HAXDEIf.I 



GEOLOGY COLOSADO RANGE. 21 



stated, liowever, that the absence or presence of large deposits of gyp- 

 sum seems in no way to affect the beds of limestone, although the gyp- 

 sum occurs in greater or less quantities all along the eastern sides of 

 the mountains wherever the group of red beds is exposed. There is a 

 very plaiu thinning out of the calcareous rocks as we proceed south- 

 ward. North of the railroad the Carboniferous limestones are quite 

 thick in some places ; near Fort Fettermau they reach a thickness of 

 1,000 to 1,500 feet, while south of the railroad the aggregate of the 

 limestone layers would not amount to more than 25 to 50 feet. The 

 beds or deposits of gypsum are much more extensive to the northward, 

 around the Black Hills, reaching a thickness of 40 to 60 feet, amor- 

 phous, and of a snowy whiteness. 



The central portion of this anticlinal is a rather low grassy valley, 

 with one or two low ridges rising 2 to 4 feet above the surface, the 

 brick-red edges of the softer intervening strata outcropping here and 

 there where the rock is rounded. The intervening valleys between the 

 ridges are of various widths, depending upon the thickness of the indu- 

 rated calcareous sandstones. From their irregularity in weathering 

 there must be a very great variability in the texture of these brick-red 

 sandstones from point to point at not very great intervals. This differ- 

 ence is shown all along the base of the mountains. Sometimes the 

 ridges are very high and the upturned i^ortions form a belt of consider- 

 able width, composed of quite compact sandstones and quartzites ; then, 

 within the space of a few miles, the greater portion of the group will 

 appear to be made up of indurated sands, which yield readily to atmos- 

 pheric agents. The irregularity in the wearing down of these ridges is 

 undoubtedly due in part to other causes, which will be discussed in an- 

 other portion of this report. In the valley between the two sides of the 

 anticlinal are several lime-kilns, and the limestone is taken from a bed 

 2 feet in thickness, rising above the surface a few feet and standing 

 nearly vertical. On either side of the limestone the arenaceous lime- 

 stone and sandstone are very cellular, looking much like a spring de- 

 posit of tufa. It is certainly very gypsiferous all the way through. 

 There are two or three of these thin beds of limestone about the middle 

 of the red group, but no fossils have ever been detected in them. This 

 anticlinal valley, although so short, is very beautiful. At its upper end 

 it is about half a mile wide, tapering to a point on the south side of the 

 Big Thompson, so that it is about three miles in length. The west por- 

 tion of this anticlinal forms also the east side of the beautiful synclinal 

 valley, through a i)ortion of which the stream flows. The eroding 

 agents have smoothed out this concave synclinal valley, which is 

 almost entirely grassed over, so that no formations can be seen newer 

 than Ko. 1. The quartzite wall on the east side, which stands 

 SOc* at least, must have been broken off, so that the under- 

 ground portions cannot be very deep, but pass beneath the valley and 

 rise up on the opposite side, inclining at a very moderate angle. This 

 peculiar arrangement of the ridges produces a very curious drainage 

 for the Big Thompson and its branches. On the south side of the val- 

 ley of the Big Thompson the long, low. Cretaceous ridges can be seen 

 from a high point extending across the surface to the north, but stop- 

 ping abruptly at the creek, while the sjnclinal interval gradually closes 

 up to the northwest, in the valley of the East Fork. South of the 

 main stream, and on the west side of the synclinal valley, the No. 1 

 ridge is very prominent, and runs up to Big Thompson close above 

 the Bed Stone Creek, as seen on the maps. Between the end of the 

 anticlinal south of Big Thompson, and the same ridge as it rises on 



