larger tributaries. Upon returning
from this point by land a wide view of the country was gained from the
four-thousand-foot ridge of vertical Carboniferous limestone, in which
it appeared that low and irregular strike ridges continue the features
of the Tertiary displayed along the mountain front far northward as well
as eastward, to a point where the higher ridges and low mountains of
older rock again appear--the last outliers of the Andean system in Peru.
Unfortunately time enough was not available for an extension of the trip
to these localities whose geologic characters still remain entirely
unknown. From the topographic aspects of the country, it is, however,
reasonably certain that the whole intervening depression between these
outlying ranges and the border of the main Cordillera, is filled with
inclined and now dissected and partly covered Tertiary strata. The
elevation of the upper surface does not, however, remain the same; it
appears to decrease steadily and the youngest Tertiary strata disappear
from view below the sediments of either the Pleistocene or the present
river gravels. In the more central parts of the depression occupied by
the Urubamba Valley, only knobs or ridges project here and there above
the general level.
_The Coastal Tertiary_
The Tertiary deposits of the Peruvian desert region southwest of the
Andes have many special features related to coastal deformation, changes
of climate, and great Andean uplifts. They lie between the west coast of
Peru at Camaná and the high, lava-covered country that forms the western
border of the Andes and in places are over a mile thick. They are
non-fossiliferous, cross-bedded, ripple-marked, and have abundant lenses
of conglomerate of all sizes. The beds rest upon an irregular floor
developed upon a varied mass of rocks. In some places the basement
consists of old strata, strongly deformed and eroded. In other places it
consists of a granite allied in character and probably in origin with
the old granite-gneiss of the Coast Range toward the west. Elsewhere the
rock is lava, evidently the earliest in the great series of volcanic
flows that form this portion of the Andes.