THE RED BEDS OF COLORADO 199 



save from the Cache la Poudre River to Colorado Springs, but in 

 this strip they are so homogeneous that a vertical section is unnec- 

 essary. Their contact with the Fountain is sharply defined, since 

 they consist essentially of fine-grained red or pink to white sand- 

 stone, at times cemented to an orthoquartzite — a process still 

 continuing. Under the microscope, the grains, except for a little 

 comminuted feldspar and muscovite, are seen to be quartz, are 

 notably rounded and uniform, and in diameter vary from o.i mm., 

 or less, to i mm. The color is due to films of ferric oxide coating 

 the grains. The thickness of the formation ranges from 850 feet 

 at Colorado Springs to a few hundred feet north of Lyons. Other 

 major characteristics important for interpretation are the type 

 of bedding (including cross-bedding) and the occurrence of ripple 

 marks and (rarely) of amphibian tracks. Of minor significance are 

 beautiful dendritic markings due to manganese oxide and small, 

 rounded cavities, marking spots where limonitic concretions have 

 fallen out. Locally, as at Colorado Springs, a heavy conglomerate, 

 with rounded bowlders 2 feet in diameter, occurs near the middle of 

 the formation. 



The significance of the true bedding is not that it is, to the casual 

 glance, massive, or, when more carefully examined, either thinly 

 platy or slabby, hence good for flagstones, but rather the fact that 

 close scrutiny reveals that all the beds consist of laminae from ^^ 

 inch to \ inch in thickness. These commonly alternate in light 

 and dark bands (disguised by the general pink of the formation), 

 the bands being minutely cross-bedded in uncertain patterns. 

 Much more prominent is cross-bedding on such a scale that its 

 interpretation always attracts comment. The true bedding now 

 dips eastward at very steep angles, and in places between a 10- to 

 15-foot thickness of such slabs will appear an equal thickness of 

 cross-bedding, abruptly truncated above and below, and dipping 

 eastward at an angle always less than that of the true bedding. Nor is 

 this all. Less conspicuous, but seldom lacking, is another set of 

 large-scale cross-beds dipping at present steeply westward. A mo- 

 ment's reflection will show that, if the original dip of the beds were 

 restored as a gentle eastward slope, the cross-bedding which now 

 dips eastward would dip gently westward and that which now dips 



