J. LeConte—Ancient Glaciers of the Sierra Nevada. 137 
in this vicinity, therefore, must have run out into the water six or 
seven miles, and doubtless formed icebergs at their point, and, 
therefore, formed there no terminal moraine. 
That the glaciers described about Lake Tahoe and Lake 
Mono ran out far into the water and formed icebergs I think 
is quite certain, and that parallel moraines open below are 
characteristic signs of such conditions I also think nearly 
certain. 
Ff. Glacial Erosionn—My observations on glacial pathways 
in the high Sierra, and especially about Lake Tahoe, have 
greatly modified my views as to the nature of glacial erosion. 
Writers on this subject seem to regard glacial erosion as mostly, 
if not wholly, a grinding and scoring ; the débris of this erosion 
as rock-meal; the great boulders, ieee are found in such 
immense quantities in the terminal deposit, as derived wholly 
from the por a cliffs above the glacial surface ; the rounded 
boulders, which are often the most numerous, as derived in 
precisely the same sae only they have been engulfed by 
crevasses, or between the sides of the glacier and the bounding 
wall, and thus carried between the moving ice and its rocky 
agenc 
oN a if such be the true view of glacial erosion, evidently its 
effect in mountain sculpture must be small indeed. Roches 
moutonneés are recognized by all as the most universal and 
characteristic sign of a ane bed. heerente these beds are 
glacial agency. "But a as gqite satisfied, from my own obser- 
vations, that this a not the only nor the principal mode of 
glacial erosion. I am convinced that a glacier, B, by its —— 
ressure and resistless onward movement, is constantly breaki: 
off large blocks from its bed and bounding walls Tas reins is 
not only a grinding and scoring, but also a crushing and break- 
ing. It makes ee sed ihe not only rock-meal, but also 
arge rock-chips a glacier is constantly breaking off 
blocks and mates eprideat surfaces, and then grinding off 
= angles both of the fragments and the bed, and thus forming 
unded boulders and moutonneés surfaces. Its erosion is a 
