96 



IOWA STUDIES IN NATURAL HISTORY 



> 









— c 



lished, if the diastro- 

 piiic warping takes 

 place so slowly that 

 the streams can de- 

 grade the up-warped 

 areas as fast as they 

 are uplifted, and thus 

 hold their courses as 

 conditions change, or 

 (2) by the uniiorm 

 UDlift of a surface 

 which has been re- 

 duced to grade and on 

 which the streams 

 have reached a final 

 stage of adjustment, 

 flowing by the most 

 direct routes to the 

 sea, provided again 

 the streams hold their 

 courses during and 

 after uplift. 



By study of the 

 Driftless Area it be- 

 comes reasonably cer- 

 tain that the tilting 

 and slight warping 

 which the strata have 

 undergone antedated 

 the establishment of 

 the courses of the 

 present streams, for 

 all the structures are 

 bevelled by the Dodge- 

 ville and Lancaster 

 plains. Either the 

 streams of the Drift- 

 less Area are in har- 

 mony Vv^ith conditions 



