124 IOWA STUDIES IN NATURAL HISTORY 



drift extending eastward beyond the mapped border of 

 Kansan drift on the west side of the Driftless Area in Iowa, 

 almost and in many places quite to the Mississippi river. 

 There is also at least one area of this upland drift in Illi- 

 nois^ Whereas the Kansan drift within the mapped area 

 lies at all levels of the bedrock topography from the tops of 

 the highest hills to the bottoms of the deepest valleys, this 

 drift beyond the Kansan border, with the exception of a 

 tongue of supposedly Kansan drift near McGregor, is found 

 most abundantly on the Lancaster plain, sparingly on the 

 slopes above the Lancaster plain, and still more sparingly 

 on the Dodgeville plain. Of the several hundred isolated 

 remnants of this drift which are now known, not a single 

 patch is in place in the valleys below the Lancaster plain. 

 If this drift were Kansan and deposited after the deep val- 

 leys were cut, it would seem difficult to explain why it would 

 all have been removed from the valleys and valley benches 

 so that the drift now extends farther east on the narrow 

 divides than in the broad, open, terraced valleys. Still more 

 difficult would it be to explain, if the deposition of this drift 

 took place after the valleys were cut, how a glacier thick 

 enough to fill valleys 600 feet and more deep so as to spread 

 over the divides could have advanced, deposited this drift 

 and retreated without so having changed the profiles of the 

 valleys or so having modified the divides or so having 

 marked the rock surfaces as to have left some trace on the 

 surface below the Lancaster plain. When it is recalled that 

 almost wherever known this oldest Pleistocene drift lies on 

 high divides or benches above valleys believed to have been 

 cut after the deposition of the drift, as in New Jersey-. 

 Montana'', and the San Juan mountains of Colorado*, there 

 is nothing new nor radical in the supposition that in the 

 Driftless Area also, it antedated in its deposition the for- 

 mation of the deep valleys. This upland drift has the ap- 

 pearance of great age but perhaps not of greater age than 



1. Trowbridge, A. C. aiul Shaw, E. W., Bull. III. Gcol. Surv.. No. 26, p. 87. 



2. Salisbury, R. D., Ann. Rcpt. State Geologist of 'New Jersey for 1893, pp. 73-123, 

 especially p. 87. 



3. Alden, W. C. and Stebinger, Eugene, Bull. Geol. Soc. Am., Vol. 24, pp. 529-572. 



4. Atwood, W. W. and Mather, K. F., Jour. Geol., Vol. 20, pp. 385-409. 



