50 THE WHITE RIVER BADLANDS 



It has more recently been shown that the waters were not 

 only not saline, but that they were eminently fitted for the 

 support of aquatic life and in fact in some localities did 

 support such life, both plant and animal in great abundance. 

 It seems that the topography of the plains region dur- 

 ing deposition of the badland materials was nearly level, 

 the slope then as now being very gentle from the Eocky 

 Mountains and the Black Hills. Broad streams found 

 their way slowly across this great tract and developed upon 

 it a net work of changing channels, backwaters, lagoons 

 and shallow lakes interspread here and there with reed- 

 bearing marshes and grass-covered fiats. Climatic changes 

 gradually brought about conditions of aridity, the rivers and 

 other water bodies dwindled and wind-driven materials be- 

 gan to assert their prominence. Thus the clays, sandstones, 

 conglomerates, fullers earth, eolian-sands and even the 

 volcanic dust, wind-borne from far away craters in the 

 Eocky Mountains or the Black Hills, are all accounted for 

 and the life conditions of the time are in reasonable measure 

 made plain. 



GEOLOGIC HISTOEY 



The rocks of the earth's crust retain to a marked ex- 

 tent a record of their history. Sometimes this is indicated 

 by composition, sometimes by manner of erosion, some- 

 times by relation to one another, sometimes by fossil con- 

 tents, et cetera. Often several such characters are avail- 

 able in the same formation. In 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 conditions and activities discussed elsewhere 

 may be better understood. 



Preceding the deposition of the Tertiary rocks, that is 

 during 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 or- 

 ganisms 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 con- 

 ditions the Black Hills region was subjected to disturbance, 



