464 



N. H. DARTOK PALEOZOIC AKD MESOZOIC OF WYOMING 



Geologic History 



I\ GENERAL 



The rocks of central ^Yyoming are tlie products of jMiddle Cambrian to 

 Recent time, l)nt several important periods are not represented and parts 

 of the geologic histor}' are ver}' obscure. 



To the southeast, wliere the pre-Cambrian rocks are directly overlain 

 by later Carboniferous deposits, the history of the long Cambrian, Ordo- 

 vician, Silurian, and early Carboniferous periods is not apparent. 



EARLY PALEOZOIC CONDITIONS 



Judging by absence of deposits and by overlap relations, a large portion 

 of the Eocky Mountain province was a land surface in earlier Paleozoic 

 times. The lands -were probably large islands, which were to some degree 

 coextensive with the present moi^ntain province, but the peripheral shores 

 are not even approximately determined for any one epoch and the rela- 

 tions of land and sea varied greatly from time to time. Doubtless there 

 were intervals of partial submergence with deposition of materials Avhich 

 have been more or less completely removed in succeeding uplifts. 



CAMBRIAN SEA 



The great interior seas of Cambrian time apparently did not cover all 

 f)f the Eocky Mountain province, and even in the deepest submergence in 

 the Middle Cambrian the present Laramie range and soine adjoining re- 

 gions were islands probably in an extensive archipelago. In central 

 Wyoming, however, there Avere marine conditions for a while, as shown 

 ])\ several hundred feet of ^Middle Cambrian sediments. Much local ma- 

 terial Avas deposited in this epoch, especially in the earlier stages, Init finer 

 grained muds were also laid down. The limestone conglomerates at the 

 top of the Deadwood formation indicate recurrence of shallow water con- 

 ditions, probably the beginning of emergence which lasted through the 

 latter part of Cambrian and the earliest part of Ordovician time. During 

 this epoch the land area extended widely over the northern Eocky Moun- 

 tain province. How much of the Cambrian was eroded is not known, 

 and therefore it is not feasible even to suggest the location of the shore- 

 lines of Middle Cambrian deposition. 



ORDOVICIAN SUBMERGENCE 



During the Trenton epoch much of Wyoming was submerged and a 

 thick bodjf of limestone laid down. The southern limit of deposition 



