GLACIAL EROSION IN ALASKA in 



surface. This valley wall extends completely across the mouth of the 

 hanging valley, forming a rock lip seven hundred feet high (Fig. 7). 

 Climbing to the crest of this lip, one is able to look up the hanging 

 valley to its mountain-walled head (Fig 8). It is found to be a broad, 

 U-shaped valley with a flat floor and moderate grade. 



The ice-born stream which flows along the bottom of this valley 

 has cut only a shallow trench in the rock floor, through which it flows 

 with moderate grade until the lip of the hanging valley is reached, 

 when its grade abruptly increases and it tumbles down the main valley 

 wall, as a succession of waterfalls, in the bottom of a gorge so shallow 

 that the entire series of cascades, from the crest of the lip to its 

 bottom, is plainly visible from the fiord. The stream has begun to 

 lower its grade to harmonize with the main valley; but it has not had 

 time yet to carry the process very far. That there is no possibility of 

 the presence of a drift-filled valley of earlier date is proved by the 

 fact that bed rock outcrops across the entire lip. 



On any assumption of stream rejuvenation, it is utterly incredible 

 that all the time required to deepen the main trough of Nunatak Fiord, 

 and to broaden it into the form of maturity which it possesses (Figs. 

 9 and 14), should have been too short to have permitted the stream in 

 the hanging valley to cut a more profound gorge, on such a steep slope, 

 and to attain a better approximation to that accordance of grades toward 

 which all tributaries tend in their relation to the main streams. Wher- 

 ever one critically examines a hanging valley in its relation to the 

 main trough, the same conclusion is necessitated. 



A fifth explanation that has been proposed is faulting. It is of 

 course admitted that a block fault, by dropping down the bottom of a 

 main valley, would leave the tributary valleys hanging. Although 

 admitted as a possibility for individual cases, the application of such 

 an explanation to Alaskan conditions in general, fails utterly to account 

 for the facts. It would not explain the truncated spurs on both sides, 

 nor the U-shape of the main and lateral valleys. Furthermore, with- 

 out the introduction of complicated secondary faulting, it would not 

 account for the difference in level at which the valleys hang above the 

 main trough to which they are tributary (Fig. 10). Another fact which 

 ordinary block faulting would fail to explain is the frequent presence 

 of a condition of double hanging valleys, — a lateral hanging above the 

 main valley, and a tributary of this lateral hanging above it. 



Such a condition of double hanging valleys may be illustrated by the 

 case of Eussell Valley (Fig. 10) which enters the lower end of Disen- 

 chantment Bay, a part of the Yakutat Bay inlet. This valley has a 

 moderate slope and a remarkably well-developed U-shape (Fig. 11). 

 Where it joins the fiord it has- built a gravel delta, so that there the 

 actual rock bottom is not visible; but about a mile back from the 

 fiord, bed rock occurs in the valley bottom near its center. Extending 



