468 



N. H. DAETON PALEOZOIC AND MESOZOIC OF WYOMING 



cates a uniform condition of sedimentation over an area of many thou- 

 sands of square miles. The shale of the Benton was followed by several 

 hundred feet of sediments, now constituting the Niobrara formation. 

 Tlie material was mostly impure chalk in the central portion of the prov- 

 ince, but this gave place to shale and sandstone farther northwest. This 

 epoch was succeeded by the Pierre, in which a thick mass of shale 

 was deposited under very uniform conditions. The retreat of the late 

 Cretaceous sea began in the later Montana or Fox Hills epoch, when a 

 widespread mantle of sands was laid on the great series of clays. T\"ith 

 farther retreat of the sea extensive land surfaces were exposed, diversified 

 by large bodies of brackish of fresh water, which received the sands, clays, 

 and marsh deposits of the end of the Cretaceous period. Marine condi- 

 tions recurred locally in later Montana time, as shown by recurrence 

 of the characteristic fauna in shales overlying the earlier coal measures. 

 "\Aliether or not the late Cretaceous sediments were deposited over the 

 area now occupied hj the Laramie mountains and the similar iiplifts is 

 not definitely known, but it is possible that they were, as they are up- 

 turned along the sides of the various uplifts. The conditions in central 

 Wyoming during Fort Union, Denver, and Arapahoe times are not 

 known. 



EARLY TERTIARY MOUNTAIN GROWTH 



There was extensive uplift in the Eocky Mountain province in early 

 Tertiary time. This fact is clearly indicated in most of the mountain 

 regions by the occurrence of Eocene and Oligocene deposits lying on 

 eroded surfaces having the general outlines of the present configuration. 

 A very great amount of material was eroded from the higher parts of the 

 uplifts, and while a portion of it is represented by various formations, 

 much has disappeared. Broad areas were baseleveled at this time, and it 

 is believed that the old plain on top of Laramie mountains is of early 

 Tertiary age, and has since been considerably uplifted. There are no 

 traces of the Wasatch and Bridger formations east of the Laramie moun- 

 tains, and they have not been identified with certainty in the Laramie 

 basin. It is probable that there was some deposition in southeastern 

 Wyoming in that portion of early Eocene time, but if so the deposits 

 were removed there prior to the Oligocene time; so that the local condi- 

 tions are not known. 



OLIGOCENE TO PLIOCENE DEPOSITION 



In later Tertiary times, after the outlines of the great mountain ranges 

 had been developed, there was a long period in which streams of moderate 



