PHYSIOGRAPHY. 33 



5 or 6 miles in breadth. The hills between which the stream flows are not 

 very high above the river bed, the maximum elevation being 175 feet. 

 The few rock outcrops are in all cases found on the tops and flanks of these 

 hills, where they have been exposed by denudation. At one jjoint only 

 has rock been found in situ near the river bed, and that is toward the mouth 

 of the river. The conclusion is natm-al, since the river is 175 feet below 

 these exjjosed rocks and has not reached rock, that it must be flowing 

 through a preexisting depression or valley partly filled by the drift of the 

 Grlacial epoch. 



The partial filling of this valley at the time of the retreat of the ice 

 to the northeast was accompanied by the filling of the depressions in the 

 drift by the water flowing from the front of the melting glacier. After 

 the depressions were filled, the ovei-flowing water naturally followed the 

 general southeastern slope, which exists throughout the area and is shown 

 by the topographical maps and by the flow of the rivers. The immediate 

 course of the water was determined by the former valley, Avhich was not 

 completely obliterated by the drift deposit. Drift barriers across the valley 

 separating the ponded water, or lakes, from one another were cut tln-ough, 

 the material eroded being spread over the bottoms of the lakes below. Thus 

 was formed a chain of lakes, connected usually by narrow streams; the 

 processes by which the channels were cut out and the lakes drained and 

 filled up with the debris were going on at the same time. The result has 

 been to obliterate the lakes to a great extent and to accentuate the char- 

 acter of the stream. 



The final effect of the processes, briefly outlined, would be to destroy 

 the lakes entirel}^ and produce a stream. 



By following on PI. VIII the Deer River from its mouth to its source, 

 we may see the several stages in its development, which are also typical for 

 other streams of the glaciated poi'tions of the world. The river is about 20 

 miles long and has a width near where it enters into the Michigamme of 

 20 to 30 yards. Near its mouth it is a slow-flowing, sluggish stream, which 

 has nearly reached its base-level of erosion, and like many of the older 

 streams of the Coastal Plain region of the United States is gradually filling 

 portions of its channel with the silt and vegetable matter broug-ht down 

 from above. 



A short distance from its mouth it resembles such streams also in the 



MON XXXVI 3 



