158 GEORGE D. HUBBARD 



that when the valley in the Tllinoian drift was first formed it 

 extended to a greater depth than the present stream has cut, 

 because the latter, in the few miles of double gorge, is not yet 

 running on lUinoian drift, not having succeeded in cutting 

 through a subsequent filling of the valley in the lUinoian. 



This leads to a consideration of the nature of the inner, lower 

 bluffs which accompany the present stream some miles beyond 

 the terminal moraine, and are represented by the second bottom 

 terrace front in Fig. 3, 5". They are of gravels and sands and, 

 farther south, clayey, fine sands, and partially sorted and strati- 

 fied drift material corresponding in composition and stage of 

 weathering with that of the earlier Wisconsin, being similar also 

 to deposits on the uplands spread out as an apron in places along 

 the southern margin of the moraine. The second bottoms 

 between the inner and outer bluffs on each side of the stream, 

 where they are not blended into one bluff, are made of the same 

 materials. Wells, on these bottoms, show a section of the same 

 materials to a depth of fifteen to thirty feet. No deeper wells 

 were found in it. Furthermore, the relation of this terrace 

 material, thick near the moraine and feathering out to nothing 

 southward, points to the same conclusion — namely, that the 

 materials of the walls of the inner gorge are layers of overwash 

 material carried out beyond the terminal moraine by the over- 

 loaded stream flowing from the melting ice, and deposited by 

 that stream in order to aggrade its course, and thus increase its 

 carrying power to a strength equal to its remaining load. Since 

 this material is of Wisconsin age and is laid down within a valley 

 already formed, that valley cannot be postglacial. The third 

 and only remaining alternative is that the valley represented by 

 the outer bluffs is one formed between the deposition of the lUi- 

 noian and that of the Wisconsin drift-sheets, and hence may be 

 called an interglacial valley. Its width and general maturity 

 differentiate it from the known postglacial valleys, while its 

 relative lack of greater maturity, even though in unconsolidated 

 materials, declares it not to belong to the preglacial drainage. 

 The same facts speak for an interglacial time period consider- 

 ably longer than the postglacial period has been. 



