Il8 ALASKA GLACIERS 



but a few general propositions may be advanced. In the 

 first case a study of the district should discover independ- 

 ent evidence of the maturity of the preglacial topography. 

 In the second and third there should be independent evi- 

 dence of rejuvenation of preglacial streams. The third 

 could not arise unless the main troughs follow the strike. 



The practical problem is further complicated by the fact 

 that the type of initial topography, when determined, leads 

 to only a limiting value for the total erosion, and also by 

 the complexity of glacial history as dependent on climatic 

 variation. A glacier which begins erosive work by 

 broadening and rounding the cross-profile of a stream 

 gorge does not cease activity when that result is attained. 

 One which falls heir to a weak-rock strike valley, fairly 

 adjusted to its conditions of flow, may carry the work of 

 excavation far beyond the grade limit of the ancestral 

 stream, and hollow out a lake basin or fiord trough. And 

 the coordinated system of grades and channel forms toward 

 which the erosive work of grouped glaciers tends, is itself 

 modified by every change of the general volume of ice. 



But despite all qualifications the hanging valley is the 

 most important witness yet discovered to the magnitude 

 of the work accomplished by the alpine glaciers of the 

 Pleistocene. 



The hanging valleys of Alaska are illustrated by many of 

 the figures and plates of this volume. The mouth of one 

 overlooking Hidden Glacier is imperfectly shown in plate 

 v and figure 28, and an alpine valley truncated below by 

 erosive action of the glacier appears in plate vi. Figure 

 30 shows the mouth of a hanging valley above Nunatak 

 Fiord; a glacier issuing from a hanging valley north of 

 Nunatak Glacier is shown in figure 31 ; and a glacier cas- 

 cading from a hanging valley of the south side of the 

 same trough in figure 32. Figure 43 shows a tributary to 

 Yale Glacier issuing from a hanging valley, and figures 44 



