458 University of California Publications in Geology [Vol. 10 



sioual forces acting at right angles to this general trend. The small 

 folds mapped may be due to minor forces or reactions which acted 

 in a nearly north-south direction, producing minor flexures along the 

 axis of some of these larger folds. This might account for the ar- 

 rangement of the axes en echelon and for the steeper dips on the west 

 limb of the more northerly anticline and on the eastern limb of the 

 more southerly anticline. 



GEOLOGIC HISTOEY 



The variegated shales and cross bedded sandstones of the Cloverly 

 formation represent shallow water conditions, and the plant remains 

 found in it in other districts indicate that it is of fresh water origin. 

 The Dakota is a shallow water deposit and represents the encroach- 

 ment of the Upper Cretaceous sea. The Thermopolis shales and thin 

 sandstones represent progressive subsidence and deposition in only 

 moderately deep water. The hard siliceous Mowry shale with its 

 numerous fish scales indicates a rather sudden influx of marine verte- 

 brates and the precipitation of silica. Sufficient data are not at hand 

 to determine what climatic factors brought about this change. The 

 sandstones and sandy shales of the Frontier formation indicate a 

 return to rather shallow marine conditions ; the formation marks the 

 most important break in shale deposition between the top of the 

 Dakota and the base of the Mesaverde, and may represent a period 

 in which subsidence did not keep pace with deposition. The dark 

 shales, sandy shales and the lenticular sandstones of the Cody for- 

 mation represent moderately deep water in which subsidence and 

 deposition were almost balanced. Following the deposition of the 

 Cody shale there was a gradual withdrawal of the sea and the sand- 

 stones, sandy shales, carbonaceous shales, and coal deposits of the 

 Mesaverde were laid down in brackish water along a low coast. This 

 was followed by the deposition of fresh water beds which are not 

 represented in this area. At the close of the Cretaceous extensive 

 uplift and folding took place and the present structural features were 

 formed. A period of erosion followed and the folds were truncated. 



Tertiary Period. — In the early Tertiary the products of the erosion 

 of the high mountains to the west were deposited along their flanks 

 and well out into the Bighorn Basin, forming the Wasatch formation, 

 which is a continental deposit laid down on a surface of moderate 

 relief. These beds rest with marked unconformity on the older for- 



