Page:Popular Science Monthly Volume 68.djvu/395

Rh valleys of the Alps, Rockies and Sierra Nevada; and associated with it is a condition of gorges and falls below the lips of the hanging valleys. It is a notable fact that these regions, like that of the Finger Lakes, are regions formerly occupied by powerful glaciers; and it is even more noteworthy that hanging valleys and associated steepened, straightened main valley walls are practically absent in regions not formerly occupied by glaciers. So general is this association; so like the work of ice erosion, as we conceive it, is the anomalous valley form; and so unlike is it to the valleys of stream erosion origin, that physiographers are now quite generally agreed that the hanging valley and the broadened and straightened main valley are the result of glacial erosion.

There are still some who question this conclusion, but none of them has offered a satisfactory substitute or advanced a vital objection to the ice erosion explanation. It is a relatively new point in interpretation of land sculpturing and naturally is not universally accepted at the start. The same was true when the origin of ordinary valleys by stream erosion was proposed in place of the current explanation of catastrophes. In fact, some of the most ardent supporters of the ice-erosion theory are recent converts to it.

When the hanging valleys of the Finger Lake region were first recognized, and ice erosion proposed in explanation of them and of the main lake valleys, there were few who accepted the conclusions advanced; but now the great majority of American physiographers accept the ice erosion explanation for this region, as well as for others. The literature of glacial erosion is now extensive, and the fact of profound ice erosion in valleys freely followed by glaciers seems established; but it would be aside from the purpose of this paper to state the full argument for glacial erosion, Which, in fact, others have already done. Suffice it to say that glacial erosion will explain the conditions in the Finger Lake valleys, and no other theory so far proposed will do so. Moreover, these valleys were a highway for glacial motion, as is proved by the presence of pronounced moraines along their sides and at their heads.

When the glacial erosion theory was first applied to these valleys it was supposed that the erosion was simple and of a single period; but the discovery of other facts led first to a question whether some other explanation than ice erosion might not be necessary, and later to the