GEOLOGIC HISTORY 465 



apparently passed east and west across central Wyoming from the south- 

 ern part of the Bighorn uplift to the south end of the Wind Eiver range. 



SILURIAN-DEVONIAN HIATUS 



From the close of the Trenton to early in Carboniferous time central 

 Wyoming presents no record, the later Ordovician, Silurian, and Devonian 

 being absent. Whether the region was land or sea, or alternated from 

 one to the other condition, is not laiown, but there is no evidence of exten- 

 sive uplift or deposition. Any deposits laid down are gone now in the 

 wide area in which the Carboniferous lies on Middle Cambrian or older 

 rocks. 



CARBONIFEROUS SEA 



Early in the Carboniferous period there was deep marine submergence 

 in a large part of the northern Eocky Mountain province, and a thick de- 

 posit of limestone resulted. It covered Ordovician sediments in northern 

 and north central Wyoming, but overlapped onto Middle Cambrian and 

 pre-Cambrian rocks to the south. The su1)mergence did not reach the 

 area noM^ marked by the Laramie range lantil late in Mississippian time, 

 and then only covered its northern end. The sediments were mostly car- 

 bonate of lime. In later Carboniferous (Pennsylvanian) time, however, 

 tlie waters covered most, if not all, of the province, and many of the sedi- 

 ments were the coarse materials of an advancing shoreline. In these 

 limes the old crystalline rocks of the lands, having been deeply eroded, 

 decomposed, and completely oxidized, furnished a large amount of coarse 

 material and much red clay. When the waters had deepened somewhat 

 limestones were deposited, but there were intervals in which sands and 

 red muds were the principal deposits, especially far to the south and 

 southwest, where these materials predominated because of shallower water 

 and stronger currents in that direction. 



RED GYPSIFEROUS SEDIMENTS 



In the later part of Carboniferous ti7ne, and proljably during the Per- 

 mian also, there was widespread emergence, resulting in shallow basins 

 with very wide mud-flats which occupied a large portion of the Eocky 

 ^Mountain province. In these regions were laid down the last deposits of 

 the Pennsylvanian division and the great mass of red clay and sands con- 

 stituting the Chugwater formation. These beds probably were largely 

 deposited by saline water under arid climatic conditions and accumulated 

 in a thickness of 1,000 feet or more. The waters were shallow much of 



