NO. 4 PHYSIOGRAPHY OF ROCKY MOUNTAINS — LEE 39 



SUMMARY OF PHYSIOGRAPHIC CONDITIONS IN LATE 

 JURASSIC TIME 



I picture the physiographic conditions of this time something as 

 follows: The Rocky Mountain region had become degraded in 

 late Jurassic time to a peneplain so low that it furnished little sedi- 

 ment. West of the present mountain system the broad valley pre- 

 viously developed had been filled chiefly with sand (lower La Plata) 

 and graded so that its surface was a continuation of the peneplain. 

 This sand brought from the western mountains thinned out toward 

 the east, where it accumulated only locally in the low-lying portions 

 of the Southern Rocky Mountain Province, the beds thinning out in 

 some places on the slopes of the higher portions. 



These deposits now constitute the lower sandstone of the La Plata 

 group — that is, the Vermilion Cliff, White Cliff, Wingate, Exeter, 

 and possibly other sandstones of the eastern foothills region, which 

 have been included by some geologists in the " Red Beds," and by 

 others in the Morrison. These continental deposits seem not to have 

 extended northward far into Wyoming. 



At a slightly later date, if not during the time these continental 

 deposits were accumulating, the marine waters entered the old valley 

 from the north and spread over the lowest portions of the area for- 

 merly occupied by the ancient Rocky Mountains. In Wyoming the 

 sea covered the eroded surface of the older " Red Beds," but in Utah 

 and western Colorado it covered the older non-marine deposits of 

 Jurassic age. Water suitable for the support of marine animals 

 probably extended southward as far as Arizona and spread over some 

 such area as that shown in figure i. Sea water, which by evaporation 

 became too saline for these animals, extended farther south and east 

 over the lower parts of the peneplain and gathered in partly inclosed 

 shallow basins or " pocket seas," where they deposited gypsum in 

 much the same way that saline deposits are being formed now in 

 some parts of Great Salt Lake. Doubtless there were many of these 

 pocket-seas formed by local warping of the surface and in other 

 ways, in which concentration of water did not reach the degree at 

 which gypsum may be deposited. In still others the supply of fresh- 

 water was sufficient to expel the marine. In such basins limestone 

 formed. In some places the presence of fossils proves this to be 

 fresh-water limestone, but in most places no fossils of any kind are 

 found in it. It is a matter of observation that at some localities this 

 limestone occurs at the same general horizon as the gypsum. No 

 gypsum is present in the layers with the marine fossils, although 



