THE ORIGIN OF RED BEDS 245 



outcrop in wide areas in various parts of the ranges. The Maroon 

 conglomerates include also fragments of quartzite and limestone 

 from older sediments. The composition of these formations 

 renders it certain that their materials were not carried far from their 

 sources; it is therefore certain that there were highlands, and there 

 may have been mountain ranges of no insignificant relief in various 

 parts of Colorado during Red Beds time — -which here probably 

 was included for the most part within the Pennsylvanian and 

 Permian periods. 



The coarser beds of the Cutler and Dolores formations in south- 

 western Colorado show by their composition that they also were 

 derived very largely from igneous and metamorphic terranes. 1 

 The studies of Cross 2 have shown that the northern part of the 

 San Juan region itself, as well as the neighboring Uncompahgre 

 Plateau, was exposed to erosion between early Cutler and Dolores 

 time — at or near the beginning of the Mesozoic era. This upland 

 may have furnished sediment to a considerable part of the plateau 

 province to the west and southwest. Cross is of the opinion that 

 the absence of the Red Beds on the Uncompahgre Plateau is due 

 to post-Dolores erosion; 3 but the conglomeratic character of the 

 Red Beds in the San Juan Mountains demands that a source for 

 those sediments be found close at hand. In the absence of any 

 conclusive evidence that the Red Beds ever were deposited over 

 the plateau in question, it may be regarded at least provisionally 

 as the probable site of that source. 



The sediments of the Plateau province, on the whole, do not 

 indicate mountainous topography in the vicinity; but their great 

 thickness (maximum more than 5,000 feet, excluding non-clastic 

 beds) calls for the existence of a land area contributing sediments 

 to this region for a long period of time. Such an area may well have 

 existed toward the south and southwest, in Mexico, southwestern 



1 See Whitman Cross and others, in the following folios of the Geol. Atlas of the 

 U.S., U.S. Geol. Survey: Telluride (No. 57), 1899; LaPlata (No. 60), 1899; Silverton 

 (No. 120), 1905; Needle Mountains (No. 131), 1905; Ouray (No. 153), 1907; Engineer 

 Mountain (No. 171), 1910. 



2 Whitman Cross, " Stratigraphic Results of a Reconnaissance in Western Colorado 

 and Eastern Utah," Jour. Geol., XV (1907), 648-49, 654-56. 



3 Ibid., pp. 648-49. 



