CHANNELED SCABLANDS OF THE COLUMBIA PLATEAU 641 



Further evidence that glacial waters so filled Washtucna 

 Coulee that the former valley became simply a channel is found in 

 the upper limit of glacial stream gravels and of scablands. At the 

 head of Devils Canyon, the highest scabland is 1,250 feet A.T., 

 250 feet above the brink of the cliffs of the canyoned channel. 

 At Estes, a gravel bar deposited by the glacial stream lies 250 feet 

 above the Coulee floor and about 125 feet above the rock terrace 

 which marks the old valley floor. Near Sulphur, the highest scab- 

 land surface, at the base of the steep loessial bluffs, is between 1,100 

 and 1 , 1 50 feet A.T. , 1 00 to 1 50 above the rock terrace. Northwest of 

 Connell a terrace of sand and fine gravel lies at 1,000 feet A.T. It 

 marks the upper margin of the scabland here and probably is a deposit 

 of the glacial stream. It is 100 feet above the broad rock terrace. 

 The canyoned channel here is cut 150 feet below the rock terrace. 



Crab Creek Valley, below Odessa, received more water than it 

 could carry away, at least before its central canyon had been 

 eroded. It overflowed southwestward, by way of Black Rock 

 Coulee and its associated scabland, to the Quincy Basin which 

 Crab Creek itself entered farther north. Measurements here are 

 only approximate but they indicate the order of magnitude of this 

 glacial stream. Scabland and glacial stream gravel along the 

 southern edge of Crab Creek Valley lie 300 feet or more above the 

 present stream and extend a mile and a half back on the upland from 

 the margin of the preglacial valley. This valley had been canyoned 

 more than 100 feet by the glacial stream which, on the basis of these 

 figures, was 200 feet deep at its inception. 



The Telford scabland tract, 13 miles wide and 20 miles long, 



has been swept almost completely bare of the loessial deposit. 



The relief in a cross-section of the basalt surface now exposed, 



aside from the minor canyons, is about 50 feet. To have been so 



denuded, this tract must have had a sheet of running water of this 



depth completely over it.^ 



' That the ice sheet did not advance over the Telford denuded tract is shown by the 

 presence of a few isolated loessial hills with characteristically steepened marginal 

 slopes. One such group lies 7 or 8 miles north of Telford.- It has a maturely eroded 

 topography and a dark loessial soil without rock fragments of any kind. But it is cut 

 by channels of glacial waters which eroded to the basalt. The fact that these waters 

 went through the group, though the surface immediately north of them drops off into 

 the deep canyons of Hawk Creek, a tributary of Spokane River, proves that glacial 

 ice must have crowded up against the northern side of the group. 



