GLACIAL EROSION IN ALASKA 109 



there is marked lateral erosion is generally admitted by all believers 

 in glacial erosion; but that this is the dominant form of glacial erosion 

 would require for its acceptance much better evidence than has been 

 presented. It may fairly be asked, if there is such pronounced lateral 

 erosion, why should there not also be vertical erosion of equal or greater 

 amount? Even if excessive lateral erosion should be granted as a 

 possibility, of which there is no proof, it alone would fail to account for 

 all the conditions observed. It would fail to explain why remnants of 

 valley spurs are left side by side with pronounced hanging valleys; for 

 in such cases the spurs should certainly be rubbed completely away. 

 But, even more fatal than this is the fact that if the grade of the 

 hanging valley is projected out into the main valley, it will, in a vast 

 number of cases, fall far short of meeting the main valley at grade. 

 Consequently, if glacial erosion is admitted at all, the element of ver- 

 tical erosion must be granted as a prominent part of the process. 



A third explanation proposed for the hanging-valley condition 

 is that of capture and diversion of tributary streams. No one would 

 deny that the diversion of a stream by capture might leave it hanging 

 above the valley to which it was originally tributary. But to attempt 

 to apply • such an explanation to the multitude of known cases of 

 hanging valleys would not be so generally accepted. It would require 

 a marvelous development of stream capture in special localities and, 

 strangely enough, almost entirely in regions of former glaciation. 

 Before this hypothesis could be seriously considered as a general ex- 

 planation of hanging valleys, it would be necessary to account for the 

 fact that this process has operated so extensively in glaciated regions, 

 whereas it so rarely operates in unglaciated countries. But even if this 

 explanation were otherwise probable for hanging valleys, it still leaves 

 unexplained the associated phenomena of aligned spurs, steepened lower 

 slopes and general U-shape of the main troughs. 



A fourth hypothesis proposed is that of rejuvenation. By this it 

 is assumed that the main and lateral valleys had an accordance of 

 grade during an earlier cycle of development, but that recent uplift, or 

 other cause, gave to the streams a new power of cutting, making them 

 young again, or rejuvenating them. As a result of this there was 

 rapid cutting, the main streams working much faster than the laterals 

 and leaving them hanging. This explanation is totally inadequate for 

 the Alaskan conditions. It fails to account for the truncated spurs; 

 it gives no explanation of the difference in level at which the laterals 

 are hanging; and, moreover, even if it operated, it could not possibly 

 produce the other results observed. Such rejuvenation would not 

 develop a broad main valley, but a narrow gorge. But, even if we were 

 to admit, which physiographers would not, that such deepening and 

 broadening of the main valley would be possible without corresponding 

 deepening at the mouths of the laterals, it is inconceivable that, during 



