382 Geology of Artesian Basin in South Dakota. 



of things west. The blue clay is frequently absent and the drift 

 becomes only a thin sheet of bowlders and gravel. At Pierre it is 

 very light, and at that particular point it does not cross the Mis* 

 souri, though above and below it extends a few miles over on the 

 reservation. 



This observation on the real distribution of South Dakota 

 drift will help us to see the position and relations of the under- 

 lying formations. 



It shows us a heavy denudation of Cretaceous shales and 

 chalks over all the Coteau region east of the James. If the 

 "Cement rock" at the top of the Wessington Hills — 2,000 feet 

 above the sea — is Niobrara or Benton, either, its position shows a 

 former wide extension eastward over all the region now where 

 the drift lies directly on Pre-Cambrian rocks of either Algonkian 

 or Laurentian age. It is a fact that the blue clays of this eastern 

 portion are largely made up of disintegrated Cretaceous shales, 

 and that this material below carries most of the fossil Baculites 

 and Belemnites, while the upper morainic material carries quan- 

 tities of diorite and syenite bowlders, with occasional slabs of 

 limestone, many of them massive, and carrying characters of 

 Silurian and Cambrian ( ?) fossils. One lying at the top of the 

 Wessington Hills, 500 feet and more above the plain, will weigh 

 more than 30 tons, and lies directly on a bed of fine ripple-marked 

 sand. How a fact like that would have made the iceberg cham- 

 pions smile 10 or 20 years ago ! We have got now where we can 

 look at *^both sides of the shield," and the berg of silver and 

 the glacier of gold belong to the same scientific armor of truth. 



We come now to the underlying geological section of the 

 Artesian basin. The facts are fairly well made out. We 

 know that the Pre-Cambrian rocks occupy wide areas beneath the 

 heavy drift of the eastern Coteaus. Southward it is always 

 Huronian quartzyte which crops out in force in all the lower 

 Sioux valley, from Dell Rapids to Sioux Falls, westward in the 

 Vermillion valley this same quartzyte appears at the surface. 

 Outcrops are found m McCook and Turner counties, and also on 

 Firesteel creek, near Mitchell. There is probably a heavier west- 

 ward extension of this formation than was supposed. The fact is, 

 all the borings show that both the granite and quartzite have a 

 wider western area than was suspected. It is more than likely 



