Geology of Artesian Basin in South Dakota. 381 



bution. They are not uniform, either in thickness or material or 

 in order of deposit. 



The earHer observations of Todd and others on the drift of 

 South Dakota inclined to the conclusion that the Coteau region 

 west of the James river was analogous to that lying east, between 

 the Sioux valley and the James. The observers found the drift 

 deposits very heavy in eastern South Dakota, sometimes as thick 

 as 250 or 300 feet, with the usual order of glacial material: a 

 base of quicksand, overlaid by blue clay, with unpolished frag- 

 ments of rock; above that till with polished bowlders inclined to 

 stratification ; over this assorted or modified drift, sand or gravel, 

 with flow and plunge structure; over this sometimes loess or 

 other sedimentary deposits. It is still a question in glacial study 

 whether the later drift lies above the loess in South Dakota. 

 (Chamberlain, Geology of Wisconsin, volume III, page 395.) 

 This is the typical drift section. It is freely illustrated in the 

 eastern Coteau region, but in the western we have very different 

 conditions. The order of the eastern glacial deposits was carried 

 by analogy to the western Coteau region. There was a similarity 

 in outline and elevation. It was an easy and natural conclusion 

 that the structure of the formation was the same. The fact is 

 there is a marked difference in the drift of the Missouri Coteau 

 and that of eastern portions of the state. In the first place the 

 western deposit is very light as compared with that of the east. 

 Rarely has the drift been found over 100 feet thick west of the 

 James river. On the highest elevations east the drift is heaviest. 

 On the highest elevation west it is lightest. At Helland, Kings- 

 bury county, near Arlington — the top of the Coteau — it is 310 

 feet thick. At Harold, west of Highmore, in the valley of Med- 

 icine creek, the drift shows a thickness of but 125 feet. This is 

 the heaviest typical drift section I have observed west of the 

 James river. Seventy feet of this is blue clay. 



Yet east, and south, and north of Harold are much higher 

 altitudes where the drift is very light. At the very top of the 

 Ree Hills the chalk is simply capped with a light deposit of 

 morainic bowlders, yellow clay and gravel, with no blue clay. 

 The same thinness of this deposit appears at Wessing^on Springs, 

 where the "Cement rock" crops out within a few feet of the sum- 

 mit, with only a few feet of sand and gravel overlying. This 

 point is fully 2,000 feet above the sea. This is the general state 



