IO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. OQ 



exposed to erosion ; and some change in the relation of highlands and 

 lowlands effected, which caused the streams to spread out, over a 

 wide area, the sand, gravel and mud of the Shinarump conglomerate 

 and other rocks of late Triassic age, both west and east of the moun- 

 tains. (4) The period was brought to a close by the rise of a land 

 mass of continental proportion in the Pacific Coast region, which 

 persisted through all of Jurassic and Cretaceous time and furnished 

 the enormous quantities of fragmental rocks which make up these 

 two systems. This rise seems to have affected the Rocky Mountain 

 region but little, for erosion continued there until stopped by the 

 accumulation of younger sediments on the peneplain. 



CLOSE OF TRIASSIC PERIOD 



There is little known from the Southern Rocky Mountain region to 

 indicate the events which closed the Triassic period, for this region 

 was one of erosion during most of Triassic and Jurassic time. For 

 evidence of these events we must look farther west, and here also 

 much of the record has been destroyed by later erosion. However, 

 an examination of the Jurassic formations described later indicates 

 that the vast quantities of material composing them came from the 

 west (see accompanying sections) ; hence it seems certain that the sea 

 which had extended from the Pacific Ocean eastward into Nevada 

 and Utah was blotted out and a land mass of great magnitude formed 

 in its place. Further, the physical characteristics of the sedimentary 

 rocks of the La Plata group indicate accumulation under desert con- 

 ditions. It seems probable that the mountains of this western con- 

 tinent were high enough to precipitate the moisture from the westerly 

 winds, just as the Sierra Nevada does at the present time, and that 

 the streams thus formed washed rock debris into the Jurassic desert 

 where it was reworked by the winds. This elevation of land to the 

 west seems to have formed a broad valley similar to the Mississippi 

 Valley between the ancient Rocky Mountains, now greatly reduced, 

 and the new western highlands. It was in this valley that the desert 

 sands accumulated and were later covered by the Jurassic sea, hence 

 it is with this valley and its filling that we are much concerned in 

 working out the physiographic history of. the Jurassic period. 



JURASSIC PHYSIOGRAPHY 



INTRODUCTORY STATEMENTS 



With the Jurassic we approach the main subject of this paper. The 

 evidence from the sedimentary rocks is still meager, but enough to 

 make some of the history of the period plain. The ancient Rocky 



