Malott: The "American Bottoms'' Regioji 



57 



which continually keeps the throat of the sink in a clogged 

 condition. Thus the water in passing over the logs, trash, 

 and other material densely packed with silt at the entrance 

 has no opportunity to cut down the opening to the level of 

 the limestone passageways below. The second suggestion 

 arises from the nature of the top of the limestone as com- 

 pared with its deeper portions. The upper few feet of the 

 Beecli Creek limestone in the "American Bottoms" region, it 

 wdll be recalled, consist of coarse yellow limestone, containing 

 considerable sand and clay. This upper portion seems to dis- 

 solve much more readily because of these properties (Fig. 2). 

 It may be noted that the subterranean drainage which is 

 developed along Cliffy Creek does not begin until the top of 

 the limestone is entirely below the level of the stream. The 

 water on the lower side comes out about the middle of the 

 formation. Everywhere that the yellow upper edges are pres- 

 ent, the solution joints are much larger than in the purer 

 limestone below. So far as Cliffy Creek is concerned, the 

 yellow upper portion is much more favorable for development 

 of subterranean drainage than the lower portion. Certainly 

 if the middle and lower portions were as favorable, there 

 should be considerable subterranean drainage beneath the high 

 narrow ridge in the constriction southeast of the center of 

 section 35, whei'e the water at present strikes directly against 

 the limestone. The yellow upper portion is apparently not 

 present at this place. 



The "American Bottoms" Basin Specially Preserved. The 

 "American Bottoms" basin is one of the very unusual physio- 

 graphic phenomena of southern Indiana, and perhaps has no 

 parallel anywhere in the Mississippi valley. It presents a 

 series of special conditions all of which are specially adjusted. 

 The position of the "American Bottoms" as a filled valley in 

 front of the Illinois Glacial Lobe is not uncommon, but the 

 case becomes special when one recalls that the valley was 

 filled by a particular outwash apron from the ice sheet to 

 the west. Valleys with the same combination of strata are 

 common enough in the Chester Series of southern Indiana, 

 and several of them are filled valleys; but no other has the 

 filling at just the critical height with respect to the Beech 

 Creek limestone, to favor the development of subterranean 

 drainage. The fact that the impounded water which was 



