6 A. G. LEONARD 



The thickness of the sandstone varies widely at different points 

 but is commonly 200 to 300 feet. In the asylum well at Jamestown it 

 is 200 feet thick, and at Valley City 300 feet of Dakota sandstone 

 were penetrated, while at Aberdeen, South Dakota, the same thick- 

 ness was encountered. In the eastern part of the state the Dakota 

 sandstone has a westward dip of about 8 feet per mile. 



After the deposition of the Dakota sandstone, marine conditions 

 were brought about through the submergence of a large part of 

 North America. The Gulf of Mexico invaded the continent, and 

 finally stretched north to the Arctic Ocean. All of North Dakota 

 was covered by the waters of this great inland sea, which extended 

 east as far as central Minnesota. In it were deposited the muds, 

 clays, calcareous shales, and sands which form the Benton, Niobrara, 

 Pierre, and Fox Hills formations. 



Benton shale. — The oldest formation which outcrops anywhere 

 in North Dakota is the Benton shale (Fig. 2). It is exposed only in 

 the deep valley of the Pembina River, in the northeastern corner 

 of the state, where the shale outcrops at intervals for a distance of 

 6 or 7 miles, and rises 150 feet above the river. It is also found in 

 many of the deep wells in different parts of the state. The Benton 

 is a dark gray, almost black, fissile shale, containing a considerable 

 amount of carbonaceous material. Small ferruginous concretions 

 are abundant, and pyrite and gj^sum are found in small quantity. 

 The shale weathers to a very plastic clay and has been used in the 

 manufacture of brick. One very marked difference between the 

 Benton and the overlying Niobrara is its low lime content, from 

 I to 2 per cent, while the Niobrara contains from 30 to 80 per 

 cent of lime carbonate and generally carries over 50 per cent. 



The thickness of the Benton shale in North Dakota is not defi- 

 nitely known, since in wells it is seldom distinguished from the 

 overlying Cretaceous shales. In the Morden, Manitoba, well the 

 thickness of the Benton is given as 105 feet, and the well at Delo- 

 raine, just north of the Turtle Mountains in the same province, 

 showed a thickness of 205 feet, but the well did not reach the under- 

 lying Dakota sandstone, so that this does not represent the entire 

 thickness of the formation.^ In North Dakota it probably ranges 



' J. B. Tyrrell, Trans. Royal Soc. Canada, VIII, sec. 4 (i 891), 93, 98. 



