22 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
In its late geological history the region was covered by great glacial 
deposits derived mainly from the adjacent mountains. These de- 
posits consist of clay, gravel, or sand, often somewhat stratified. 
Pure deposits of each 30 meters thick or more are common. The 
total thickness of the glacial deposit has been estimated at from 
150 to 300 meters. 
The whole region is densely timbered with the exception of a 
series of small gravelly plains. These are largest and most abundant 
in the central part of the basin, but similar ones occur near Van- 
couver, and on Whidby and other islands. Likewise the tips of 
many of the points projecting into Puget Sound have the same 
gravelly soil, accompanied by a characteristic flora and fauna. These 
' gravelly prairies are plainly formed by flowing water, and are gen- 
erally considered to be deposited by post-glacial streams. Very 
similar prairies occur along the Willamette Valley. They form, in- 
deed, an interrupted series from the middle part of that valley north- 
ward to Vancouver Island. Owing to the very gravelly soil of these 
prairies, they partake of a semiarid condition. Indeed, the flora 
contains many species identical with those of eastern Washington. 
The drainage of the basin is mainly into Puget Sound, the princi- 
pal rivers coming from the Cascades, but the Cowlitz River and 
various smaller streams in the extreme southern part of the basin, 
flow into the Columbia. 
These streams for the main part originate in glaciers, and all of 
them have formed rather narrow valleys largely of glacial detritus. 
THE CASCADE MOUNTAINS. 
These mountains vary in breadth from 100 to 125 kilometers (80 
to 100 miles), traversing the State in a course a little easterly of a true 
north direction. The altitude of the main uplift varies from 1,800 
to 2,100 meters, (6,000 to 7,000 feet). The important peaks which 
conspicuously exceed this altitude are Mount Baker, in Whatcom 
County, altitude 3,335 meters (10,825 feet); Glacier Peak, Snoho- 
mish County, said to be 3,214 meters (10,436 feet) high; Mount 
Stuart, Kittitas County, 2,903 meters high (9,479 feet); Mount 
Rainier, on the dividing line of Pierce and Thurston counties, the 
highest peak of the Cascade system, 4,475 meters high (14,530 feet) 5 
Mount Adams, Klickitat County, altitude 3,819 meters (12,401 feet), 
and Mount St. Helens, Skamania County, 2,947 meters high (9,570 
feet). These tall peaks are all capped with perpetual snow, and rise 
far above the limits of ordinary plant life. With the exception of 
tlacier Peak and Mount Stuart they are all volcanic cones. 
The Cascade Mountains form the most important topographical 
feature of the State as affecting the distribution of plant life. The 
prevailing southwest winds from the Pacific are by them deprived 
