LAND SCULPTURE BY MOUNTAIN GLACIERS
=Contrasted sculpturing of continental and mountain glaciers.=—In
discussing in a previous chapter the rock pavement lately uncovered by
the Greenland glacier, we learned that this surface had been lowered
by the processes of plucking and abrasion, the combined effect of
which is always to reduce the irregularities of the surface, soften
its outlines, and from sharply projecting masses to develop rounded
shoulders of rock—_roches moutonnées_.
Though the same processes act in much the same manner beneath
mountain glaciers, though here upon all parts of the bed, they are,
in the earlier stages at least, subordinated to a third process more
important than the two acting together. Sculpture by mountain glaciers,
instead of reducing surface irregularities and softening outlines,
increases the accent of the relief and produces the most sharply
rugged topography that is known. In nearly all places where Alpinists
resort for difficult rock climbing, mountain glaciers are to be seen,
or the evidence for their former presence may be read in unmistakable
characters.
=Wind distribution of the snow which falls in mountains.=—Until quite
recently students of glaciation have concerned themselves but little
with the work of the wind in lifting and redistributing the snow after
it has fallen. We have already seen that, for the continental glaciers,
wind appears to be the chief transporting agent, if we except the
marginal lobes where glacier flow assumes large importance. In the case
of mountain glaciers, also, we are to find that for the earlier stages
particularly wind is of the first importance as a redistributing agent.
In the higher levels snow is swept up from the ground by all high
winds, and does not find a resting place until it is dropped beneath an
eddy in some irregularity of the surface; and if the inherited surface
be relatively smooth, this will be found in most cases upon the lee of
the mountain crest.
In normal cases at least the inherited irregularities of the higher
zones of mountain upland are the gentle depressions which develop at
the heads of streams. These become, then, the sites of snowdrifts that
are augmented in size from year to year, though at first they melt away
in the late summer.
[Illustration: FIG. 390.—Snowdrift hollowing its bed by nivation and
building a delta (at the left). Quadrant Mountain, Yellowstone National
Park.]
=The niches which form on snowdrift sites.=—Wherever a drift is
formed, a process is set in operation, the effect of which is to hollow
out and lower the ground beneath it, a process which has been called
_nivation_. The drift shown in Fig. 390 was photographed in late summer
at an elevation of some 9000 feet in the Yellowstone National Park.
The very gently sloping surface surrounding the drift is covered with
grass, but within a zone a few feet in width on the borders of the
drift no grass is growing, and in its place is found a fine brown soil
which is fast becoming the prey of the moving water derived by melting
of the drift. This is explained by the water permeating the crevices of
the rock and being rent by the nightly freezing. Farther from the drift
the ground is dry, and no such action is possible. With each succeeding
spring the augmented drift as it melts carries all finely comminuted
rock material down slopes beneath the snow to emerge at the lowest
margin and be there deposited in the form of a delta. By the operation
of this process of nivation the higher parts of the drift site are
lowered as deposition goes on upon the lower. The combined effect is
thus to produce a _niche_ or faintly etched amphitheater upon the slope
of the mountain (Fig. 391).
[Illustration: FIG. 391.—Amphitheater formed on a drift site in
northern Lapland (after a photograph by G. von Zahn).]
=The augmented snowdrift moves down the valley—birth of the
glacier.=—In still lower air temperatures the drifts enlarge with
each succeeding year until they endure throughout the summer season.
From this stage on, an increment of snow is left from each succeeding
season. No longer entirely wasted by melting, the time soon comes when
the upper snow layers will by their weight compress the lower into ice,
and the mass will begin to creep down the slope along the course of the
inherited valley. The enlarged snowdrift which feeds this ice stream is
called the _névé_ or _firn_.
Against the sloping cliff which had been shaped by nivation at the
upper margin of the snowdrift, that snow which is not of sufficient
depth to begin a movement towards the valley separates from the moving
portion, opening as it does so a cleft or crevasse parallel to the
wall. This crack in the snow is called by its German name _Bergschrund_
or _Randspalte_, and may perhaps be referred to as the marginal
crevasse (Fig. 392).
[Illustration:
FIG. 392.—The marginal crevasse or Bergschrund on the highest margin
of a glacier (after Gilbert).]
=The excavation of the glacial amphitheater or cirque.=—It has been
found that the marginal crevasse plays a most important rôle in the
sculpture of mountains by glaciers, for the great amphitheater which
is everywhere the collecting basin for the nourishment of mountain
glaciers is not an inherited feature, but the handiwork of the ice
itself. This was the discovery of Mr. W. D. Johnson, an American
topographer and geologist, who, in order to solve the problem of the
amphitheater allowed himself to be lowered into such a crevasse upon
the Mount Lyell glacier of the Sierra Nevadas in California.
[Illustration:
FIG. 393.—Niches and cirques in the same vicinity in the Bighorn
Mountains of Wyoming. _A, A_, unmodified valleys; _B, B_, niches on
drift sites; _C, C_, cirques on small glacier sites (after map by F. E.
Mathes, U. S. G. S.).]
Let down a distance of a hundred and fifty feet, he reached the bottom
of the crack, and in a drizzling rain of thaw water stood upon a floor
composed of rock masses in part dislodged from a wall which extended
some twenty feet upwards upon the cliff side of the crevasse. It was
evident that the warm air of the day produced the thaw water which was
constantly dripping and which filled every crack and cranny of the rock
surface. With the sinking of the sun below the peaks the sudden chill,
so characteristic of the end of the day in high mountains, causes this
water to freeze and thus rend the rock along its planes of jointing.
Broad and thin plates of ice, loosened by melting at the walls, could
be extracted from the crevices of the rock as mute witnesses to the
powerful stresses developed by this most vigorous of weathering
processes.
[Illustration:
FIG. 394.—Subordinate small cirques in the amphitheater on the west
face of the Wannehorn above the Great Aletsch Glacier of Switzerland.]
In short, the rock wall above the glacier, which in its initial stage
was the upper wall of the niche hollowed beneath the snowdrift, is
first steepened and later continually both recessed and deepened by
an intensive frost rending which is in operation at the base of the
marginal crevasse. The same process does not go on as rapidly above the
surface of the névé for the reason that _the necessary wetting of the
rock surface does not there so generally result from the daily summer
thaw_. At the bottom of the marginal crevasse alone is this condition
fully realized. Intensive frost action _where the rock is wet with thaw
water daily_ is thus a fundamental cause, both of the hollowing of
the early drift site to form the niche, and of the later enlargement
of this niche into an amphitheater or cirque when the drift has been
transformed into the névé of a glacier. Inasmuch as the crevasse forms
where the snow and ice pull away from the rock toward the middle of the
depression, the cirque wall in its early stage has the outline of a
semicircle. In the Bighorn Mountains of Wyoming, all stages, from the
unmodified valley heads to the full-formed cirque, may be seen near
one another (Fig. 393). It will be noted that wherever a glacier has
formed, as indicated by the cirque, there is a series of lakes which
have developed in the valley below (see p. 412).
[Illustration: FIG. 395.—“Biscuit cutting” effect of glacial sculpture
in the Uinta Mountains of Wyoming (after Atwood).]
[Illustration:
FIG. 396.—Two intersecting inverted cones representing glacial cirques
of different sizes, to show that their intersection is the arc of a
hyperbola, the curve to which the col approximates.]
=Life history of the cirque.=—In its earliest stage the cirque is more
or less uniformly supplied with snow from all sides, and so it enlarges
by recession in a manner to retain its early semicircular outline. In
a later stage a larger proportion of the snow reaches the cirque at
its sides so that its further enlargement causes it to broaden and to
flatten somewhat that part of its outline which represents the head of
the valley (Fig. 389, p. 364). As the territory of the upland is still
further invested by the cirques, their nourishment becomes still more
irregular, and the circular outline gives place to a scalloped border,
as the amphitheater becomes differentiated into subordinate smaller
cirques, each of which corresponds to a scallop of the outline (Fig.
398 and Fig. 394).
=Grooved and fretted uplands.=—The partial investment by cirques of a
mountain upland yields a type of topography quite unlike that produced
by any other geological process. The irregularly connected remnants
of the inherited upland resemble nothing so much as a layer of dough
from which biscuits have been cut (Fig. 395). The surface as a whole,
furrowed as it is below the cirques,
may be described as a _grooved upland_ (plate 19 A). A further
continuation of the process removes all traces of the earlier upland,
for the cirques intersect from opposite sides and thus yield palisades
of sharp rock pinnacles which rise on precipitous walls from a terraced
floor. This ultimate product of cirque sculpture by glaciers is called
a _fretted upland_ (plate 18 A and 19 B).
┌─────────────────────────────────────────────────────────────────────┐
│ PLATE 18. │
│ │
│ [Illustration: _A._ Fretted upland of the Alps seen from the summit │
│ of Mount Blanc.] │
│ │
│ [Illustration: _B._ Model of the Malaspina Glacier and the fretted │
│ upland above it (after model by L. Martin). │
└─────────────────────────────────────────────────────────────────────┘
┌──────────────────────────────────────────────────────────────────┐
│ PLATE 19. │
│ │
│ [Illustration: _A._ Contour map of a grooved upland, Bighorn │
│ Mountains, Wyoming (U. S. Geol. Survey).] │
│ │
│ [Illustration: _B._ Contour map of a fretted upland, Philipsburg │
│ Quadrangle, Montana (U. S. Geol. Survey).] │
└──────────────────────────────────────────────────────────────────┘
=The features carved above the glacier.=—The ranges of pinnacles
carved out by mountain glaciers have become known by various names of
foreign derivation, such as _arête_, _grat_, _aiguille_ mountains,
“files of _gendarmes_”, etc. They may, perhaps, be best referred to as
_comb ridges_, and according to their position they are differentiated
into main and lateral comb ridges, as will be clear from the second map
of plate 19.
[Illustration: FIG. 397.—A col shaped like a hyperbola between Mount
Sir Donald and Yogo Peak in the Selkirks (after a plate by the Keystone
Plate Co.).]
With the gradual invasion of the upland upon which the cirques have
made their attack, the area from which winds may gather up the snow
is steadily diminished, and hence cirque recession is correspondingly
retarded. Cirques which have approached each other from opposite sides
of the ridge until they have become tangent at one point may, however,
still receive nourishment at the sides and so continue to cut down the
intervening rock wall to form a pass or _col_. The theoretical curve
which results from this intersection is that known as the hyperbola, of
which an illustration is afforded by Fig. 396. An approximation to this
form is clearly furnished by most of the mountain passes in glaciated
mountain districts, and a particularly good illustration is furnished
from the vicinity of Glacier on the line of the Canadian Pacific
Railway (Fig. 397).
[Illustration:
FIG. 398.—Diagrams to illustrate the progressive investment of an
upland by cirques with the formation of comb ridges, cols, and horns.
I, early stage, youth; II, intermediate stage; III, late stage,
maturity.]
Upon either side of the col the land mass is left in high relief,
rising from a more or less triangular base (Fig. 398, III) into a sharp
horn or tooth. An illustration of such a _horn_ is furnished by the
Matterhorn in the Swiss Alps, or by Mount Sir Donald in the Selkirks,
though less noteworthy examples may be found in every maturely
glaciated mountain district.
=The features shaped beneath the glacier.=—Those features which are
carved above the glacier—the comb ridge, the col, and the horn—are
all shaped as a result of intensive weathering upon the cirque wall.
The shaping at lower levels is accomplished by processes in operation
below the glacier surface, where weathering is excluded and where
plucking and abrasion work together to tear away and grind off the rock
surface. By their joint action the valley is both deepened and widened,
directly to the height of the glacier surface, and indirectly through
undermining as far up as rock extends. Thus the valley is transformed
into one of broad and flat bed and precipitous side walls—the U-shaped
section illustrated by valleys of the Swiss Alps and in fact in all
districts which have been strongly glaciated by mountain glaciers (Fig.
399).
[Illustration:
FIG. 399.—The U-shaped Kern valley in the Sierra Nevadas of California
(after W. B. Scott).]
As high up in the valley as it has been occupied by the glacier,
the bed is rounded, smoothed, and polished, and marked by the
characteristic glacial scorings or striæ which point down the valley.
Above the level of the glacier’s upper surface, on the other hand,
erosion is accomplished through undermining or sapping, a process which
always leaves precipitous slopes of ragged surface made up of the joint
planes on which the fallen blocks have separated from the cliff. Thus
there is found a sharp line which separates the smoothly rounded rock
surface below from the jagged and precipitous one above (Fig. 400).
Inasmuch as this boundary usually separates the scalable from the
inaccessible slopes above, snow is apt to lodge at this level and make
it strikingly apparent.
[Illustration:
FIG. 400.—Glaciated valley wall in the Sierra Nevadas of California,
showing the sharp line which separates the abraded from the undermined
rock surface (after a photograph by Fairbanks).]
[Illustration:
FIG. 401.—View of the Vale of Chamonix from the séracs of the Glacier
des Bossons. The alb of the opposite side is well brought out.]
If uplift of the land occurs while glaciers occupy the valleys of
mountains, an increased capacity for deepening the valley is imparted
to these ice streams, and we find, as a result, a deep central valley
of U cross section excavated within a relatively broad trough visible
above the shoulder on either side of the later furrow. Save only for
its characteristic curves, such a valley bears close resemblance to
a mature stream valley which has been rejuvenated (see p. 173). The
remnants of the earlier glacier-carved valley are, as already stated,
gently curving high terraces so common in Switzerland, where they
are known as _albs_ or high mountain meadows. These albs may be seen
to special advantage on the sides of the Chamonix valley (Fig. 401),
the Lauterbrunnen valley, or in fact almost any of the larger Alpine
valleys.
=The cascade stairway in glacier-carved valleys.=—If now, instead of
giving our attention to the cross section, we follow the course of the
valley that has been occupied by a glacier, we find that it descends
by a series of steps or terraces having many backwardly directed
treads (plate 19), whereas a normal and well-established river valley
has only forward grades. Because of these backward grades the stream
waters are impounded, and so lakes are found strung along the valley
in chains as the larger beads are found in a rosary, and these are the
characteristic _rock basin lakes_ sometimes referred to as “Paternoster
Lakes” (see p. 412 and Fig. 402).
┌───────────────────────────────────────────────────────────────────┐
│ PLATE 20. │
│ │
│ [Illustration: Map of the surface modeled by mountain glaciers in │
│ the Sierra Nevadas of California (after I. C. Russell).] │
└───────────────────────────────────────────────────────────────────┘
When the backward grades upon the valley floor are especially steep,
the rock step becomes a _rock bar_, or _Riegel_, of which nearly every
Alpine valley has its examples. In a walk from the Grimsel to Meiringen
many such bars are passed. Carrying in suspension the sharp rock sand
from the glacier deposits along its bed, the stream which succeeds
to the glacier as it vacates its valley saws its way through these
obstructions with a rapidity that is amazing, thus producing narrow
defiles, of which the Gorge of the Aar near Meiringen and that of the
Gorner near Zermatt are such well-known examples (Fig. 403).
[Illustration:
FIG. 402.—Map of an area near the continental divide in Colorado,
showing an unglaciated surface to the west of the divide, where the
westerly winds have cleared the ground of snow, and the glacier-carved
country to the eastward. Note the regular forms of the youthful cirque,
the glacier stairway, and the rock basin lakes (U. S. G. S.).]
[Illustration:
FIG. 403.—Gorge of the Albula River near Berkum in the Engadine, cut
through a rock bar by the river which has succeeded to the earlier
glacier.]
[Illustration:
FIG. 404.—Idealistic sketch showing both glaciated and nonglaciated
side valleys tributary to a glaciated main valley (after Davis).]
It is characteristic of rivers that the tributaries cut their valleys
more rapidly than does the main stream within the neighboring section,
though they cannot cut lower than their outlets—the side streams enter
_accordantly_. This is easily explained because the grades of the
tributary streams are the steeper, and, as we well know, the corrosion
of a valley is augmented at a most amazing rate for each increase of
its grade. No such law controls the processes of plucking and abrasion
by which the glacier lowers its floor, for these processes appear to
depend for their efficiency upon the depth of the ice, and the supply
of cutting tools, quite as much as upon the grade of the bed. To apply
a homely illustration, the hollowing of flagstones upon our walks is
dependent more upon the number of persons that pass over them, and upon
their size and the number of protruding nails in their boot heels, than
upon the grades upon which they are placed. At all events we find that
the main glacier valleys are cut deeper than the side valleys, so that
the latter become _hanging valleys_—they enter the main valley, not
upon its bed, but some distance above it (Fig. 404).
The U-shaped hanging valleys, like the main valley, are much too large
for the streams which now fill them, and these diminutive side streams
plunge over the steep wall of the main valley in ribbon-like falls so
thin that the wind turns them aside and disperses the water in the
spray of a “bridal veil.” Such falls are found by the hundred in every
glaciated mountain district, imparting to it one of the greatest of its
scenic charms.
[Illustration: FIG. 405.—Character profiles in landscapes sculptured
by mountain glaciers.]
[Illustration: FIG. 406.—Flat dome shaped under the margin of
a Norwegian ice cap with projecting rock knobs and moraines in
foreground.]
=The character profiles which result from sculpture by mountain
glaciers.=—The lines which are repeated in landscapes carved by
mountain glaciers are easy to recognize (Fig. 405). The highest
horizon lines are the outlines of horns which are separated by cols.
Minaret-like palisades, or “files of _gendarmes_”, often run for long
distances as the characteristic comb ridges. Lower down and lacking
the lighter background of the sky, we make out with less distinctness
the U-valley, either with or without the albs to show that the
sculpturing process has been interrupted by uplift.
[Illustration: FIG. 407.—Two views illustrating successive stages in
the shaping of tinds or “beehive” mountains.]
=The sculpture accomplished by ice caps.=—In the case of ice caps,
the only rock exposed is found in the neighborhood of the margin—the
projecting islands known as nunataks. It is essential for the existence
of the ice cap that the rock base should have relatively slight
irregularities compared to the dimensions of the cap itself. Except
in very high latitudes this base must be somewhat elevated, for like
mountain glaciers ice caps are nourished by the surface air currents,
and their snows are deposited above the snow line.
=The Norwegian tind or beehive mountain.=—Within temperate or tropical
climes the snow line lies so high that only the loftier mountains
are able to support glaciers. It follows that those which are formed
flow upon relatively high grades with correspondingly high rate of
movement and increased cutting power. Within high latitudes the snow
is found nearer the sea level, and glaciers are for the most part
correspondingly sluggish in their movements as well as less active
denuding agents.
To this condition characteristic of high latitude glaciers, there is
added in Norway another in the peculiar shape of the basement beneath
the recent and the still existing glaciers. The plateau of Norway is
intersected by a network of deep and steep walled fjords, and the
glaciers have developed as small ice caps perched upon veritable
pedestals of rock, over the margins of which their outlet tongues of
ice descend on steep slopes into the fjord. The tops of the pedestals
thus come to be shaped by the plucking and abrading processes into
flat domes (Fig. 406), while the knobs of rock, which as nunataks
reach above the surface of the ice, divide the outflowing ice tongues
at the margin of the pedestal. These tongues being much more active
denuding agents, because of their steep gradients, continually lower
their beds, thus transforming the earlier knobs of rock into high and
steep mountains of more or less circular base. Such “beehive” mountains
upon the margins of the fjords are the characteristic Norwegian _tinds_
(Fig. 407).
READING REFERENCES FOR CHAPTER XXVI
I. C. RUSSELL. Quaternary History of Mono Valley, California, 8th Ann.
Rept. U. S. Geol. Surv., 1889, pp. 329-371, pls. 27-37.
F. E. MATTHES. Glacial Sculpture of the Bighorn Mountains, Wyoming,
21st Ann. Rept. U. S. Geol. Surv., 1900, Pt. ii, pp. 179-185, pl. 23.
W. D. JOHNSON. Maturity in Alpine Glacial Erosion, Jour. Geol., vol.
12, 1904, pp. 569-578.
G. K. GILBERT. Systematic Asymmetry of Crest Lines in the High Sierras
of California, _ibid._, pp. 579-588.
EMM. DE MARTONNE. Sur la Formation des Cirques, Ann. de Géogr., vol.
10, 1901, pp. 10-16.
W. M. DAVIS. Glacial Erosion in North Wales, Quart. Jour. Geol. Soc.
Lond., vol. 65, 1909, pp. 281-350, pl. 14.
ED. BRÜCKNER. Die Glazialen Züge im Antlitz der Alpen, Naturw.
Wochenschr., N. F., vol. 8, 1909.
WILLIAM H. HOBBS. Characteristics of Existing Glaciers, pp. 1-96.