39© C. W. TOM LIN SON 



MIDDLE AND UPPER ORDOVICIAN 



The Trenton submergence. — About the end of Black River time 

 the sea readvanced into the central Rocky Mountain region, 

 probably from the west. The area known to have been covered 

 by this inundation in Utah and Wyoming is approximately the 

 same as in the case of the Late Cambrian submergence (see Fig- 9) . 

 Locally, in Wyoming a little sand was deposited in depressions of 

 the surface as the sea-border transgressed the region; but the 

 main body of limestone which represents this Trenton submergence 

 is wholly free from arenaceous and shaly matter. The seas were 

 even more persistently clear than in Upper Cambrian time. 



The masses of calcareous algse which make up much of the 

 Lower Bighorn and Lower Fish Haven dolomites bear witness 

 to the prevalent shallowness of the water. 



The post-Trenton emergence. — Sedimentation was interrupted by 

 emergence between Trenton and Richmond times, but the erosion 

 accomplished during this interval seems to have been slight. If 

 any considerable thickness of strata was removed at this time, no 

 remnants of them have been recognized. It is probable that the 

 land was not uplifted much above the base level of the streams which 

 drained its surface. 



The Richmond submergence.- — With the return of the sea in the 

 Richmond epoch the processes of deposition were resumed under 

 conditions similar to those which obtained during the Trenton 

 epoch. The seas remained clear until the end of the Ordovician 

 record. As in the case of the Upper Cambrian formations, the 

 Bighorn and Fish Haven dolomites present no evidence, aside from 

 the sandstone locally found at the base of the Bighorn, of proximity 

 to shore lines. The original extent of the Bighorn formation was 

 probably much greater than the area in which it now is found. The 

 long period of emergence which followed its deposition afforded 

 ample opportunity for erosion. 



SILURIAN 



Emergence.- — During the early part of the Silurian period 

 (Medina and Clinton epochs) the entire central Rocky Mountain 

 region was above sea. The emergence, like the two or three next 



