NO. 4 PHYSIOGRAPHY OF ROCKY MOUNTAINS — LEE 19 



Ouray, the limestone is described as 6 to 16 feet thick and varies in 

 character from black and massive to thin-bedded and shaly. Near 

 Placerville, situated in this same general region, the limestone 

 between the two sandstones of the La Plata is described orally by 

 Frank L. 1 Tess, who has examined it, as consisting of small masses 

 which seem to occupy channels eroded after the lower La I data sand- 

 stone was formed. Still farther to the north, according to members 

 of the Colorado Survey (personal communication), the upper sand- 

 stone of the La Plata group is absent in some places. The forma- 

 tions included in these sections have been identified by Gregory in 

 Arizona, as shown by the lines connecting the Arizona and Rico 

 sections in figure 5, and Cross and Larsen have traced them eastward 

 to the base of the Rocky Mountains. In Piedra Valley in southern 

 Colorado * these observers recognized the two sandstones of the 

 La Plata group, separated by dark-colored thin-bedded bituminous 

 limestone having a maximum thickness of 30 feet. In some places 

 this limestone is distinctly brecciated. The lower La Plata is normal 

 in thickness and character and overlaps older sedimentary beds onto 

 the Archean. The upper sandstone of the La Plata group is absent 

 in some places. 



Still farther to the southeast, on Chama River, N. Mex., the upper 

 La Plata seems to be represented by 75 feet of sandstone, the middle 

 La Plata by a bed of gypsum, and the lower La Plata by the Wingate 

 sandstone. 2 This section may be regarded as characteristic of the 

 western foothills region of southern Colorado and northern New 

 Mexico. Similar beds outcrop in the foothills east of these moun- 

 tains. In the Pueblo section, which has been selected to represent 

 this eastern region, all rocks between the Purgatoire and the underly- 

 ing " Red Beds " were formerly classed as Morrison." However, beds 

 of gypsum in the lower part of these rocks may represent the marine 

 Jurassic to the north and the middle La Plata to the west. If the 

 stratigraphic relations have been correctly interpreted here, the Mor- 

 rison of the Pueblo region overlaps Carboniferous beds (Fountain) 

 onto the Archean. 



In Purgatoire Canyon in southeast Colorado the Morrison is pres- 

 ent and in some places, but not in all, there are thick beds of gypsum 

 between it and the typical " Red Beds." I described this gypsum 



1 Cross, Whitman, and Larsen, E. S., Washington Acad. Sci. Jour. vol. 4, 

 P- 237, K>T4. 



2 Darton, N. H., Unpublished manuscript. 



'Gilbert, G. K.. U. S. Geol. Survey Geol. Atlas, Pueblo folio (No. 36), 1897. 



