no. 4 



PHYSIOGRAPHY OF ROCKY MOUNTAINS LEE 



II 



Mountain region was still a highland, but was reduced before the 

 close of the Jurassic to a peneplain, and thin deposits of sediment 

 accumulated on it toward the close of the period. But the interpreta- 



Fig. i. — Sketch map of area occupied by sedimentary rocks of Jurassic age. 

 (The numerals 1-33 denote location of sections used in figures 2-6.) 



tion of Jurassic events is based chiefly on a study of the deposits in 

 the old valley. 1 



In order to determine the physiographic conditions under which 

 the stratified rocks were formed, it is necessary to observe their litho- 



1 In order to make the study complete the entire filling of the valley should 

 be considered. But as this paper deals chiefly with the Rocky Mountain 

 region, little is said of the principal deposits of the La Plata group, which 

 are situated in the western part of the old valley. 



