44 



The Bad land Formations of the Black Hills Region 



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 calcareous layers. 

 Their thickness is estimated at ioo feet. Near the top is a. layer 

 ot dark-gray volcanic ash, two feet thick."*" 



THE UPPER MIOCENE 



The Nebraska Beds. The Nebraska beds, so named by 

 Scott, are represented in various areas not yet carefully mapped 

 along the Niobrara river, where they immediately overlie the 

 Harrison beds. Farther south they pass beneath the Oglalla 

 formation, which covers so much of western and southwestern 

 Nebraska. They have been studied by Hatcher and by Peter- 

 son, t 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 pro- 

 trude as hard undulating or shelving masses from the underlying 

 and overlying softer materials. Peterson states that the thick- 

 ness 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 interesting fossils of vertebrates, 

 and Osborn states that the fauna is one of the best known, most 

 widely distributed, and most characteristic in all the Tertiary 

 series. 



CONCRETIONS AND SAND-CALCITE CRYSTALS 



A concretion is a spherical, cylindrical, elliptical, or nodu- 

 lar body produced by the tendency of certain mineral constiuents 

 to orderly aggregate about a common center within an embed- 

 ding rock mass. The discovery in the Badlands several years 

 ago of what are known as "sand-calcite crystals" has added 

 greatly to our knowledge of concretionary development and has 

 served well to indicate the local conditions with reference to 

 these abundant and interesting forms. 



Concretions vary greatly in size, shape, composition, man- 

 ner of distribution and method of growth. They are common 

 in the Black Hills region. In some of the Cretaceous and Ter- 



*Matthew, W. D, and Cook, H. J. A. Pliocene Fauna from West- 

 ern Nebraska. Am. Mus. Nat. Hist., Bull., Vol. 2 6, pp. 3 61-414. 



t Hatcher, J. ,B- Origin of the Oligocene and Miocene Deposits 

 of the Great Plains. Am. Phil. Soc. Proc, Vol. 41, 1902, pp. 113-131. 



Peterson, O. A. 'Osteology of Oxidactylus. Carnegie Museum 

 Annals, Vol. 2, 1903-'04, pp. 434-476. 



