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
cañons 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 cañons 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 cañons,
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, cañons 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 cañon 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 cañons, 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
cañons 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 névé-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, cañons, and _roches moutonnées_, 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 névé-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 cañons, 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 cañon 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 cañon glaciers, and which have been overflowed or pastflowed,
while in many of the smaller valleys _roches moutonnées_ 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.