AN INTERGLACIAL VALLEY IN ILLINOIS I 57 



sheet? Third, is it interglacial, formed after the advance of the 

 ice that left the Illinoian and before that which left the Wiscon- 

 sin drift? If it can be shown that the valley is neither preglacial 

 nor postglacial, but is subsequent to the deposition of the Illi- 

 noian drift and prior to that of the Wisconsin, its position will be 

 established. The three possibilities will be taken up in the 

 order mentioned. 



Since this valley, walled by the second bluffs, is cut in the 

 Illinoian drift-sheet, it must have been formed subsequent to 

 the laying down of that till-sheet. The character of the walls 

 also precludes the possibility of its being due to rejuvenation 



Fig. 3. — Looking N. W. from above east bluff at south end of the Wisconsin over- 

 wash in the interglacial valley. R = Embarras River, F = present flood plain, S == 

 "second bottoms" — the terrace top, U = upland surface. Vertical scale very greatly 

 exaggerated. 



just prior to the first advance of the ice, as may be the case with 

 those gorges cut in rock, but more or less filled with drift of a 

 late age, such as have been reported in New York. Again, since 

 the valley is so rarely cut down to bed-rock, when the latter is 

 usually very near the surface outside the valley, it is probably in 

 a preglacial, well-matured valley, whose limits are reached only 

 when the present valley encounters ledges. Furthermore, since 

 the valley in the Illinoian drift apparently lies within the pregla- 

 cial valley, the latter must have been nearly, but not completely, 

 filled and obliterated by the drift-sheet of this early ice invasion. 

 Had it been completely filled, how could the stream now be so 

 continuously within it ? It is also true that no large percentage of 

 the drift-filling has been subsequently removed, else the walls of 

 the preglacial rock valley would have been more frequently dis- 

 covered. Likewise, it is reasonable, if not imperative, to suppose 



