606 J. H. BRETZ GLACIAL DRAIXAGE '».%- COLVMBLl PLATEAU 



to have been carried, here in floating ice. Either the upper limit of the 

 submergence was greater over the plateau than has been thought or there 

 has been post-AVisconsin upwarping in the vicinity of Trinidad. The 

 latter seems the more probable. 



Pardee has described a deposit of silt with sand and gravely the Xes- 

 pelem formation/^ in the Columbia Valley above Grand Coulee. This 

 he believes to be of Wisconsin age and to have been caused by a lowering 

 of the region such that the upper surface of the deposit (1,700 feet above 

 tide) records the sealevel of that time. What is taken to be a part of 

 this formation lies on the floor of Grand Coulee about Steamboat Rock 

 and the granite hills. Its upper surface here is about 1,650 feet above 

 tide and it is seasonally banded. From its position, it obviously was 

 deposited after the margin of the Okanogan lobe and the diverted glacial 

 Columbia had abandoned Grand. Coulee. It therefore was deposited after 

 the berg-borne debris had been carried through the coulee. N^one of this 

 silt has been recognized in the Quincy Valley or in the Columbia Valley 

 below the Okanogan lobe. It may be a record, of the submergence, as 

 are the berg-carried erratics. It also may be related to the ponding of 

 the Columbia by the Okanogan lobe, or by the large Wisconsin Valley 

 train from the Okanogan Valley, which Pardee describes and which Avas 

 formed during the retreat of the Cordilleran ice-sheet. 



Most of the fill of the Quincy structural basin, as shown by well 

 records, is clay and silt. The Pleistocene boreal mollusks reported from 

 the upper part of the clay^' suggest that it was deposited while a glacial 

 climate prevailed, but probably not when the glacial waters were being 

 discharged across the northern rim of the plateau, for the gravels over- 

 lying the clays were then carried into the basin. 



There appears to be one great summit plane of the gravels, now dis- 

 sected into four parts. The altitude of the northern part of each terrace 

 is about 1,250 feet above tide. The surface (restored) slopes toward the 

 Frenchman Springs and Potholes cataracts on the western margin of the 

 basin and toward the Drumheller plexus on the southern margin. A 

 continuous grade exists westward into the bottoms of the eroded channels 

 at the head of the cataracts, but to the south the gravel terrace is 150 

 feet above the floor of Crab Creek Valley immediately adjacent and as 

 high as the basalt buttes among the channel heads. 



This gravel fill probably dates back to the Spokane epoch, though some 

 of it may have been aggraded during the early part of the Wisconsin 



1" .T. T. Pardee: Geology and mineral deposits of the ColviUe Indian reservation, 

 Washington. U. S. Geol. Survey, Bull. 677, 1918, pp. 28-29 and 47-50. 



^' Schwenneson and Meinz^r : Ground water in Quincy Valley, Washingrton. U. S. 

 Geol. Survey. Water Supply I'aper 42.:) E. 1018, pp. 143-144. 



