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 continental 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 occupied 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 features 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.