30 J. E. TODD HYDROGRAPHIC HISTORY OF SOUTH DAKOTA 



Traces of supposed wave action are not infrequently found.* The ex- 

 tensive deposit of fine material may have been deposited in lakes, as well 

 as by atmospheric action, and the rarity of freshwater mollusks may be 

 attributed to the muddy character of the waters. 



While, therefore, the complete establishment of the earlier lacustrine 

 view is not claimed, nevertheless by reason of the numerous marks of 

 water action, the mapping has been .done in accordance with that view. 



That the Miocene deposits reached into the James River valley is in- 

 ferred from the general slope and altitude of the deposits west of the 

 Missouri and their identification at corresponding altitudes in the Ree 

 hills and Bijou hills, and less certainly in the Wessington hills. It is 

 possible that the Tertiary may not have extended much north of the 

 Great Cheyenne river, except in the northwestern part of the state, where 

 it is finely developed in the Slim buttes and Short Pine hills. In such 

 a case the streams in the eastern part of the state probably flowed south- 

 west into the Tertiary basin. Traces of deltas and other fluviatile de- 

 posits of the Oligocene epoch should be sought for under the eastern 

 portion of the Loup Fork beds. 



Movements in the Pliocene 



outlining of existing streams 



With the Pliocene we enter on surer ground and recognize the outlin- 

 ing of many of our existing streams. By this time lake Cheyenne, as 

 King called it, had become filled and tilted, so as to drain toward the 

 southeast. This we may attribute to the gradual rise of the Rocky Moun- 

 tain axis. The same movement at an earlier stage would account for 

 the shifting of the lakes toward the east during the Miocene. Streams 

 that had begun in the Black hills as early as the Laramie had, during 

 the Miocene, built their deltas across the lakes in spite of their shifting 

 farther east, so as to form eastward courses for themselves, until they 

 reached the bottom of the geosynclinal fold near the eastern edge of the 

 Tertiary deposit. In this way our state simply shared in the movements 

 common to the whole west side of the Missouri valley. The valley of 

 the James in this state corresponded to the Missouri in Nebraska and 

 Kansas. 



PERSISTENT EASTWARD DRAINAGE 



That the drainage was eastward across the present South branch of 

 the Cheyenne is abundantly attested, first, by numerous erratics from 

 the Black hills capping extensive areas in the valleys of the White river 



* Cf. Bulletin 2, South Dakota Geological Survey, p. 62. 



