GLACIAL EROSION IN ALASKA 117 



do scour their beds at all, as every one admits, and that there is plenty 

 of time available, as is well known to be the case. 



Accepting ice erosion as a doctrine now established, as it seems 

 to me we must, we will briefly examine some of the consequences 

 of such erosion. Hanging valleys, U-shaped valleys, aligned spurs, 

 and steepened valley slopes are among the more prominent of these con- 

 sequences. From their existence we must of necessity infer enormous 

 vertical as well as lateral erosion, such erosion occurring in places 

 where actively moving streams of ice were concentrated in valleys along 

 relatively narrow lines. Along the Inside Passage, and in Yakutat 

 Bay, the two sections immediately under consideration in this paper, 

 the amount of erosion which must be deduced from the evidence is in 

 places not less than two thousand feet vertically; and erosion of this 

 magnitude has occurred along hundreds of miles of fiords. 



In discussions of the significance of hanging valleys, it has been 

 rather common to speak as if the main valleys were eroded while 

 the tributaries were left undeepened. This has been done here, as 

 doubtless in other writings, in order not to introduce an unnecessary 

 complexity into the discussion. It would, however, be entirely errone- 

 ous to suppose that the lateral valleys were not eroded also. It re- 

 quires only an examination of the photographs accompanying this 

 paper to see that the normal cross-section of the hanging tributary 

 valleys has the same curve as that of the main valleys; that is, the 

 curve which glacial erosion produces. 



From the statement just made, it follows that the level at which 

 a lateral valley now hangs above the main trough is not to be taken 

 as the full measure of vertical erosion along the main valley. That 

 this is true is indicated by the fact that of several valleys tributary to 

 a main trough, no two usually hang at exactly the same level. There 

 may be, and in many cases are, wide differences in the hanging levels 

 of neighboring valleys (Fig. 10) ; some being perched far up on the 

 mountain side, others so far lowered that the sea water enters and 

 drowns their mouths (compare Figs. 2 and 5), which, however, are 

 still hanging above the bottom of the fiord. Such differences in the 

 hanging level are, in the main, a measure of the difference in amount 

 of erosive work performed by glaciers in the several hanging laterals. 



In general, those valleys occupied by the largest glaciers have been 

 lowered most ; and it may be stated as a law that, other conditions being 

 equal, the height of a hanging valley above the bottom of the main 

 trough varies inversely with the size of the glacier. The ojDeration of 

 this law is, of course, modified by the influence of varying rock texture, 

 slope and other causes which tend to modify the rate of ice erosion. 

 We are not yet in full enough possession of the facts relating to the 

 process of glacial erosion to warrant an attempt at a full statement 

 of the nature and result of the various influences which tend to modify 



