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Indiana University Studies 



taken place on account of the extension of the mouth of the 

 Mississippi River several hundred miles southward from its 

 position in early Pleistocene time. This extension was 

 brought about by the glacial floods, mainly, which in the 

 Pleistocene period descended from the north. 



THE PHYSIOGRAPHIC INFLUENCE OF THE ILLINOIS 

 GLACIAL LOBE 



General Obstruction cf the Streams in Front of the Illinois 

 Glacial Lobe. The third and fourth conditioning factors in 

 southern Indiana physiography, as stated in the introductory 

 part of this paper, are glaciation on two sides of a triangular 

 unglaciated area, and the fact that the main elements of drain- 

 age are almost exclusively to the west and south and that 

 this drainage bears a peculiar relation to the glaciated parts 

 on the two sides of the unglaciated part. These two factors 

 which give form and expression to the area here involved 

 are treated together, tho only the first mentioned one is an 

 active factor or involves active factors. 



The ice of the Illinois Glacial Epoch pushed in from the 

 west and northwest upon the drainage systems leading toward 

 it on the one hand, and on the other it pushed in from the 

 east and north down the drainage systems leading away from 

 it. Thus, there are only areas of inconsiderable size entirely 

 immune from the influences of the Illinois Glacial Lobe in 

 southern Indiana. Any particular area must be considered 

 with respect to its relation to the particular glacial flank con- 

 trolling its modification. The "American Bottoms" region is 

 located on the west, and consequently the great ice and debris 

 barrier invaded from the west and north into the drainage 

 systems leading toward it. The streams of the region are 

 not thru streams ; that is, they are small and head in the 

 unglaciated region only a few miles from where they flow 

 against the position of the ice front. 



It may be noticed that the glacial ice advancing from the 

 west and northwest vigorously over-rode the pre-glacial val- 

 ley of the west fork of White River, having crossed it every- 

 where almost at right angles, and that it advanced several 

 miles beyond, going somewhat farther to the south than to 

 the north. In transgressing somewhat beyond the pre-glacial 

 White River valley, it advanced up the drainage lines coming 



