SOUTH DAKOTA SCHOOL OF MINES 47 



MIDDLE MIOCENE 



The Middle Miocene, so far as I am aware, has not been 

 identified within the area covered by the Black Hills map, 

 except in the southern part, chiefly in Nebraska. Strata of 

 this age have been studied fifteen or twenty miles south- 

 southwest of Agate Springs, and they have there yielded a 

 limited fauna. Matthew and Cook designate them as the 

 Sheep Creek beds, and describe them briefly, as follows: 

 "They consist of soft fine-grained sandy 'clays' of a light 

 buff color, free from pebbles, and containing harder cal- 

 careous layers. Their thickness is estimated at 100 feet. 

 Near the top is a layer of dark-gray volcanic ash, two feet 

 thick." 



UPPER MIOCENE 



The Nebraska Beds. The Nebraska beds, Nebraska 

 formation as designated by Scott, are represented in various 

 areas not yet carefully mapped along the Niobrara river, 

 where, as widely scattered river channel and flood plain 

 deposits, they immediately overlie the Harrison beds. Fur- 

 ther south they pass beneath or blend into the Oglalla for- 

 mation, which covers so much of western and southwestern 

 Nebraska. They have been studied by Hatcher and by 

 Peterson. Hatcher describes them as consisting of a series 

 of buff colored sandstones of varying degrees of hardness 

 and unknown thickness, with occasional layers of siliceous 

 grits, which protrude as hard undulating or shelving masses 

 from the underlying and overlying softer materials. Peter- 

 son states that the thickness cannot be greater than 150 or 

 200 feet, and he gives a section near the Nebraska- Wyoming 

 line showing only 70 feet. The beds have afforded many in- 

 teresting fossils of vertebrates, some of which are described 

 elsewhere in this publication. 



PLIOCENE 



Pliocene strata are found irregularly distributed on the 

 eroded surfaces of Upper Miocene beds bordering Little 

 White river valley and the valley of the Niobrara. They 

 contain important fossils but the beds have not been care- 

 fully mapped. As a consequence local names have been 

 used to designate the beds in the several localities where 

 fossil hunting has been carried on. Among these names 



