SOUTH DAKOTA SCHOOL OF MINES 49 



MANNER OF DEPOSITION 



Geologists who first studied the badland formations of 

 the western plains early formulated the theory that the 

 deposits were collected by streams from the highlands of 

 the Rocky Mountains and the Black Hills and were laid 

 down as sediment in great fresh water lakes. These lakes 

 were thought to have varied in position and extent in the 

 different periods of time during which the several forma- 

 tions were being deposited. They were believed in general 

 to have had their origin in certain structural changes, 

 either a slight depression along the western side or the 

 elevation of some drainage barrier on the east, and to have 

 been obliterated by the development of new drainage chan- 

 nels accompanied possibly by general uplift, and by the 

 progressive aridity of the climate. 



More recently doubts began to be entertained as to the 

 accuracy of this attractive lacustrine theory, more detailed 

 study disclosing many facts at variance with the usual 

 conditions of lake deposition, both with reference to the 

 physical character of the deposits and to the nature, con- 

 dition, and distribution of the fossil remains found in them. 

 There now seems to be abundant evidence for the belief that 

 the deposits were of combined lagoon, fluviatile, floodplain 

 and possibly eolian origin instead of having been laid down 

 over the bottom of great and continuous bodies of standing 

 water as was first supposed. 



The lacustrine theory originated in the earlier accepted 

 idea that all horizontally bedded sedimentary rocks were 

 deposited in bodies of comparatively still water, either 

 marine, brackish, or fresh. It was believed that the fine- 

 grained banded clays were deposited in the quiet deeper 

 waters of the lake, that the sandstones and conglomerates 

 were deposited along the shores and about the mouths of 

 tributary streams, and that the wide distribution of the 

 animals now found as fossils was accomplished by the drift- 

 ing about in the lake of the decaying bodies washed down 

 by the inflowing streams. The fossils obtained by the 

 earlier students of the region showed a general lack of an 

 aquatic fauna. As a result the idea developed that the 

 waters of this great lake although receiving the drifting 

 bodies of land animals were themselves of such a saline or 

 alkaline nature that they were incapable of supporting life. 



