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.
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