Preliminary report of the United States Geological Survey of Wyoming, and portions of contiguous territories : (being a second annual report of progress), under the authority of the Secretary of the Interior . l of the Laramie range, which I have already- described,they sometimes repose with a slight discordance on the older rocks; some-times, as near the Laramie Peak, they rest directly on the granites, andentirely conceal, for a distance of forty or fifty miles, all the unchangedrocks of older date; but a careful study of the eastern flank, from RedButtes to Longs Peak, will reveal all thefo
Preliminary report of the United States Geological Survey of Wyoming, and portions of contiguous territories : (being a second annual report of progress), under the authority of the Secretary of the Interior . l of the Laramie range, which I have already- described,they sometimes repose with a slight discordance on the older rocks; some-times, as near the Laramie Peak, they rest directly on the granites, andentirely conceal, for a distance of forty or fifty miles, all the unchangedrocks of older date; but a careful study of the eastern flank, from RedButtes to Longs Peak, will reveal all theformations that are known to exist in thispart of the West, inclining from the sidesof the granitic nucleus at various angles. Figure 14 will illustrate the surface fea-tures of the Monument Creek Group. Therocks are composed mostly of decomposedgranites, a feldspathic paste holding somepebbles of quartz or feldspar. The columnsthat are left standing over a large area,are capped with a hard layer of rusty yellowsandstone, as shown in the cut. This groupcovers the divide between the South PJatteand the Arkansas Rivers, and,is supposed to be of upper miocene or pliocene age. Monument Creek, c oiorado. 11 G. 162 GEOLOGICAL SURVEY OF THE TERRITORIES. Figure 15 Tvill serve to show the appearance of the ridges of upheaval,or hog backs, as they are sometimes called iu the west. These ridgesoccur to a greater or less extent all along the eastern flanks of the KockyMountains. They sometimes rise like steps to the crest, so that onemay walk from the tertiary formations in the plains, holding a horizon-tal position, across the uplifted edges of all the formations to the granitenucleus. Between these ridges are, in many instances, beautiful grassyvalleys, varying from a few hundred yards to half a mile in width. When exposed only on one side of a range I have called them mono-clinal ridges and valleys. Fig. 15.
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