THE RED BEDS OF COLORADO 20i 



Fountain sediments until the orthoclase was decomposed, the 

 muscovite comminuted, and the quartz both comminuted and 

 rounded to a marked degree. The puzzling cross-bedding which 

 now dips eastward at a gentler angle than that of the true bedding 

 he explains more particularly. At certain points of the coast line, 

 he argues, notably at Boulder and Golden, there were elevated 

 east-west arches, separated by bays. Across these bays longshore 

 currents built bars. The cross-bedding represents the originally 

 shoreward dip of this sand-bar.^ 



Interpretation: the writer^s desert hypothesis. — Aside from the 

 fact that the marine hypothesis ignores that other large-scale type 

 of cross-bedding which the writer has described, that which under 

 approximate original conditions would dip eastward at a higher 

 angle than the true bedding, many difficulties confront Fenneman's 

 viewpoint. Why do the Lyons sands contain no coarse material 

 from the assumed arches ? How many bars were there, inasmuch 

 as the cross-bedding is not limited to the Boulder and Golden locali- 

 ties? Are the regular laminae, of which all the beds ultimately 

 consist, consonant with a hypothesis which requires violent waves 

 and currents to comminute and round the quartz grains ? Above 

 all, if the trough continued to subside, how could quartz grains, the 

 largest less than i mm. in diameter, come to lie in such sharp con- 

 tact with coarse arkose ? And why, if the grains were rolled about 

 under water, did they acquire such films of ferric oxide that the 

 whole formation looks pink to red ? Or is the Lyons color entirely 

 secondary ? 



A very different hjpothesis^ — that the climate grew more arid, 

 that the rivers of Fountain time shrank to nothingness, in short, 

 that desert conditions prevailed when the Lyons sediments were 

 laid down — ^may not solve all of the difficulties, but it is here 

 advanced as more plausible than the marine hypothesis. 



There is certainly no evidence of uplift of the western land at 

 the close of Fountain time. It would accordingly have been lowered 

 by erosion, the rivers would have headed farther back, and they 

 would have brought less and finer materials to the forelands. Winds 

 would thus have had an opportunity to whip about the diminished 



^ N. M. Fenneman, op. cit., pp. 58-60. 



