590 J. H. BRETZ GLACIAL DRAINAGE OX COLUMBIA PLATEAU 



consin glaciation of northern Douglas County was more extensive than 

 the Spokane and obliterated all records left b}' the Spokane ice. 



There are scablands in the northern part of some of these spillways, 

 notably between Davenport and Eocklyn and in the vicinity of Telford. 

 The detailed relations of the largest, at and southwest of Telford, are 

 not yet well known : but concentration in preexisting valleys in basalt 

 was completed in most cases within 10 miles or so of the edge of the ice, 

 and rock-walled canyons with elongated lakes in the bottoms are the rule. 

 The altitudes of the heads of these spillways are not the same, and had 

 not the ice been hard against the unglaciated hills south of the basalt 

 plain, marginal drainage would have carried all waters to the lowest of 

 the group. 



Gravel deposits and scattered granite boulders and cobbles are well 

 distributed in these coulees. Gravels are chiefly of basalt and appear as 

 fresh as in the scablands of the Palouse drainage. The terrace forms are 

 indefinite, wide-open gullies have dissected them, and they are but rem- 

 nants, perhaps miles apart, of former probably continuous fillings. This 

 also is comparable to the gravels of the main Palouse channels. 



Because of the prominence of cliffs in these canyons of Crab Creek 

 drainage, another feature is well presented — the talus piles which have 

 formed since the glacial streams abandoned their channels. The cliffs 

 are, without exception, composed of the Columbia lava, and, with very 

 few exceptions, the flows are horizontal or but gently inclined and possess 

 columnar jointing. The conditions of origin produced essentially ver- 

 tical cliffs, and the rock structures and arid climate have maintained 

 vertical faces during all subsequent wasting of the cliff and growth of 

 the talus. 



Except in Grand Coulee and Moses Coulee, which carried Wisconsin 

 glacial waters also, these canyons have been occupied by inconsequential 

 streams since the Spokane glacial floods subsided. The present talus, 

 therefore, is a measure of all post-Spokane disintegration in the vertical 

 walls of jointed lava. In the great majority of cases the talus extends 

 from the rock-floor of the canyon three-fourths to four-fifths of the way 

 to the top (see figures -k and 7). In the lower cliffs it has climbed even 

 nearer the summit, and rarely the cliff has become obliterated by the 

 mounting waste. The talus slopes range between 20 degrees and 30 de- 

 grees, with the average probably between 25 degrees and 30 degrees. 



Since conditions of origin, of rock structure, and of climate are suffi- 

 ciently alike in the Crab Creek and Palouse drainage areas, it follows 

 that relative proportion of talus on cliff's of comparable height may be a 

 valuable criterion to establish more firmly the contemporaneity of glacial 



