NO. 4 PHYSIOGRAPHY OF ROCKY MOUNTAINS — LEE 25 



attention to the minutiae of his problem until he grows scientifically 

 nearsighted and fails to see that some line of evidence other than 

 his own may have an important bearing on his problem. 



The meager information as to physiographic events in the Rocky 

 Mountain region in late Triassic time is widely scattered through 

 geologic literature. A brief summary is probably all that the present 

 discussion calls for. Sedimentation which had been in progress both 

 east and west of the mountains was terminated at the end of the 

 period for some reason not now known and the Triassic rocks sub- 

 jected to erosion. Before sedimentation was renewed in the Jurassic 

 period this erosion appears to have reduced large parts of the region 

 to a nearly level plain. It cut away the mountains, truncated domes 

 and anticlines, and removed such large parts of the older sedimentary 

 rocks that the sediments of the La Plata group were spread out on a 

 floor consisting of all the older rocks of the region from Triassic 

 down to Archean. This plain, completed over a vast area in early 

 Jurassic time, may be called the La Plata peneplain, for on it the sedi- 

 ments of the La Plata group were laid down. 



The peneplaination continued throughout Jurassic and Lower Cre- 

 taceous times in areas not covered by La Plata, the central portions of 

 Colorado being the last of the uplands in the Southern Rocky Moun- 

 tain Province to disappear. It was on the lowest parts of this pene- 

 plain that sediments began again to accumulate in late Jurassic time. 



Farther west, Jurassic sedimentation began earlier, possibly at the 

 beginning of the period, when the sands of the Vermilion Cliff began 

 to accumulate in the old valley. By the time the accumulations had 

 pushed across the valley to the present mountainous region of Colo- 

 rado, the Jurassic period was well advanced, and only thin, isolated 

 representatives of the La Plata sandstone were formed there. 



JURASSIC DEPOSITS 



Years ago the marine sedimentary rocks of late Jurassic age were 

 regarded as the oldest representatives of the Jurassic system in the 

 Rocky Mountain region, the underlying " Red Beds " being referred 

 to the Triassic. But for several years there has been a growing 

 tendency to include in the Jurassic system some of these unfossil- 

 iferous older rocks. In some places in the mountain region these are 

 thin; in other places they are absent. But west of the mountains, 

 rocks which seem to be the age equivalents of these thin beds are very 

 thick and persistent over a large area. For this reason they must be 

 considered in connection with those of the mountains proper. They 



