50 E. C. ANDREWS 



along the lines of preglacial drainage. Just below the point where 

 two profound canyons junction to form one deep valley, comparable 

 only in width with each of its feeders, the ice-masses converged, and, 

 by analogy v^ith viscous fluids, would thereby have their velocity 

 increased. Thus during the height of the ice-flood, they ripped oft" 

 spur ends, straightened the canyon walls, finally undermining the 

 sides and scooping out deep rock basins thousands of feet -below base- 

 (sea-) level. 



During the height of the glacial phase (flood stage), aggradation 

 would progress hand in hand with canyon lowering and widening. 

 Thus, wherever "narrows" in the canyons opened out into "broads," 

 glacial action — so strong just below the points of canyon convergence — 

 would here receive a decided check. This would also be very notice- 

 able even during the rock-basining stage in the narrows. Again, 

 as in the case of ordinary streams, we should, on this theory of ice- 

 action, expect practical ice-stagnation at the majority of points at 

 distances from the centers of the main drainage lines. Thus ice- 

 masses swarming over the canyon rims, and flooding the stream- 

 dismantled plateau, would find their counterparts, in stream-action, 

 in the "back-water"- of a flooded river. It might be, of course, that, 

 in the case of ice, some old, deserted plateau channel might be found 

 along which corrasion could be effected ; or it might be that the mass 

 would start corrading some steep sea declivity and form "hanging" 

 valleys, the return of warm conditions checking ice-corrasion at this 

 stage. 



After the ice- flood came recession of the glaciers. As with ordinary 

 streams, aggradation and smoothing (along declivities) now became 

 the work of the ice-drought glaciers. 



Along the rock basins, undercut walls, etc., which marked the 

 work accomplished by the ice-flood near and below baselevel, the 

 rapidly diminishing glaciers would be mainly employed in obliterating 

 the traces of their former handiwork. 



1. They would partly or wholly efface and smooth over their 

 original deep rock groovings, formed during vigorous ice-thrusts. 



2. Smoothing of rocks would later practically cease and aggrada- 

 tion commence. 



3. Ice-stagnation or overriding of gravels would finally ensue. 



