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Now what would be the normal history of a stream bed filled in this 
rythmical fashion, when the source of special supply was cut off ? 
Obviously erosion must have begun on the crests of the high deposits. 
The material so eroded must have been carried forward and deposited 
in the lower stretches just as material is now being deposited in the 
bottom of Lake Pepin, whose existence is attributed to the deposits of the 
Chippewa River that enters just below it. Clearly the eroded material 
could not be carried over the next rise until a common degradation gradi- 
ent for the whole section was established. A period, therefore, followed 
the close of the glacial action during which the high deposits were cut 
down and ¢he low deposits built up. In the present case the Beaver 
deposits were presumably cut down and the Brilliant deposits built up. 
How long this period continued it is impossible to estimate accu- 
rately, for it was dependent on several uncertain conditions. (1) From 
the very nature of the hypothesis, it is impossible to determine what 
was the status of filling of the low parts at the close of glacial action. 
Almost any degree of filling or lack of filling may be assumed. (2) 
The retarding effects of the Champlain depression are quite unknown. 
(3) The deposits in the valleys of the glacial tributaries participated in 
the action to an undetermined degree. As soon as the high deposits 
at their mouths began to be cut down, their gradients, already high, 
would be increased and their material would be carried into the main 
stream and would check further degradation until it was disposed of. 
It was even possible, under certain conditions, for these tributaries to 
continue to build up the deposits at their mouths after the cessation of 
glacial action. 
How much the Brilliant terrace was built up during the establish- 
ment of acommon plane of degradation seems to be quite indeter- 
minable. Taking the present Ohio as a basis of reference, and 
supposing the surface of the Brilliant terrace to represent the plane of 
equation (neither of which is beyond question) there were heights of 30 to 
50 feet, in addition to the contributions of the tributaries, to be brought 
down, and the low parts to be built up from unknown depths. The 
shallow depth of the implement, eight feet, seems a very modest figure 
to assign as the possible, or even probable, upbuilding of the Brilliant 
deposits. 
There is still another item in the history. After the establishment 
of the plane had been accomplished, there was a temporary stage of 
equilibrium in the parts built up, followed by the initial stages of 
