561 Emery — Green River Desert Section, Utah. 



La Plata Group (Jurassic). 



The La Plata group, which consists of three formations, 

 includes all the beds between the Chinle and McElmo and 

 is probably limited both above and below by unconform- 

 ities. The lowest formation of the group is the Wingate 

 sandstone, about 900 feet thick and composed of massive 

 cross-bedded, quartz-sandstone beds. Above it lie 100-300 

 feet of shale and gypsum, with a thin marine limestone 

 containing fossil invertebrates of Jurassic age near the 

 base, which are believed to represent the Todilto forma- 

 tion of the Navajo Reservation. Massive cross-bedded red 

 sandstone and thinner bedded red sandstone and sandy 

 shale aggregating about 700 feet in thickness constitute 

 the Navajo sandstone, which lies above the Todilto (!) 

 and is the highest formation of the La Plata group. The 

 group as developed in Colorado, Arizona, and New Mex- 

 ico has been referred to the Jurassic on account of strati- 

 graphic position, and that reference is now confirmed by 

 the distinctive marine Jurassic invertebrates in the 

 Todilto (?) formation. 



Wingate Sandstone (Jurassic). 



The Wingate sandstone, the lowest formation in the 

 La Plata group, embraces all the massive cross-bedded 

 sandstone between the Chinle and the overlying To- 

 dilto (?) formation. It is widely exposed in Green 

 River Desert, forming the canyon walls of Green River 

 from the mouth of San Rafael River to Horseshoe Canyon 

 and capping The Ledge from there to the Dirty Devil. 

 The wonderfully impressive crags of San Rafael Reef 

 are formed from the Wingate sandstone. 



The Wingate is a remarkably uniform series of 

 extremely massive, very cross-bedded, medium to fine- 

 grained sandstones, averaging in this region about 900 

 feet in thickness. The name Orange Cliffs, which 

 appears on the Land Office base (1915) along what is 

 locally and more expressly known as The Ledge, is a 

 descriptive term derived from the color of the lower part 

 of the Wingate which forms these cliffs. The upper part 

 of the formation near The Ledge is white. A similar 

 color scheme is seen in the Wingate in San Rafael Reef 

 in the vicinity of San Rafael River, but at other places 



