THE PRE-WISCONSIN DRIFT OF NORTH DAKOTA 529 



CHARACTERISTICS OF THE PRE-WISCONSIN DRIFT 



The topography of the older drift and the large amount of ero- 

 sion it has suffered compared with the Wisconsin till have been 

 mentioned on a previous page, where it was shown that in many 

 places only the coarser materials of the drift — the pebbles and bowl- 

 ders — remain as evidence that the ice sheet once covered the region. 

 But the color of the pre-Wisconsin till is generally unlike that of 

 the Wisconsin drift. The latter is commonly light yellow to light 

 gray in color as exposed in railroad cuts or along stream valleys, 

 but where the deeper till appears in fresh excavations it is seen to 

 have the blue color of 

 the unoxidized clay. 

 The pre-Wisconsin till, 

 on the other hand, 

 where best exposed 

 on Sand and Clear 

 creeks in northeastern 

 McKenzie County 

 (Fig. 8), is dark gray 

 in color throughout the 

 maximum observed 

 thickness of over 100 

 feet. 



Except in the morainic areas the thickness of the pre-Wisconsin 

 drift is not great. West of the Missouri River it is seldom as much 

 as 8 or 10 feet and generally the thickness is not over 2 or 3 feet or 

 less. The thinness of the older drift is due partly to erosion which 

 has removed much of the glacial material and over a large part 

 of the area left only bowlders or a thin veneer of gravel, but partly 

 perhaps also to the fact that the drift may never have been very 

 thick in this region. 



Fig. 8. — The valley of Clear Creek where it 

 cuts through the moraine of the pre-Wisconsin 

 drift, northeastern McKenzie County. 



BOUNDARY OF THE PRE-WISCONSIN DRIFT 



Chamberlin and Salisbury many years ago noted the presence 

 of an older drift beyond the Altamont Moraine and its approximate 

 boundary was shown on their map. 1 The more detailed work of 



1 "Terminal Moraine of the Second Glacial Epoch," Third Ann. Report U.S. 

 Geol. Surv., pp. 291-402; also Plate XXXV. 



