the john muir exhibit - writings - steep_trails - chapter 15
Chapter 15
Glacial Phenomena in Nevada
20
The monuments of the Ice Age in the Great Basin have been greatly
obscured and broken, many of the more ancient of them having perished
altogether, leaving scarce a mark, however faint, of their existence
-- a condition of things due not alone to the long-continued action of
post-glacial agents, but also in great part to the perishable
character of the rocks of which they were made. The bottoms of the
main valleys, once grooved and planished like the glacier pavements of
the Sierra, lie buried beneath sediments and detritus derived from the
adjacent mountains, and now form the arid sage plains; characteristic
U-shaped canyons have become V-shaped by the deepening of their
bottoms and straightening of their sides, and decaying glacier
headlands have been undermined and thrown down in loose taluses, while
most of the moraines and striae and scratches have been blurred or
weathered away. Nevertheless, enough remains of the more recent and
the more enduring phenomena to cast a good light well back upon the
conditions of the ancient ice sheet that covered this interesting
region, and upon the system of distinct glaciers that loaded the tops
of the mountains and filled the canyons long after the ice sheet had
been broken up.
The first glacial traces that I noticed in the basin are on the
Wassuck, Augusta, and Toyabe ranges, consisting of ridges and canyons,
whose trends, contours, and general sculpture are in great part
specifically glacial, though deeply blurred by subsequent denudation.
These discoveries were made during the summer of 1876-77. And again,
on the 17th of last August, while making the ascent of Mount
Jefferson, the dominating mountain of the Toquima range, I discovered
an exceedingly interesting group of moraines, canyons with V-shaped
cross sections, wide neve amphitheatres, moutoneed rocks, glacier
meadows, and one glacier lake, all as fresh and telling as if the
glaciers to which they belonged had scarcely vanished.
The best preserved and most regular of the moraines are two laterals
about two hundred feet in height and two miles long, extending from
the foot of a magnificent canyon valley on the north side of the
mountain and trending first in a northerly direction, then curving
around to the west, while a well-characterized terminal moraine,
formed by the glacier towards the close of its existence, unites them
near their lower extremities at a height of eighty-five hundred feet.
Another pair of older lateral moraines, belonging to a glacier of
which the one just mentioned was a tributary, extend in a general
northwesterly direction nearly to the level of Big Smoky Valley, about
fifty-five hundred feet above sea level.
Four other canyons, extending down the eastern slopes of this grand
old mountain into Monito Valley, are hardly less rich in glacial
records, while the effects of the mountain shadows in controlling and
directing the movements of the residual glaciers to which all these
phenomena belonged are everywhere delightfully apparent in the trends
of the canyons and ridges, and in the massive sculpture of the neve
wombs at their heads. This is a very marked and imposing mountain,
attracting the eye from a great distance. It presents a smooth and
gently curved outline against the sky, as observed from the plains,
and is whitened with patches of enduring snow. The summit is made up
of irregular volcanic tables, the most extensive of which is about two
and a half miles long, and like the smaller ones is broken abruptly
down on the edges by the action of the ice. Its height is
approximately eleven thousand three hundred feet above the sea.
A few days after making these interesting discoveries, I found other
well-preserved glacial traces on Arc Dome, the culminating summit of
the Toyabe Range. On its northeastern slopes there are two small
glacier lakes, and the basins of two others which have recently been
filled with down-washed detritus. One small residual glacier lingered
until quite recently beneath the coolest shadows of the dome, the
moraines and neve-fountains of which are still as fresh and unwasted
as many of those lying at the same elevation on the Sierra -- ten
thousand feet -- while older and more wasted specimens may be traced on
all the adjacent mountains. The sculpture, too, of all the ridges and
summits of this section of the range is recognized at once as glacial,
some of the larger characters being still easily readable from the
plains at a distance of fifteen or twenty miles.
The Hot Creek Mountains, lying to the east of the Toquima and Monito
ranges, reach the culminating point on a deeply serrate ridge at a
height of ten thousand feet above the sea. This ridge is found to be
made up of a series of imposing towers and pinnacles which have been
eroded from the solid mass of the mountain by a group of small
residual glaciers that lingered in their shadows long after the larger
ice rivers had vanished. On its western declivities are found a group
of well-characterized moraines, canyons, and roches moutonnees, all of
which are unmistakably fresh and telling. The moraines in particular
could hardly fail to attract the eye of any observer. Some of the
short laterals of the glaciers that drew their fountain snows from the
jagged recesses of the summit are from one to two hundred feet in
height, and scarce at all wasted as yet, notwithstanding the countless
storms that have fallen upon them, while cool rills flow between them,
watering charming gardens of arctic plants -- saxifrages, larkspurs,
dwarf birch, ribes, and parnassia, etc. -- beautiful memories of the Ice
Age, representing a once greatly extended flora.
In the course of explorations made to the eastward of here, between
the 38th and 40th parallels, I observed glacial phenomena equally
fresh and demonstrative on all the higher mountains of the White Pine,
Golden Gate, and Snake ranges, varying from those already described
only as determined by differences of elevation,
relations to the snow-bearing winds,
and the physical characteristics of the rock formations.
On the Jeff Davis group of the Snake Range, the dominating summit of
which is nearly thirteen thousand feet in elevation, and the highest
ground in the basin, every marked feature is a glacier monument
-- peaks, valleys, ridges, meadows, and lakes. And because here the
snow-fountains lay at a greater height, while the rock, an exceedingly
hard quartzite, offered superior resistance to post-glacial agents,
the ice-characters are on a larger scale, and are more sharply defined
than any we have noticed elsewhere, and it is probably here that the
last lingering glacier of the basin was located. The summits and
connecting ridges are mere blades and points, ground sharp by the
glaciers that descended on both sides to the main valleys. From one
standpoint I counted nine of these glacial channels with their
moraines sweeping grandly out to the plains to deep sheer-walled
neve-fountains at their heads,
making a most vivid picture of the last days of the Ice Period.
I have thus far directed attention only to the most recent and
appreciable of the phenomena; but it must be borne in mind that less
recent and less obvious traces of glacial action abound on ALL the
ranges throughout the entire basin, where the fine striae and grooves
have been obliterated, and most of the moraines have been washed away,
or so modified as to be no longer recognizable, and even the lakes and
meadows, so characteristic of glacial regions, have almost entirely
vanished. For there are other monuments, far more enduring than
these, remaining tens of thousands of years after the more perishable
records are lost. Such are the canyons, ridges, and peaks themselves,
the glacial peculiarities of whose trends and contours cannot be hid
from the eye of the skilled observer until changes have been wrought
upon them far more destructive than those to which these basin ranges
have yet been subjected.
It appears, therefore, that the last of the basin glaciers have but
recently vanished, and that the almost innumerable ranges trending
north and south between the Sierra and the Wahsatch Mountains were
loaded with glaciers that descended to the adjacent valleys during the
last glacial period, and that it is to this mighty host of ice streams
that all the more characteristic of the present features of these
mountain ranges are due.
But grand as is this vision delineated in these old records, this is
not all; for there is not wanting evidence of a still grander
glaciation extending over all the valleys now forming the sage plains
as well as the mountains. The basins of the main valleys alternating
with the mountain ranges, and which contained lakes during at least
the closing portion of the Ice Period, were eroded wholly, or in part,
from a general elevated tableland, by immense glaciers that flowed
north and south to the ocean. The mountains as well as the valleys
present abundant evidence of this grand origin.
The flanks of all the interior ranges are seen to have been heavily
abraded and ground away by the ice acting in a direction parallel with
their axes. This action is most strikingly shown upon projecting
portions where the pressure has been greatest. These are shorn off in
smooth planes and bossy outswelling curves, like the outstanding
portions of canyon walls. Moreover, the extremities of the ranges
taper out like those of dividing ridges which have been ground away by
dividing and confluent glaciers. Furthermore, the horizontal sections
of separate mountains, standing isolated in the great valleys, are
lens-shaped like those of mere rocks that rise in the channels of
ordinary canyon glaciers, and which have been overflowed or
pastflowed, while in many of the smaller valleys roches moutonnees
occur in great abundance.
Again, the mineralogical and physical characters of the two ranges
bounding the sides of many of the valleys indicate that the valleys
were formed simply by the removal of the material between the ranges.
And again, the rim of the general basin, where it is elevated, as for
example on the southwestern portion, instead of being a ridge
sculptured on the sides like a mountain range, is found to be composed
of many short ranges, parallel to one another, and to the interior
ranges, and so modeled as to resemble a row of convex lenses set on
edge and half buried beneath a general surface, without manifesting
any dependence upon synclinal or anticlinal axes -- a series of forms
and relations that could have resulted only from the outflow of vast
basin glaciers on their courses to the ocean.
I cannot, however, present all the evidence here bearing upon these
interesting questions, much less discuss it in all its relations. I
will, therefore, close this letter with a few of the more important
generalizations that have grown up out of the facts that I have
observed. First, at the beginning of the glacial period the region
now known as the Great Basin was an elevated tableland, not furrowed
as at present with mountains and valleys, but comparatively bald and
featureless.
Second, this tableland, bounded on the east and west by lofty mountain
ranges, but comparatively open on the north and south, was loaded with
ice, which was discharged to the ocean northward and southward, and in
its flow brought most, if not all, the present interior ranges and
valleys into relief by erosion.
Third, as the glacial winter drew near its close the ice vanished from
the lower portions of the basin, which then became lakes, into which
separate glaciers descended from the mountains. Then these mountain
glaciers vanished in turn, after sculpturing the ranges into their
present condition.
Fourth, the few immense lakes extending over the lowlands, in the
midst of which many of the interior ranges stood as islands, became
shallow as the ice vanished from the mountains, and separated into
many distinct lakes, whose waters no longer reached the ocean. Most
of these have disappeared by the filling of their basins with detritus
from the mountains, and now form sage plains and "alkali flats."
The transition from one to the other of these various conditions was
gradual and orderly: first, a nearly simple tableland; then a grand
mer de glace shedding its crawling silver currents to the sea, and
becoming gradually more wrinkled as unequal erosion roughened its bed,
and brought the highest peaks and ridges above the surface; then a
land of lakes, an almost continuous sheet of water stretching from the
Sierra to the Wahsatch, adorned with innumerable island mountains;
then a slow desiccation and decay to present conditions of sage and sand.
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