But not only does the ice move.  There have been other forces set in motion as well as that of the ice.  The fierce attacks of the storms, the insidious forces of frost, of expansion and contraction, of lightning, etc., have shattered and loosened vast masses of the mountain summits.  Some of these have weathered into toppling masses, which required only a heavy wind or slight contractions to send them from their uncertain bases onto the snow or ice beneath.  And the other causes mentioned all had their influences in breaking up the peaks and ridges and depositing great jagged bowlders of rock in the slowly-moving glaciers.

Little by little these masses of rock worked their way down lower into the ice-bed.  Sometime they must reach the bottom, yet, though they rest upon granite, and granite would cleave to granite, the irresistible pressure from above forces the ice and rock masses forward.  Thus the sharp-edged blocks of granite become the blades in the tools that are to help cut out the contours of a world’s surface.  In other words the mass of glacial ice is the grooving or smoothing plane, and the granite blocks, aided by the ice, become the many and diverse blades in this vast and irresistible tool.  Some cut deep and square, others with flutings and bevelings, or curves, but each helps in the great work of planing off, in some way, the rocky masses over which they move.  Hence it will be seen that the grooving and marking, the fluting and beveling, the planing and smoothing processes of the ice are materially aided and abetted by the very hardness and weight of the granite and other rocks it carries with it.

Now let Joseph LeConte take up the theme and give us of the rich treasure-store of his knowledge and observation.  In the American Journal of Science and Arts, Third Series, for 1875, he discussed the very field we are now interested in, and his fascinating and illuminating explanations render the subject perfectly clear.  Said he: 

Last summer I had again an opportunity of examining the pathways of some of the ancient glaciers of the Sierra.  One of the grandest of these is what I call the Lake Valley Glacier.[1] Taking its rise in snow fountains among the high peaks in the neighborhood of Silver Mountain, this great glacier flowed northward down Lake Valley, and, gathering tributaries from the summit ridges on either side of the valley, but especially from the higher western summits, it filled the basin of Lake Tahoe, forming a great “mer de glace,” 50 miles long, 15 miles wide, and at least 2000 feet deep, and finally escaped northeastward to the plains.  The outlets of this great “mer de glace” are yet imperfectly known.  A part of the ice certainly escaped by Truckee Canyon (the present outlet of the Lake); a part probably went over the northeastern margin of the basin.  My studies during the summer were confined to some of the larger tributaries of this great glacier.

[Footnote 1:  This is the name given by Dr. LeConte to the Basin in which Lake Tahoe rests and including the meadow lands above Tallac.]