THE SPOKANE GLACIATION ' 587 



end of the lake. Eock Lake exists because the ancient river here exca- 

 vated more deeply in its bed than it did a little farther down stream. 

 Rock Lake is typical of most lake basins of the Columbia Plateau in 

 Washington, in that it is a much elongated true rock-basin in an ancient 

 river channel in a scabland area. The only part played by glacial ice in 

 the formation of these basins is that of supplying the stream. Factors 

 essential for development of these rock-basins made by streams are (1) 

 large volume abruptly introduced, (2) high gradient, and (3) rock which 

 is closely and vertically jointed. The excavation was accomplished by 

 plucking, rather than by grinding. 



Vicinity of Kalilotus. — About 25 miles south of the latitude of Eock 

 Lake the scabland is more restricted. So far as now known, all channels 

 but one entered Snake Eiver in the vicinity of the present junction of 

 the Palouse. The one channel which escaped the control of the Palouse 

 system is traceable westward, by way of Washtucna, through Kahlotus, 

 to Esquatzel Coulee at Connell. It is a splendid abandoned channel, in 

 places 250 to 300 feet deep. Throughout its length it has all the char- 

 acters of the channels of the scabland tract to the east and northeast 

 except that it does not lie in such a tract. It probably was sufficiently 

 deep at the inception of the flood to contain the waters which came its 

 way. A few miles above the junction of Washtucna Coulee, Esquatzel 

 Coulee has none of these characters, but possesses the broad, graded 

 slopes of maturity. 



Vicinity of Sprague. — The westernmost channels among the Palouse 

 Hills near Cheney which discharged to the Palouse system enter the 

 broad scabland between Sprague and Fishtrap. The roughness of these 

 channel floors, due to the gashed Basalt, is in striking contrast with the 

 smooth flowing contours of the inclosing hills. One needs but little 

 imagination to see again from these hilltops the torrents of glacial waters 

 invading from the north and following the lower valleys southward to 

 join the scabland between Sprague and Lament. These hills, like those 

 in the Palouse wheat lands, are composed largely of a fine-te.ytured 

 unindurated sediment, probably of lacustrine origin."^ 



It does not seem probable that any water, other than that which occu- 

 pied Washtucna Coulee, escaped westward from the Palouse drainage 

 during the Spokane epoch. There are two possible routes, however, of 

 such discharge, namely, one by way of Keystone, near the lower end of 

 Colville (Sprague) Lake and Eitzville; the other by way of Balston. 

 Both enter Lind Coulee at Lind. The altitude at the head of the Eitz- 



■ M. R. Campbell : Guidebook of the Western r'nited States. Tart A : The Northern 

 I'acltic Route. U. S. Geol. Survey, Bull. 611, 1015, p. 168. 



