580 j. h. bretz— glacial drainage ox columbia plateau 



Glaciation 



Probably the entire highland area north of the plateau was Imried be- 

 neath the Cordilleran ice-sheet during the Pleistocene giaciations. At 

 least three times the Cordilleran ice forced a crossing of the Columbia 

 Valley and advanced onto the plateau. The latest of these, of Wisconsin 

 age, has been described ; the earlier ones thus far have been but noted in 

 the literature. Farther east, where Spokane Eiver serves as the bound- 

 ary, only two glacial invasions of the plateau are known, both of which 

 are pre-Wisconsin, and only brief notes exist regarding them.- The 

 latter of these two giaciations is well recorded in the topography. With 

 it and the Wisconsin glaciation on the northwestern part of the plateau 

 this paper chiefly deals. 



The Spokane Glaciation 

 general statement 



Observers for years have been noting the existence of granite boulders 

 scattered over the basalt plateau of Washington far beyond the limits 

 reached by the Wisconsin ice. They are now known to belong to four 

 categories: (1) derived from knobs of granite buried in or projecting 

 above the basalt flows, (2) berg-drifted in a widespread ponding during 

 late Wisconsin time,^ (3) glacier-borne during giaciations earlier and 

 more extensive than the Wisconsin, and (4) berg-carried in great streams 

 born of one of these earlier ice-sheets. 



The precise limits reached by the earlier ice-sheets are as yet un- 

 mapped, but enough is known to mdicate that the latter of the two 

 crossed the Columbia and Spokane valleys from the northern highlands, 

 pushed well up on the rim of the plateau, and in the vicinity of Spokane 

 advanced south of the rim divide as far as Spangle and nearly to Cheney. 

 Xo prominent morainal margin now exists, if one ever was formed, but 

 the till, the striated erratic boulders, the deposits of outwash material, 

 and the drainage channels constitute adequate evidence of the glaciation. 

 It is here named the Spokane glaciation. Details which are essential to 

 a conception of the character of the Spokane glaciation, so far as it is 

 now known, are here presented. 



- Frank Leverett : Bull. Geol. Soc. Am., vol. 28, 1917, p. 143. 



M. M. Leighton : Bull. 22, Wash. Geol. Survey, 1910. 



Thomas Large : Science, vol. oG, 1922, p. 335. 



.1. T. Pardee : Science, vol. 56, 1922, p. G8G. 

 3 J. H. Bretz : The late I'leistocene submergence in the Columbia Valley of Oregon 

 and Washington. .Tour. Geol., vol. 27, 1919, pp. 489-50G. 



