CORRASION BY GRAVITY STREAMS. 297 



Tliese lines of glacial erosion were not observed in a 

 valley "broad," such as one finds in the middle or lower 

 portion of Rock Creek Canon. On the contrary, one would 

 look here for aggradation, since the diminished stream 

 could only corrade comparatively slightly, even on the 

 declivities of the " narrows." Moreover evidence of heavy 

 and continuous morainic deposition is actually here abun- 

 dant, the moraines ever closing in on the central channel 

 in terrace form as the glaciers diminished in volume. Such 

 rapid decrease in the measure of lateral corrasive strength 

 for the waning glaciers is admirably shown in Hock Creek 

 Canon. As diminution in volume progressed, quarrying 

 grew ever weaker, and the once powerful action of the 

 glaciers appears to have resolved itself into the scratching, 

 and, finally into the smooth polishing only of rock surfaces. 



Then came the present stage of ice drought. The glacial 

 volumes have shrunken so much that their weight now can 

 barely induce ice flow. The resulting stagnancy is due in 

 great measure to the flat channel bottoms which had been 

 produced by the previous ice floods. Precipitation probably 

 went on as heretofore, but little ice was formed, the stream 

 now being of relatively mobile material. Therefore re-ad- 

 justment of channel grades ensued, and such re-adjustment 

 of grades is still going on. It is this re-adjustment phase 

 which has caused difficulty in grasping the significance of 

 the phenomena of glaciation here, as elsewhere. Never- 

 theless, from mechanical principles it is evident that 

 aggradation during the decadent phases of stream action 

 will be at just those points where, on the hypothesis of 

 valley over deepening by ice, that corrasion of valley floors 

 and sides would have been expected to occur. Thus in 

 valley "broads" and on cutting curves determined by the 

 ice floods, the waning stage of glaciation should be marked 

 by moraine deposition, the "laterals" approaching the 

 centre in terrace form as the glacier shrinks in size. 



