THE RED BEDS OF COLORADO 195 



Less significant, perhaps, are the red color, the presence of cal- 

 careous as well as hematitic and siliceous cement, and (except as 

 above noted) the absence of fossils. 



The cross-bedding is scarcely of a type recognized by the text- 

 books. The scale is medium, the cross-bedded horizons seldom 

 occupying more than 2 feet vertically. In general, the dips are 

 at low angles, though by no means always so, and run in all direc- 

 tions, both parallel to the dip of the true bedding, at right angles 

 to it, and obliquely. The cross-bedded layers are truncated both 

 by horizontal beds, as in "torrential" cross-bedding, and by other 

 and highly irregular cross-beddings. 



The thickness of the formation also has a bearing on its inter- 

 pretation. At the Cache la Poudre, the thickness is 600-725 feet. 

 Thence southward, the thickness is 900-1,075 feet west of Fort 

 Collins; 1,000 feet at Lyons; 800-1,500 feet near Boulder; 500 

 feet at Morrison; 4,500 feet, at the maximum (the average being 

 2,000 feet), at Colorado Springs; 1,000 feet at Canyon City; 250 

 feet a mile south of the Arkansas River; 200 feet at a point 6 miles 

 northwest of Badito. If Fountain beds are present in the Purga- 

 toire Valley southwest of La Junta, recent well borings would indi- 

 cate there a thickness of about 1,000 feet. 



Interpretation of conditions of origin: the marine hypothesis. — As 

 far as the writer knows, discussions of the origin of the Fountain 

 sediments make them marine. The most complete interpretation 

 is that of Fenneman (1905), accepted by Butters and George (1913) 

 and by Henderson (1908) — though the latter (personal communica- 

 tion) is now in accord with the present writer. Fenneman's view, 

 briefly, is this. In late Pennsylvanian time, a north-south shore 

 line, of unstated length, existed 10 to 12 miles west of the present 

 foothills of the Front Range. This land was of granite and of very 

 low relief,^ though, as Fenneman deduced from the absence in the 

 Fountain sediments of "residual soil" material, the land was not a 

 peneplain in the usual sense. On this "featureless" land, the sea 

 encroached, its advance and resultant planation of the shore keep- 



^ N. H. Darton, U.S. Geol. Surv. Prof. Paper 52, p. 16, differs from Fenneman at 

 this point, by emphasizing relief. Relief is certainly conspicuous on the "old plain," 

 as now visible near Red Moimtain, close to the Wyoming line. 



