22 The Badland Formations- of the Black Hills Region 



showing the greatest tropographic complexity. It is continuous 

 with the larger and greatly important area south of White 

 river. This latter extends eastward, beyond the area of the 

 map into Rosebud Indian reservation and southwestward and 

 westward alongside the upper White river forming the high 

 Pine Ridge escarpment which extends through northwestern 

 Nebraska into Wyoming. 



The typical badland topography is of little importance 

 south of Pine Ridge within the area covered by this, paper. 

 However, since the geology and paleontology there is closely 

 related to the geology and paleontology of the adjoining 

 areas in South Dakota and since the southern slope of Pine 

 Ridge marks the approximate southern structural limit of the 

 Black Hills uplift the map is made to include the area southward 

 to and a little beyond the Niobrara river. 



CLASSIFICATION AND CORRELATION 

 The history of the earth since the advent of life on its sur- 

 face is commonly divided into certain time-divisions called eras. 

 Beginning with the oldest, these are the Archeozoic, the Protero- 

 zoic, the Paleozoic, the Mesozoic, and the Cenozoic* Each of 

 these eras is divided into shorter time-divisions known as 

 periods, varying somewhat among authors. For example the 

 Paleozoic may be divided into the Cambrian, Ordovician, Silur- 

 ian, Devonian, Mississippian, Pensylvanian, and Permian 

 periods; the Mesozoic into Triassic, Jurassic and Cretaceous; 

 the Cenozoic into the Tertiary and Quaternary. The periods 

 may in turn be divided into epochs, as for example, the Tertiary 

 into the Eocene, the Oligocene, the Miocene, and the Pliocene 

 epochs; the Quaternary into the Pleistocene or Glacial epoch, 

 and the Recent or Human epoch. The rocks laid down during 

 the various epochs or periods are spoken of as being grouped 

 into formations (not to be confused with the ill-defined expres- 

 sion often used for any natural oddity) the name of each forma- 

 tion being usually derived from some town, stream, tribe of peo- 

 ple, or other feature of local interest where the formation was 

 first carefully studied and described. The- following section in 

 order of deposition, the oldest being at the bottom, shows the 

 various formations for the Black Hills region. 



*I regret the seeming necessity of following conservative class- 

 ification rather than joining present events with probable future con- 

 ditions anidi adding the (beautifully (expressive term "Psychozoic era,'* 

 the Age of Man, introduced by Prof. Joseph LeConte many years ago 

 and used by him in the various editions of his Elements of Geology. 



