THE CRETACEOUS GROUPS. 153 



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chiefly to this cause, as they are not usually covered on this side of the 

 range by unconformable Tertiaries. The exceptions will be noted hereafter. 



On the eastern end of the Uinta Mountains, between the Po Canon 

 district and the Junction Mountain region, the upturned edges of Jura Trias 

 rocks occasionally protrude through the overlying unconformable beds of 

 Brown's Park age, but these' protruding masses are too small to be shown 

 on the map. 



About the Junction Mountain uplift the eroded edges of the Trias are 

 sometimes buried beneath unconformable beds of Brown's Park acre. 



o 



On the south side of the Uinta Mountains the area of outcrop of the 

 Jura Trias groups is very much greater. Near the head of Ashley's Creek 

 the three upper groups of the Jura Trias are buried by unconformable rocks 

 of Tertiary Age. Farther west, beyond the area covered by the map, these 

 unconformable Tertiary rocks ride high up on the groups of Carboniferous 



beds. 



THE CRETACEOUS GROUPS. 



The Henry's Fork Group, which is the lowest Cretaceous formation, 

 has an outcrop parallel and approximately co-extensive with the several 

 groups of Jura Trias; that is, like those groups it was brought up by the 

 great Uinta upheaval and the elevation of the Yampa Plateau. The same 

 is true of the higher Cretaceous groups, but the latter ate also brought into 

 view in the Aspen Mountain upheaval ; and hence, in the discussion of the 

 geographic distribution of these formations it is necessary to refer to a dis- 

 trict of country not heretofore mentioned in connection with the Carbon- 

 iferous or Jura Trias groups. 



I shall call this the Aspen Mountain district. 



HENRY'S FORK GROUP. 



Nothing further need be said of the geographic occurrence of this form- 

 ation. The group is composed of sandstones, indurated arenaceous shales, 

 and conglomerates. These shales ring under the hammer, and are of steel- 

 gray color, and rarely afford footing to vegetation, and can be traced in a 

 bright band everywhere through the outcrop of the formation. The con- 

 glomerates contain many gravels and bowlders of pre-existing schistic rocks. 



