I 
DAVIS: GLACIAL EROSION. 309 
glaciers, existing or extinct, can be reached only by a complete 
analysis of the conditions under which they began to work, of the 
energy with which they worked, of the part of a cycle during which 
they worked, and of the complications of climatic change or of 
crustal movements by which their work was modified in this way or 
in that. A partial analysis may suffice for a particular instance ; but 
the explorer will be better equipped for the explanation of all the 
instances that he discovers if he sets out with a well-elaborated con¬ 
ception of the ideal glacial cycle of denudation, and of the compli¬ 
cations it is likelv to suffer. However extensive and definite this 
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conception may be, exploration will probably require its further 
extension and definition; however brief exploration may be, it will 
probably be aided by an orderly examination of all pertinent knowl¬ 
edge previously accumulated. 
As a practical instance of the value of the glacial cycle, we may 
consider the aid given toward the solution of certain problems by 
the careful reconstruction — or at least the conscious attempt at 
reconstruction — of the form of the land surface on which the pleis¬ 
tocene glaciers began their work, and by the legitimate deduction of 
the characteristics of maturity in the cycle of glacial erosion. Beyond 
the mature stage, we may seldom have occasion to go, as there do 
not seem to be actual examples of more advanced glacial work. The 
initial form on which pleistocene glacial action began is in no case 
known to be that implied in the opening paragraphs of the section 
on the Glacial Cycle; namely, a land mass freshly uplifted from 
beneath the sea and not previously carved by the streams of an 
ordinary or normal cycle of erosion. In central France, for example, 
the initial form was an uplifted and submaturely dissected peneplain, 
in which valleys with incised meanders had been habitually devel¬ 
oped. It was there of the greatest assistance to carry into the 
glaciated area a clear picture of its preglacial form, as determined by 
generalizing the adjacent non-glaciated area. At the same time, the 
ideal picture of a maturely developed glacial drainage system, with 
smooth-sided troughs, was seen to represent a much more advanced 
condition than was attained in the rugged valley of the Rhue; and 
thus a tolerably definite idea was gained of the youthful stage of 
glacial development, somewhere between its beginning and its 
maturity, and of the amount of destructive work needed to reach 
this youthful stage. This elementary example illustrates a method 
embodying the cycle of glacial denudation that ought to be applied 
whenever possible. 
