586 J. H. BRETZ- — GLACIAL DRAINAGE OX COLUMBIA PLATEAU 



some iiiuisnal way. The basalt of the scablands is the firm and resistant 

 foimdation on Avhich the hills stand. The overlving sedimentarY deposit 

 is the formation whose etched surface constitutes the hills. 



The scabland does not extend north of the Palouse Hills about Cheney 

 and Medical Lake, though it does extend among them in great river 

 channels to their northern limits. Farther north, there is neither scab- 

 land nor Palouse Hills. The area is a basalt plain with widely spaced 

 mature valleys and broad, low, flat divides. The whole is thinly covered 

 with glacial drift, some of it a stony till with hummockv morainic topog- 

 raphy, some of it washed and stratified glacial sand and gravel. The 

 ice-sheet clearly covered this plain. It, however, did not extend south- 

 ward into the scabland area, except for a minor lobe between Moran Peak 

 and the Cheney Hills, which reached as far south as Spangle. The asso- 

 ciation of broad undissected divides with shallow, mature valleys, all in 

 resistant rock (for example. Deep Creek and its tributaries) is anoma- 

 lous. It is believed to be the result of the glaciation of a tract which 

 ])ossessed a typical Palouse mature topography, developed largely in a silt 

 or ash deposit, but etched in the deeper valleys into the surface of the 

 underlying basalt. The ice removed every trace of the hills in the weaker 

 formation, bringing the whole down to the surface of the basalt : in effect, 

 it cut off the hills well down to their bases without 2:reatlv modifvino- the 

 main valley bottoms. 



Vicinilij of Lamont. — Twenty miles to the southwest of Cheney the 

 escaping glacial waters had become largely concentrated between Sprague 

 and Lamont. The eroded tract here is nearly 10 miles wide and without 

 a surviving Palouse hill. A sheet of torrential water must have spread 

 completely across the scabland between these two towns. It rapidly re- 

 moved the weaker material of the Palouse Hills and then scoured out 

 channels in the basalt. By the close of its occupancy it had concentrated 

 in certain channels and eroded them approximately 100 feet below the 

 surface of the basalt. Colville (Sprague) Lake lies in one of these 

 channels. 



Between Lamont and Eock Lake, to the east, is a linear tract of 

 Palouse Hills about eiglit miles ^\i(le and traversed l^y but one valley 

 possessing bare rock in its floor and scattered granite fragments. Sepa- 

 rating this tract from the Palouse country of Whitman County is a great 

 river channel, occupied now by Rock Creek and several elongated lake^. 

 Rock Lake is the largest of these and, save one, is the largest and longest 

 lake on the Columbia Plateau of Washington. Rock Lake (see fiii'ure 5) 

 is bounded on both sides ])y sheer cliff's of l)asalt 200 feet and more in 

 lieight. There is no damming of consequence in tlie channel at the lower 



