NO. 4 PHYSIOGRAPHY OF ROCKY MOUNTAINS LEE 41 



dance ; or on still older rocks at localities where the marine Jurassic 

 is not present. It seems reasonable to believe that the waters of the 

 Jurassic sea may have been advancing up the old valley during the 

 time that the sediments of the lower La Plata were accumulating; 

 that the gypsum beds represent maximum extent of the sea water; 

 and that the upper La Plata sediments accumulated as the sea was 

 retreating. Hence the marine Jurassic beds, although relatively 

 thin, may be partly equivalent in age to the sandstones of the 

 La Plata group. , 



As gauged by the thickness of the sediments deposited in It, the 

 sea occupied the Rocky mountain region only a short time. How- 

 ever, these deposits probably represent a longer time than would the 

 same thicknesses of material deposited in a sea surrounded by higher 

 lands. The country to the east of it seems to have been so near base- 

 level that it furnished little detritus. This may explain in part the 

 fact that the Sundance is all there is along much of the eastern 

 margin of this sea to represent the thick Jurassic sediments which 

 gathered farther west. 



It was on the graded plain abandoned by the Jurassic sea and the 

 peneplained area surrounding it that the streams of early Cretaceous 

 time spread out the sediments of the Morrison formation. The story 

 of this formation and its significance is well known 1 and that of the 

 relation of the later Cretaceous formations to the Rocky Mountains 

 has already been presented." The papers describing these Cretaceous 

 events, and the present paper, which should have been published 

 first, give the sequence of events, as I picture them, by which the 

 ancient Rockies were peneplained; submerged by the Cretaceous 

 sea ; and buried by its sediments, from which they finally emerged at 

 the close of Cretaceous time. 



1 See Geol. Soc. America Bull., vol. 26, pp. 295-348, 1915. Mock, Charles C, 

 New York Acad. Sci. Annals, vol. 27, pp. 39-191, *9 l 6- 



2 Lee, W. T., U. S. Geol. Survey Prof. Paper 95-C, 1915. 



