60 The Badland Formations of the Black Hills Region 



such cases the history may be unraveled with much fulness. 



A detailed history of the Tertiary of the Black Hills region 

 may not be entered upon here, but a brief review of the general 

 physical changes is desirable in order that the setting of con- 

 ditions and activities discussed on various pages elsewhere in 

 this paper may be better understood. 



Preceding the deposition of the Tertiary rocks, that is dur- 

 ing the Cretaceous period, the Black Hills region had for a long 

 time been surrounded and largely if not wholly covered by a 

 great sea. In this sea countless marine organisms flourished 

 and died. The sea from time to time, and particularly near the 

 close of the period, tended through a brackish to a fresh water 

 nature. Approximately coincident with the full development 

 of fresh water conditions the Black Hills region was subjected 

 to a great disturbance, profound elevation took place and a 

 more active erosion was inaugurated. The waste products of 

 this earliest Tertiary erosion (Eocene) have not been preserved 

 but the trenched Cretaceous-covered surfaces, later filled with 

 Oligocene materials, indicate in a way the passing events. 



The Oligocene streams were in general of moderate decliv- 

 ity. Away from the central part of the uplift swampy con- 

 ditions for a time were prominent, and the streams were evid- 

 ently sluggish and muddy. These slow moving streams by 

 meandering developed vast flood plains across which they 

 shifted their lazy way and deposited and redeposited the debris 

 obtained from the higher lands to the west. Following the 

 Oligocene the main Black Hills uplift was raised some hundreds 

 of feet higher and erosion was correspondingly quickened, but 

 the general history continued much as before. 



The climate for a considerable time in the history of depos- 

 ition of the badland formations seems to have been moist to a 

 marked degree. Later a more arid condition prevailed. During 

 this later time transportation and deposition by wind seems to 

 have become a feature of some importance. Throughout it all 

 animal life was prodigious and varied and the bones of these 

 bygone creatures mingled with the sediments in countless 

 numbers. 



The great disturbance near the beginning of the Tertiary 

 resulting in the pronounced doming of the Black Hills region 

 and the development of the general structure as we now know 

 it was accompanied by profound intrusion in the Northern 

 Hills and in the Rocky Mountains to the west and southwest. 

 Within the Rockies some of this igneous material connecting 



