434 IOWA ACADEMY OF SCIENCE Vol. XXVI, 1919 



long puzzling pheneomena presented by arid regions throughout 

 the world. With the exportation and disposal of the dusts of the 

 desert comes their disposition elsewhere. It is in connection with 

 the last mentioned phase that Iowa plays such an important role. 



In the recent consideration of subaerial formations so many 

 novelties enter that in many an old and familiar field a new interest 

 is aroused. Prominent among such tracts is the country lying be- 

 tween the Rocky mountains and the Mississippi river. Both for 

 the origin of the plains surface itself, and the unconsolidated de- 

 posits which immediately underlie it, no very satisfactory explana- 

 tion is found except recently. 



On a grand scale the Great Plains seem to introduce to us a mode 

 of terranal genesis that has long passed unrecognized. Continental 

 deposits thus begin to assume in this country an importance never 

 before accorded them. The constant aggradation of the region 

 appears to be due mainly to the potent activity of the winds.''" 



Concerning epirotic, or continental deposits several essential 

 points are to be noted. They are as important as either marine or 

 lacustrine terranes. On the whole American eolic deposits are of 

 vast extent. They are being formed under conditions whereby 

 they may be preserved through the geologic ages as effectually as 

 any of the Cambrian formations have been. In this new ceiitury the 

 theory of eolic planation, transportation and deposition promises to 

 be one of the great and novel thoughts in the domains of geology. 



PRE-GLACIAL CROSS DRAINAGE OF IOWA 



Assuming with Powell that of all physiographic features the 

 rivers are the most permanent one looks about for clues to what 

 might be the drainage expression of ovu* state before the continental 

 ice-sheets covered the land. Present drainage is, we know, entirely 

 a post-Glacial consequence ; so there is manifestly no relation be- 

 tween it and that which existed prior to Glacial times. Except the 

 small area around Dubuque every vestige of the ancient river 

 courses is more or less deeply hidden by the till. In order to get at 

 what prevailed before the coming of the glaciers we have to remove 

 in fancy the great drift mantle. 



Some insight into the character of Iowa's pi'e-Glacial water- 

 ways is obtained by consideration of the present streams which are 

 outside of the drift-mantled area. In Tertiary times the lowest 

 line of the continental interior depression was no doubt occupied 



"Bull. Geol. Soc. America, Vol. XXII, pp. 687-714, 1911. 



