GLACIAL EROSION IN ALASKA 119 



of ice erosion, for, as in a river, the current naturally at time impinges 

 upon one side with greater force than on the other, as, for example, 

 when by the entrance of a tributary the ice current is pushed against 

 the opposite side of the -valley. 



A prominent feature in regions of former glaciation, both of con- 

 tinental glaciers and mountain-valley glaciers, is the presence of 

 through valleys, that is, valleys in which there is now no pronounced 

 divide. Such valleys abound in the Finger Lake region of central 

 New York, and they are common also in Alaska, and, as Penck has 

 shown, in the Alps. The evidence points to the conclusion that many 

 of these through valleys owe their characteristics to the passage of ice 

 across divides, and the consequent lowering of the divides by glacial 

 erosion. In some places in Alaska, as in the Yakutat Bay region, the 

 ice is still pouring across such divides; in other cases, owing to the 

 shrunken state of present-day glaciers, the through valleys are now oc- 

 cupied by glaciers which flow both ways from a low, flat divide area 

 across which, at a higher stage of the ice, through glaciers once passed. 

 So far as seen in the Yakutat Bay region, none of the through valleys 

 are entirely free from ice ; but in many cases the glaciers are so shrunken 

 as to expose the valley form, which is distinctly that characteristic of 

 glacial erosion. In central New York, where the work was performed 

 by continental glaciers instead of valley tongues, and where the ice 

 is entirely gone, the character of these through valleys is easily observed. 

 They are often U-shaped, steep-sided, straight-walled, and possess 

 hanging valleys. 



The acceptance of the conclusion that glaciers have been powerful 

 agents of erosion, and doubtless still are where now in active operation, 

 seems a necessary result of a candid consideration of the evidence. 

 Once this conclusion is reached, a number of remarkable phenomena, 

 otherwise not satisfactorily explained, find ready explanation. The 

 belief in glacial erosion carries with it stupendous consequences, for it 

 assigns to glacial action some of the most striking topographical fea- 

 tures of regions formerly occupied by actively moving ice. Nowhere 

 is the evidence clearer, or the results more striking, than along the 

 Inside Passage to Alaska, and in the fiords northwest of this, such as 

 Yakutat Bay. For those who still doubt the effectiveness of ice erosion, 

 a trip through these fiords is strongly recommended instead of a study 

 of the weak termini of small, dwindling Alpine glaciers. 



