(504 J. H. BRETZ GLACIAL DRAIXAGE OX COLL^MBIA PLATEAU 



aggTaded floor. The northwest sides of these hills are notably smoothed 

 and rounded and some bear striae. The orientation varies, as would be 

 expected on rugged rock hills overridden by ice, but is not far from 

 northwest-southeast. The southeast sides are steep and jagged. The 

 sheeted structure of the granite apparently has lent itself to plucking l)y 

 the ice. Furthermore, the summits of the basalt bluffs on the western 

 side of the coulee also are striated in approximately the same directions, 

 and stria? and abundant large granite erratics are reported on the top of 

 Steamboat Rock (figure 12). 



The giaciation of these granite hills and the basalt hill south of them 

 occurred at the maximum deployment of the AVisconsin ice on the pla- 

 teau. It can not be a record of the Spokane giaciation, for these granite 



FiGiKE 12. — ^Steatuboat Rock 



Steamboat Rock is in the middle of the Grand Coulee and near the head. The talus is 



of Dost-Wiscottsin age. 



hills tht^n were buried hundreds of feet l)eneath basalt and have been 

 exhumed by the erosion of both Spokane and Wisconsin glacial drainage. 

 If it l)e argued that this exhumation occurred during the early stages of 

 the Spokane giaciation, the ice later advancing into the head of Grand 

 Coulee, the answer is that these exposed glaciated surfaces would have 

 l)een obliterated during the interglacial interval. 



What became of the Wisconsin drainage when the Okanogan lol^e 

 crowded down into the head of Grand Coulee? Since the Spokane spill- 

 ways to Pa louse River are lower than all others on the plateau except 

 Grand Coulee, and since there was an eastward route open to them in 

 front of the Wisconsin ice, it might be expected that for a short time 

 glacial drainage would be diverted to the Palouse. But reexamination 

 of tliese spillways has found no signs of such occupation. Furthermore. 



