122 IOWA ACADEMY OF SCIENCES. 



NEW LIGHT ON THE DRIFT IN SOUTH DAKOTA. 



BY J. E. TODD. 



Hitherto, the writer's study of the drift of South Dakota 

 has led him to consider it to belong mainly, if not entirely, to 

 the Wisconsin epoch. The reasons briefly stated are as fol- 

 lows: 



(a) The numerous borings made in the state for artesian 

 wells, have nowhere revealed distinctly, well defined forest- 

 beds or soils, such as are found in some other regions. Though 

 limited sheets of sand have been found in till at certain points, 

 it has not been clearly proved that they are not such as might 

 have been formed by sub-glacial streams or a slight advance of 

 the ice-sheet during a single period of occupation. A few 

 exposures described herein in the eastern part of the state 

 have thrown some doubt upon this point. 



{b) The drift in northeastern Nebraska, though suggesting 

 previous advance by an ice-sheet, is, nevertheless, from its thin- 

 ness and its relation to the A Itamont moraine, thought to be due, 

 in part, to the marginal waters; with a possible sub-glacial origin 

 for a portion of it resulting from an extreme advance of the ice- 

 sheet, slightly antedating that moraine. Because this conclusion 

 seemed to disagree with those derived from other regions, the 

 writer's results of several years' work in the Missouri valley 

 have been withheld from publication for several years. 



This summer, while revising these results, the following 

 inference presented itself. It is strange that it had not sug- 

 gested itself before. 



I. Inference from the TroiigJt of the Missouri River. — ^Since 

 1884, it has been generally recognized that the relation of the 

 outer moraine and its drainage channels and attendant deposits, 

 to the Missouri river, and the narrowness of the channel of the 

 latter above Yankton, with the reflection of pre -glacial topog- 

 raphy in the ice movements, all indicate that the Missouri river 

 was displaced from the James river valley, and forced to take 



