64 SEDIMENTARY GROUPS OF THE PLATEAU PROVINCE. 



prevailed during the deposition of the lower sandstones abruptly changed, 

 leaving a part of the ancient lake dry and a part submerged, and in this 

 submerged area the limestones and shales were deposited. 



BITTER CREEK GROUP. 



The Bitter Creek beds are chiefly bad-land sandstones. The area of 

 lacustrine deposition had its greatest expansion during this time. The 

 plane of demarkation separating it from the next group in order is one of 

 great importance. When the beds of the underlying group had been laid 

 down, and before these beds were formed, that movement began which, 

 carried on through Cenozoic time, has given us the great Uinta upheaval. 

 There seems to have been a widely spread dry land condition separating 

 them, for on the flanks of the mountains the lower Bitter Creek beds rest 

 unconformably on the next group, and this unconformity is by erosion 

 and also by angle of dip. This is seen in several places. But the move- 

 ment in upheaval in the Uinta Mountains was oscillatory, and we often 

 find the upper members of the Bitter Creek series overlapping the older 

 members, and in extreme cases all of the groups of Mesozoic Age also. The 

 unconformity between the two groups away from the mountains is simply 

 represented by erosion. The hard, gray sandstone, which is the upper 

 member of the Point of Rocks Group, is often seen to have been eroded 

 into gentle or more abrupt valleys, and the shales of the Bitter Creek Group 

 were carried into and filled these valleys. 



These facts are exhibited in very many places on either flank of the 

 Uinta Mountains, and on either flank of the Aspen Mountain fold, or 

 that fold the axis of which is seen in the Salt Wells Basin ; yet there are 

 many points where the conditions of recent erosion are such that the junc- 

 tion of the two groups are more or less masked, and where the uncon- 

 formity is less apparent. 



But this epoch of change has a more important significance. The 

 group below I have classed with the Mesozoic, the group above with the 

 Cenozoic, and the change was from marine to lacustrine conditions. But 

 this change was not abrupt ; brackish water fossils are found in the lower 

 group associated with marine forms, and with these a few species of 



