EVOLUTION OF UNCONFORMITIES 347 



Montana time. This is one example of the failure of the ^^reat post- 

 Laramie unconformity" to hold its place. 



EVOLUTION OF UNCONFORMITIES 



Enough evidence and examples have been given to emphasize the fact 

 that the Upper Cretaceous in the Interior Province was an epoch of re- 

 peated differential movements which brought now one area, now another, 

 above sealevel. With such a record the question is, When did the Cre- 

 taceous period end ? The retreat of the sea from ai;iy particular local 

 area can not be taken as a criterion because, as we have seen, the time of 

 the final retreat varied considerably from place to place. Even in the 

 Denver basin this final retreat occurred before the close of the Cretaceous. 

 All are agreed in referring the Laramie of the Denver basin to the Cre- 

 taceous because it rests conformably on the marine Cretaceous Fox Hills 

 and represents a transition from marine to non-marine conditions. Its . 

 upper part was formed above sealevel. Xow as soon as an area is ele- 

 vated above the sea and, even before that, as soon as it is brought within 

 reach of strong tidal currents it necessarily becomes more or less subject 

 to erosion. For this reason an erosional unconformity in estuarine or 

 continental deposits may have very little time significance and its im- 

 portance must be tested by paleontologic and other criteria. Likewise a 

 conglomerate in continental deposits has no such essential importance as 

 a basal conglomerate of a marine formation. It may mean only a slight 

 change in the grade of a stream, or even nothing more than a change in 

 climate which has made erosion and transportation more active. 



Full consideration should be given to the physical evidence of the post- 

 Laramie unconformity in the Denver basin and to the sudden change in 

 the lithologic character of the formations which follow it; but in my 

 opinion the length of the erosion interval between the Laramie and the 

 Arapahoe, which immediately succeeds it, has been very greatly exagger- 

 ated on account of a wrong conception of the history and physiographic 

 condition of neighboring areas during later Cretaceous time. It has been 

 supposed that the post-Laramie-pre-Arapahoe interval is measured by 

 the erosion of at least 14,000 feet of sediments because the conglomerates 

 of the Arapahoe contain pebbles derived from formations stratigraphic- 

 ally that far below the top of the Laramie. But this supposition is based 

 on the assumption, which I believe to be unwarranted, that there were 

 no upward movements and no lands subjected to erosion in ad jacent areas 

 from the beginning of marine Cretaceous sedimentation in the region:, to 

 the close of the Laramie. 



Granting that the post-Laramie diastrophic movement was important, 



