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 (0,000 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) ; 

 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 

 Glacier 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 



