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[Written at Eureka, Nevada, in
November, 1878. Editor.]
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 and sage plains; characteristic
U-shaped canons 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
strioe 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 canons long after the
ice-sheet had been broken up.
The first glacial traces that
1 noticed in the basin are on the Wassuck, Augusta, and Toyabe ranges,
consisting of ridges and canons, 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, canons with V-shaped cross
sections, wide névé amphitheatres, moutonnéed 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 ValleSr, about fifty-five hundred
feet above sea-level.
Four other canons, 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 delight-
fully apparent in the trends
of the caflons and ridges, and in the massive sculpture of the név5 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 Are
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 név&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, canons, 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
stria 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 canons, 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 cañonwalls. 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 past-flowed, 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. |
