DAVIS: GLACIATION OF THE SAWATCH RANGE. 7 
must often been much greater than the depth of the bergschrund. 
Scouring and plucking are effective processes, yet it is difficult to 
understand how they can have been so effective as to excavate great 
cirques and transform normal valleys into broad and overdeepened 
troughs, whose beds are far below the mouths of their hanging 
tributaries. The reason for accepting a large measure of glacial 
erosion is therefore not because a full understanding of the methods 
of glacial erosion has been gained, but, in our admittedly incom- 
plete understanding of this process, because the forms that are found 
to be constantly associated with ancient glaciers in mountain ranges 
all over the world are on the one hand essentially beyond production 
by normal processes, and are on the other hand precisely like the 
7 
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Fic. 5.— The glacial trough and moraines of Clear creek; looking west. 
forms that glaciers might be expected to produce if they were active 
eroding agents. The contrasts of the two classes of forms have been 
frequently set fourth in recent years, and need not be again stated 
here; but the systematic association of the large-textured forms of 
apparently glacial origin in the glaciated valleys of the Sawatch 
range deserves to be better known than it is; and all the more 
because the forms of normally eroded valleys can be studied in 
the neighboring non-glaciated or less glaciated mountains, whose 
structure, height and history have in all other respects than glacia- 
tion been essentially the same as those of the strongly glaciated 
masses. 
