NO. 4 PHYSIOGRAPHY OF ROCKY MOUNTAINS — LEE 21 



several years ago 1 as a part of the " Red Beds," but on further study 

 I am inclined to believe rather that the gypsum is of the same age 

 as that which occurs in the middle of the La Plata group in many 

 places in southwestern Colorado and northern New Mexico. 



SOUTHERN UTAH TO NORTHEASTERN NEW MEXICO 



The southernmost group of sections here described extends from 

 northern Arizona, where Gregory has correlated the formations with 

 those of the type locality of the La Plata, westward through Utah 

 and eastward through New Mexico. The Shinarump conglomerate 

 is a persistent and easily recognized stratum and forms a convenient 

 datum plane for grouping the sections of Utah and Arizona. The 

 overlying beds of Triassic age (Chinle) were eroded and later cov- 

 ered with the sands of Vermilion Cliff and White Cliff. These sand- 

 stones have been supposed to constitute two separate formations, the 

 older one of Triassic and the younger of Jurassic age. They are 

 separated in some places, but not in all, by shaly beds, but the horizon 

 of the shaly parting seems to vary from place to place. Also the color 

 of the sandstone is variable, the white of the upper sandstone disap- 

 pearing entirely in some places where the brilliant colors of the Ver- 

 milion Cliff extend to the top of the sandstone. Gregory 2 correlates 

 the Todilto of northwest Arizona with the shaly beds which separate 

 the Vermilion Cliff from the White Cliff in the canyon walls along 

 Colorado River. On the other hand, Emery recognizes the marine 

 Jurassic of the Greenriver region, which is above the White Cliff 

 sandstone as probably the Todilto of the Arizona section. But the 

 marine Jurassic of southern Utah is also above the White Cliff, hence 

 the question arises again, Are there two marine Jurassic horizons or 

 is the Todilto of Arizona to be correlated with the marine beds of 

 Utah, as indicated in figure 6, rather than with the shaly beds lower 

 in the sections. 



Less uncertainty exists in the correlation of the Arizona section 

 with those of northern New Mexico and southern Colorado. Accord- 

 ing to Gregory the tripartite division of the La Plata group is even 

 more conspicuous in Arizona than it is in southern Colorado. The 

 Wingate sandstone is traceable eastward to Thoreau in New Mexico, 

 and the Navajo is probably equivalent to the two sandstones, 290 feet 

 thick, of the Thoreau section which underlie the variegated beds. 



1 Lee, W. T., Jour. Geol., vol. 9, pp. 343-352, 1901. 



2 Gregory, H. E., U. S. Geol. Survey Prof. Paper 93, 1917. 



