AN INTERGLACIAL VALLEY IN ILLINOIS I 59 



Subsequent to the formation of the overwash plain in the 

 interglacial valley and the associated retreat of the ice, the habit 

 of the stream has been changed, because its load has been greatly 

 reduced. Probably its volume has also been somewhat cut down, 

 but not to correspond with the reduction in burden ; hence, whereas 

 the stream was aggrading its course during the Wisconsin stage, it 

 has been degrading during the postglacial stage, and has so far 

 succeeded in removing the previously deposited material that the 

 stream now flows in a gorge, a half mile or more in width. The 

 stream is still widening this valley, and the second bottoms (Fig. 

 2, G) still remain to be removed. 



The second bottoms constitute terraces of better agricultural 

 soil than that of the vicinal upland. They correspond in orign 

 with those described by early writers in many New England 

 valleys, and especially those in the Connecticut and tribu- 

 taries so thoroughly studied recently by Professor W. M. Davis. 

 They may be correlated with man}^ terraces in Illinois valleys, 

 because nearly all streams heading on the new drift and flowing 

 out through the moraine are terrace-bordered. The conditions 

 differ to some extent from those reported by Professor Davis in 

 that the postglacial excavation has not proceeded sufificiently as 

 yet to encounter the walls and floors of the old valley. There- 

 fore there are no defending ledges, no series of terrace steps, 

 and very few cusps. The stream has cut back as far, or a little 

 farther, each time than on the previous swing, and hence there 

 is one long step from the second bottoms or terrace-top to the 

 present flood-plain. It is unfortunate that more attention has 

 not been given to the terraces of the Illinois, Iowa, and Wiscon- 

 sin streams. There are many such terraces, and no doubt a 

 careful study of them would yield valuable returns. 



Summary. — The history of the present Embarras River valley 

 is a complex one. North of the moraine it is entirely postglacial, 

 and probably bears little relation to its precursor of interglacial 

 time, still less to the preglacial valley. Very likely near the 

 moraine and in it where the valley is deepest the stream has cu 

 its recent channel mostly in the filling of a former valley, but the 

 occurrence of occasional rock-ledges witnesses to the errors of 



