28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



All things considered, it seems probable that in §pite of the great 

 difference in thickness the Vermilion Cliff-White Cliff sandstone is 

 equivalent to Wingate or lower La Plata ; the marine Jurassic to 

 Todilto ; while the upper La Plata has been included in McElmo in 

 some places and was eroded away in other places before McElmo 

 time. 



Much might be said of the rocks in other places, which are here 

 included in the La Plata group. But in this paper a tedious review 

 may be omitted, with the statement that I have consulted every avail- 

 able source of information and embodied the results in the correla- 

 tions shown in the accompanying figures. In brief, the sedimentary 

 rocks of Jurassic age are very thick in the plateau region of Utah 

 and thin eastward. The evidence tending to prove that the sand- 

 stones originated largely as wind-blown deposits is in harmony with 

 the supposition that the material came from the newly formed moun- 

 tains to the west and gradually thinned toward the east. 



SIGNIFICANCE OF LOWER LA PLATA CONTACT 



In southwestern Colorado the La Plata sandstone rests uncon- 

 formably on the Dolores formation which is classed as Triassic and 

 which has a known range in thickness from- something more than 

 1,700 feet to 100 feet or less. Similar relations obtain in north- 

 eastern Utah, but in southern Wyoming (fig. 2) marine Jurassic 

 rocks rest unconformably on Chugwater, which is classed by some 

 geologists as Triassic and by others as Permian. In northern, 

 central and southern Colorado west of the Rocky Mountains, rocks 

 correlated with the La Plata rest unconformably on " Red Beds " 

 and overlap these onto the Archean. Also east of the mountains, 

 sandstones which may be age equivalents of the La Plata overlap 

 Triassic and older rocks down to the Archean. It is obvious there- 

 fore that there is a broad, well-defined plain of unconformity here 

 called the La Plata peneplain separating the La Plata group from 

 older formations. 



It is further obvious that the few feet of Jurassic strata in the 

 Rocky Mountain region cannot represent the entire time required 

 for the accumulation of the deposits of this age in Utah which are 

 more than 3,000 feet thick (fig. 6, p. 22). The thinning of the rocks 

 eastward points to a western source of the sediments, and the fossils 

 of the marine beds above the White Cliff denote late Jurassic time. 

 It is therefore probable that the upland deposits — such as the sand 

 of the Vermilion Cliff and White Cliff sandstones — accumulated in 



