384 Geology of Artesian Basin in South Dakota. 



of very hard limestone all of a lead gray color." Beautiful nests 

 and single crystals of selenite are found in them. Over this 

 formation lies the Niobrara, know^n locally as "Chalk rock." It 

 contains some "marly clays," and some "thick beds of a light, 

 friable sandstone." It varies in thickness, being at Chamberlain 

 probably about 200 feet. 



Above the Niobrara comes the Pierre formation, a heavy de- 

 posit of dark, plastic "unctuous clays," with a few fossils, and 

 those appearing in the upper and lower beds. It has a total 

 thickness, according to Dr. Hayden, of 1000 feet, but probably not 

 represented in any one section by more than 350 feet. In all the 

 central and northern portions of the Artesian basin this deposit is 

 passed through in boring the wells. It is absolutely impervious to 

 water, yet when disintegrated it talces up water and forms a 

 tough, sticky mud that makes a "Gumbo" flat the terror of the 

 prairie-dweller. 



This is the now fairly well determined section of the Artesian 

 basin. 



As one passes up the Missouri river these formations pass 

 under each other with a very slight dip. If one follows this dip 

 through to the head waters of the Missouri river he will find these 

 same formations in outcrop on the slopes of the mountains, with 

 a slight dip eastward and southward along the tributaries of the 

 Missouri watershed. The Dakota sandstone is exposed in thou- 

 sands of places. Observation and calculation have shown that the 

 visible flow of the Missouri river does not represent by any means 

 the volume of water that is gathered by the Missouri watershed. 

 The position and character of the strata at the headwaters ex- 

 plain its disappearance, and the tremendous volume of over 200 

 artesian "gushers" shows that both the subterranean flow, and 

 the surface streams have the same source. It is one of the most 

 marked examples of a great river system attested by homogeneous 

 geological conditions that the world furnishes. The inference is 

 that the water beneath will flow as long as the water above, the 

 artesian wells will go dry when the river does, and that will be 

 when snow ceases to melt on the Rocky Mountains. 



Now as to the wells themselves : 



First, depth. — As I have said the local topography has 

 nothing to do with wells except to modify the depth. The 

 Dakota sandstone lies from 350 to 600 feet above sea level. Ob- 



