Page:Popular Science Monthly Volume 70.djvu/113

Rh 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 vertical 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 explanation 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