AN INTERGLA CIAL VALLEY IN ILLINOIS I 5 5 



The decrease in depth is due, of course, to the absence of the 

 newer drift-sheet and its thickened border, the moraine ; the slight 

 increase in width may doubtless be ascribed to the same cause. 

 The valley continues to widen as the stream increases in size, 

 but does not deepen much in the fifty-mile remainder of its 

 course. Its fall is about that of the general slope of the land. 

 Very rarely does the stream touch bed-rock, and at no point is 

 the valley perceptibly constricted because of encountering the 

 more resistant layers. In low water the stream quietly meanders 

 along to its mouth. At a number of points within the first ten 

 miles south of the moraine opposite valley bluffs are apparently 

 of different heights. This was discovered to be due to the fact 



A 



Fig. 2. , supposed boundary of preglacial valley; ^, interglacial valley filled 



with; B, Wisconsin overwash material; C, present valley — postglacial; D, present 

 stream bed ; E, remnant of terrace on east side ; F, present flood plain ; G, second 

 bottoms ; H, Illinoian drift sheet. Vertical exaggeration about 25. 



that the present gorge-like valley is within an older, more mature 

 valley, whose sides are reached by the inner valley only on the 

 east, and not continuously there. Hence along the west side 

 there are practically two continuous bluffs, the inner one being 

 from a few rods to three-fourths of a mile within the outer one. 

 But on the east side the stream has pushed its inner bluff much 

 nearer the outer one, reaching it at a number of points and 

 blending with it in one continuous descent from the uplands to 

 the present flood-plain. Where this stage has not been reached 

 there are two bluffs on the east side, as on the west, but they are 

 rarely far apart. When the bluffs have coalesced on one side 

 and not on the other, opposite bluffs seem to stand at discordant 

 levels. Fig. 2 illustrates the case. The second or outer bluffs 

 often have a scarp nearly as steep as will stand, say 20-25°, but 

 are usually of gentler slope. 



