268 BOTANICAL GAZETTE [APRIL 
fields which fed the glaciers moving down the valleys, transporting 
the débris that now forms the morainal heaps at 6000-7000 feet. 
The peaks and ridges, left after the débris was carried away, show 
many typical features of glaciated mountains: cirques, aretes, 
dents. A most perfect example of the last is Pyramid Peak, on 
whose north slope the Rubicon Glacier had its source, thence 
moving north, scouring out Rubicon Valley and probably debouch- 
ing into Lake Tahoe through Rubicon Pass (7150 feet) and the 
valley of McKinney Creek. 
Ice action varied in its effect upon the relief with the several 
rocks beneath the surface of the glaciers; in the andesite the 
glacial sapping is especially marked and the cirques of Twin Peak 
and Mt. Tallac are excellent examples of their kind. On the mas- 
sive granite the effect seems to have been practically limited to 
clearing away the loose material overlying the bed rock, which wa 
little, if at all, affected. 
Topography 
The region studied is embraced within the area mapped by the 
United States Geological Survey on the Truckee and Pyramid Peak 
quadrangles. It is roughly some 50 miles long and 15 wide, and 
may be considered as a typical section of the Central Sierra. This 
section of the range does not contain elevations comparable to 
those further south in the High Sierra of Fresno and Tulare coun- 
ties, nevertheless several of the peaks rise above 10,000 feet, an 
one, Freel’s Peak, is over 11,000 feet. The area is in shape 4 
trough, with average elevation of the floor about 6500 feet. The 
rim of the trough is formed on the west of the Great Western Dr 
vide, which separates those streams flowing westward to the Great 
Valley of California from those forming a part of the Great Basin 
drainage system. The eastern margin is the Carson Range, the 
most eastern of the Sierran ranges. 
The Great Western Divide is the more deeply dissected of these 
limiting mountain chains. From Round Top (10,430 feet) ™ 
Alpine County this dividing ridge runs north to Echo Lake and 
then bears west of north, continuing beyond Mt. Lola (9167 feet) 
into Sierra County. Along this height of land the more promr 
