CHANNELED SCABLANDS OF THE COLUMBIA PLATEAU 631 



scablands are located in places of exceptionally steep original 

 gradients. 



If the channeled scablands are the product of stream erosion, 

 and if all tracts are of the same age, the fact that the scabland heads 

 vary in altitude, though all but two are open to the same glaciated 

 basalt plain, can have but one satisfactory explanation. Each 

 glacial stream must have had a source of water which was uncon- 

 nected with the others. This means that the ice sheet whose melting 

 yielded these streams must have covered the basalt plain and in 

 most places must have been in contact with the mature loessial 

 hills which separate the scabland heads. It means, furthermore, 

 that but a few miles of ice front supplied the water for streams so 

 huge that they flooded over many preglacial divides of the mature 

 topography even in the southern part of the plateau. It was this 

 flooding across divides which produced the scabland plexus of the 

 plateau. 



About 40 miles of ice front in one case' yielded water sufficient 

 to denude a non-elongated tract 250 square miles in area of a loessial 

 cover about 100 feet in maximum thickness. This was done by 

 lateral planation in the preglacial drainage lines of the tract. These 

 lines were so shallowly intrenched in the basalt and the volume of 

 the water was so great that, as the loessial hills were eroded away, 

 the flood spread over the entire area, 13 miles wide. Gradients 

 were low, however, and it, did not develop canyoned channels. 

 Steeper gradients farther from the edge of the ice, and greater 

 capacity of the preglacial valleys, held the waters within the con- 

 fines of these valleys, but six such' were necessary to contain the 

 flood and they were all greatly eroded in the underlying basalt. 



Another large scabland area^ is 75 miles long and 15 miles in 

 average width. Its total descent is 1,850 feet. Its altitude at the 

 head is the lowest of all such tracts. It differs from the one above in 

 its notable linear extent, in the possession of a large number of iso- 

 lated groups of loessial hills of the older topography, in its greater 



I The Telford scabland and its dependencies. 



^ Coal Creek, Duck Creek, Lake Creek, an unnamed creek, Connawai Creek and 

 Wilson Creek. 



3 The Cheney-Hooper area, extending from Spangle and Cheney to Snake River, 

 south of Hooper. 



