644 J SARLEN BRETZ 



epoch of erosion of the basalt. Deposits on the highest scablands^ 

 were made before the deepening canyons drew the waters into more 

 restricted routes. Deposits in the deeper portions^ were made 

 during the latest stages of the episode. No gravel deposits were 

 ever as thick as the rival hypothesis would require them to be.^ 

 That view demands that the canyons be eroded, then filled com- 

 pletely with their own debris, then re-excavated in large part. 



However, there are two places on the plateau where the history 

 of deposition by glacial streams has been somewhat different. 

 One of these is Hartline gravel flat, the other is Quincy Basin. 

 Both are structural depressions, not completely filled before the 

 glacial floods arrived. The Hartline structural valley became 

 filled with debris from the cutting of Upper Grand Coulee before 

 Lower Grand Coulee had been eroded. The trenching of the 

 lower coulee, and particularly the development and retreat of 

 Grand Falls, incised the southern rim of the valley so that, by the 

 close of the episode, the gravel fill, once the floor of the glacial 

 stream, had been removed in its western part, and the remnant left 

 200 feet or so higher than the brink of Grand Falls. ^ The total fill 

 in the Harthne structural valley is about 250 feet. It is composed 

 of bowlders, cobbles, and gravel near Grand Coulee, and of sand 

 5 or 6 miles east, back from the main drainage line. An old channel 

 from Grand Coulee crosses its northern and eastern part and leads 

 into Deadmans Gulch, a distributary which spilled across the south- 

 em rim into Spring Coulee before Lower Grand Coulee had devel- 

 oped into the dominant notch that drained and dissected the flat. 

 The terrace form of Hartline flat is the result of erosion of a once 

 complete gravel fill. Its scarp is not constructional, as are those 

 of bars. But the fill and the subsequent erosion occurred because 

 of special local conditions, not because general conditions changed. 



I As about Gloyd along the Black Rock distributary from Crab Creek Valley. 



' As at the mouths of Duck Creek and Wilson Creek in Crab Creek Valley and at 

 the junction of Crab and Coal creeks. 



3 For example, gravel deposits north of the town of Washtucna lie on the slopes of 

 the coulee, 350 feet above the present floor. They antedate the deeper canyoned 

 portions of the coulee. 



t Estimated from the surviving eastern member of that complex waterfall, the only 

 part which escaped modification by the Wisconsin glacial stream. 



