SURVEY OF COLORADO AND NEW MEXICO. 27 



imnsnally massive front, and in many places, are weathered into the gro- 

 tesqne forms so well shown southwest of Denver. 



Near the head of Little Thompson the ridges are admirably well shown. 

 Two beds of sandstone, belonging to the lower eretaceous group, seem to 

 have broken off in the process of elevation, and so ti))ped over tliat the 

 upper edges are past verticality. The npper cretaceous beds re dly form 

 but one principal ridge, although made up of three or four subordinate 

 ones. The sediments of these beds are so soft and yielding that they 

 have been easily worn down smoothly or rounded off' and grassed over 

 for the most j)art. But by looking across it, it is not difficult to detect 

 the black shales of No. 4, tlie yellow laminated chalky marl of No. 3 pass- 

 ing into the alternate layers of light-gray limestone and black plastic clays 

 of No. 2. As the little streams cut through these ridges at right angles, 

 they reveal not only the dift'ereut beds, but also the dip very distinctly. 



The Little Thompson begins to show evidences of enormous drift 

 agencies in the thick deposit of gravel, the high table lands on each side 

 of the creek, with here and there a butte with the top planed oif, and 

 over the surface is strewn a vast quantity of loose material which has 

 been washed down from the mountains. Each one of the little streams 

 has worn its way through the ridges of uplieaval, usually making enor- 

 mous gorges, but sometimes producing wide open valleys. The valley 

 of St. Vrain Creek is one of these valleys of erosion, with broad table 

 ]an<ls or terraces on each side, leaving the divide in the form of a con- 

 tinuous smooth bench, extending far down into the prairie, giving to the 

 surface of the country a beautiful and almost artificial appearance. 



The banks of the St. Vrain seem to be composed of an upper covering 

 of yellow marl, which soon passes down into gravel. The soil appears 

 to derive its fertility from the eroded calcareous sediments of No. 3, but 

 it rests upon a great thickness of a recent conglomerate, cemented, in 

 part at least, with oxide of iron. The greatest width of this valley is 

 over ten miles, gradually sloping down to the bed of the creek from the 

 north. The abrupt side is on the south, wliere a bank fifty feet high is 

 cut by the channel of the stream. This basik increases in height toward 

 the mountains, but becomes lower further down the stream eastward. 

 Above this bank, southward, is a broad level plain about two miles in 

 width, and then a gentle rise leads to another broad table plain which 

 forms a bench or divide. 



On the north side of St. Yrain Creek, near the foot of the mountains, 

 there is a long ridge of rather rusty yellow and gray sandstone, with a 

 treud about north 5° east, or nearly north and south. There are also 

 two other ridges, with a dip varying between 45° and 55° east. The first 

 ridge is about one hundred feet across the upturned edges, and there is 

 then westward a grassy interval of three hundred feet, and then au other 

 ridge of about the same thickness, the harder layers projecting above 

 the grassy plain from two to thirty feet. It presents the appearance, in 

 the distance, of a high, rugged, irregular wall, or broken-down fortifica- 

 tion, and is about three-fourtlis of a mile in length. These are the lower 

 sandstones of the lignite tertiary projecting above the grassy plain. 



Near the foot-hills of the mountains, about four miles south of St. 

 Vrain's Creek, are some high cretaceous benches, extending down from 

 the base of the mountains. They usually do not extend more than one 

 or two miles in length before they break off, sometimes abruptly and 

 sometimes gradually. Not unfrequently a sort of truncated cone-shaped 

 butte is cut off from the end of some of the benches. On the summit is a 

 considerable thickness of a recent conglomerate which has been 

 cemented so as to form a tolerably firm rock. In this drift some frag- 



