MATURITY IN ALPINE GLACIAL EROSION 577 



be regarded as a zone of rigidity merely, constant only as to position, 

 and thickening, from the mobile ice below, as it is thinned by ablation 

 above. Rates of glacial motion, measured along the surface, there- 

 fore will be deceptive. On these assumptions, the line of most 

 rapid advance in the glacier mass is from near the bed, at the rear, 

 to the surface, near the front. Along the bed, motion slows forward; 

 and as pressure upon the bed diminishes in that direction, presum- 

 ably abrasive erosion is most vigorous toward the rear. The accepted 

 view as to the flow-curve of the river is that, normally, it advances 

 most rapidly at the surface. Deep rivers, however, are found to 

 advance from a point measurably below the surface. If rivers had the 

 great depth of glacial streams, possibly it would appear that the curve 

 of flow which they actually have is but the reverse curve due. to bed 

 friction, extended to the surface because the surface is near. It 

 would seem to be a safe assertion that descending grade of bed is 

 not essential to river motion, only decline of the river surface toward 

 the level of discharge; and that, in a long canyon with level floor, 

 terminating at the sea, a river, one or two thousand feet in depth 

 and maintained at that depth at its head, would advance with 

 essentially the same flow-curve as that, here attributed to the glacier. 

 The value of such speculation consists in the indication it affords 

 that appeal to the observed flow-curve of the river, in rebuttal, may 

 not be valid. 



The long ribbon meadow of the lower canyon course, no less than 

 the ponded amphitheater floor, I think, invites interpretation as the 

 manifestation of a tendency on the part of the glacier to channel 

 excessively up-stream. And in this overdeepening toward the canyon 

 head, I suspect, the two agencies of horizontal sapping and of vertical 

 corrosion powerfully co-operate. 



The ultimate effect, upon a range of high-altitude glaciation, 

 would be rude truncation. The crest would be channeled away, 

 down to what might be termed the base-level of glacial generation. 

 Where, among the determining causes of glaciation, high latitude 

 rather than high altitude is operative, the base-level of degradation 

 may lie below the sea, deepest centrally and shallowing outward. 

 Given a land area initially, the glacier itself, as degradation 

 approached its maximum, would replace the land, affording the 



