THE GEOLOGY OF NORTH DAKOTA 23 



considerably over one-half the state, including the eastern and 

 northwestern portions, and its western border is marked by the 

 wide, massive Altamont moraine, which in places has a width of 

 20 miles and over. Outside of the Altamont moraine there is in 

 northwestern North Dakota and perhaps elsewhere in the state a 

 drift which is httle older than that within the moraine, and is itself 

 marked by occasional morainic patches. This young drift, which 

 in Williams County extends 20 miles or more south of the Alta- 

 mont moraine, is probably Early Wisconsin. Beyond the Early 

 Wisconsin till and appearing at the surface for the most part west 

 and south of the Missouri River, is a distinctly older drift whose age 

 is not yet definitely determined, but which may be Kansan. This 

 pre-Wisconsin drift has been described elsewhere' and need not 

 be discussed more fully here. It forms well-developed moraines in 

 northeastern Morton and northern McKenzie counties. 



In marked contrast to the older drift the Wisconsin has suffered 

 very little erosion, its surface is poorly drained, is characterized by 

 almost countless lakes, ponds, swamps, and marshes, and by many 

 rough, hilly, morainic belts. Twelve of these moraines, including 

 the remarkably wide and massive Altamont moraine, have been 

 mapped in eastern North Dakota^ and adjoining regions. 



The drift varies in thickness from a few feet to 300 and even 400 

 feet, though the latter thickness is probably exceptional. In the 

 Red River Valley the till is commonly from 200 to 300 feet thick, 

 and in other parts of the state covered by Wisconsin drift the thick- 

 ness appears to be somewhat less, ranging usually from 150 to 250 

 feet. The pre-Wisconsin drift is much thinner, the maximum 

 thickness so far noted being 100 feet. West of the Missouri River 

 it is seldom as much as 8 or 10 feet and generally not over 2 or 3 feet 

 or less. 



Lake Agassiz silt. — In the Red River Valley the glacial drift 

 is in most places covered by a fine silt which was deposited in Lake 

 Agassiz toward the close of the Pleistocene. Associated with this 



^ A. G. Leonard, "The Pre-Wisconsin Drift of North Dakota," Jour. Geol., XXIV 

 (1916), 521-32. 



^ Warren Upham, " Glacial Lake Agassiz," U.S. Geol. Survey, Mono. No. 25, 

 Plate XVII. 



