350 ILLINOIS STATE ACADEMY OF SCIENCE 



lower than at present, and Indian Creek would enter it 

 at least 95 feet lower than at present. There would be 

 a deep depression in the southeast part of the county 

 under what is known as "Wolf prairie where drilling has 

 shown that it is 176 feet to solid rock. The wide stretch 

 of country between the Wabash and Embarrass would be 

 in places at least 100 feet lower and would no doubt be 

 rougher, and the Sand Ridge and Dubois hills, with their 

 crests as high as at present and their bases as low as the 

 rivers were then flowing, would stand up as very con- 

 spicuous hills. 



The question naturally arises as to the origin of the 

 deposits of mantle rock that have covered the rougher 

 rock topography and have given the region a compara- 

 tively gentle undulating surface. The soil under physi- 

 cal and chemical tests shows that it has not been derived 

 from the rocks upon which it rests. It contains particles 

 entirely foreign to the underlying rocks. Over the 

 upland, resting upon the rock surface, is a pebbly bowl- 

 der clay with some sand and gravel near its basal 

 portion. Its lower part is made of blue clay, but that 

 within 10 or 12 feet of the surface is light yellow clay. 

 The yellow color is due probably to weathering rather 

 than to any original differences in composition from the 

 blue clay. This formation is the glacial till of the Illi- 

 nois Drift Sheet which spreads over southern Illinois as 

 far south as the spur of the Ozarks. 



By examining the logs of a large number of oil wells 

 upon the upland, the variation in the depth of the mantle 

 rock is found to be from to 36 feet. Over the lowland 

 the material of the mantle rock is different from that of 

 the underlying rocks and very different also from the 

 glacial till of the upland. The deposits of this portion 

 are much thicker, showing a variation of from 60 to 176 

 feet. An examination of these deposits of the lowland 

 shows that they are river laid deposits. They consist of 

 sand, gravel, and silt. With such, the lowlands were ele- 

 vated from 50 to 175 feet. The evidence given by wells, 

 gravel pits, and any excavation that goes but a short dis- 

 tance below the surface of the lowland indicates that it 

 is underlain by thick deposits of gravel. Above the 



