ALASKA GLACIERS 
1 x 8 
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 
