42 



Indiana University Studies 



by broadening of the valleys by lateral corrosion; but it was 

 caused by the slow reduction of the valley train of the main 

 stream, and it took place almost entirely as the streams cut 

 down their valleys, always retaining a broad graded surface. 

 The terraces according to this explanation have always stood 

 out higher and higher above the local base level, as the down- 

 cutting of the main stream allowed the base level of the minor 

 streams to be let down. These in the future will stand higher 

 than they are at present above the wide valleys, as there is 

 no doubt but that the steep gradient of the west fork of 

 White River must be considerably reduced, tho the great load 

 the stream has to transport will allow its middle and lower 

 portions to be reduced only very slowly. 



The removal of the glacio-fluviatile deposits by the de- 

 velopment in them of ravines and streams since their deposi- 

 tion is mainly occurring in places where the pre-glacial val- 

 lays were obliterated or permanently obstructed. To a minor 

 extent, however, removal in this manner takes place in the 

 silted-up pre-glacial valleys the streams of which still main- 

 tain their original courses, as the many small tributaries in 

 the terrace deposits along some of these valleys testify. The 

 deposits which are undergoing the action of post-glacial 

 streams are mainly in valleys where the pre-glacial drainage 

 lines have been more or less completely destroyed. The lower 

 part of the pre-glacial Cliffy Creek valley and the pre-glacial 

 valley of the "American Bottoms" are illustrations of this 

 type of removal. Here the filling has been due to special 

 rather than general considerations. No two such filled val- 

 leys need be very similar, especially as regards depth of 

 filling and the readjustment of drainage necessary to meet 

 the derangement that took place. In the two valleys that 

 serve as types, it will be noted that the filling was due to a 

 local barrier in the form of an outwash plain. There are 

 many other similar cases. 



The sandy outwash deposits in the completely or partially 

 filled pre-glacial valleys are easilj^ trenched by the streams. 

 The stream heading about one-half mile south of Park has 

 broadly removed the outwash sand and gravel in a pre-glacial 

 tributary of Cliffy Creek, and then turned down the pre- 

 glacial Cliffy Creek valley itself. Many sharp precipitous 

 ravines occur as its tributaries. Most of those which have 



