REPORT OF F. V. HAYDEN, U. S. GEOLOGIST. 



H A P T E E I. 



EEMARKS OIN SUEVET— THE GEOLOaiCAL FEATURES OF THE EAST 

 SLOPE OF THE COLORADO RANGE OF THE ROCKY MOUNTAINS, FROM 

 CACHE A LA POUDEE RIVER SOUTHWARD TO PIKE'S PEAK. 



In the third annual report of the survey for 1869 I described with 

 some care the geological features of the east slope of the Eocky Moun- 

 tains, from Cheyenne to Santa Fe. My investigations were more par- 

 ticulairy confined to the sedimentary formations as they are shown by 

 their upturned edges along the immediate eastern base of the range. 

 In this chapter I can do but little more than confirm the accuracy of 

 the statements made in that report, and add a few new facts, referring 

 the reader to the more detailed reports of Mr. Marvine and Dr. Peale. 



Although the sedimentary rocks along the flanks of the mountains 

 are of great interest, yet the general outline of their geology is compar- 

 atively simple. The vast plains to the west of Cheyenne are covered 

 with the drab-yellow and light-gray sands, marls, and clays of the great 

 fresh- water lake deposit, known as the "bad lands," [Mauvaisses terres,) 

 and may be Miocene, or Pliocene, or both. This entire group of deposits 

 juts up against the foot of the mountains, not conforming to the older 

 beds, and in a horizontal position or inclining not over 5^. The Union 

 Pacific Eailroad passes uninterruptedly across this deposit from a point 

 east of Grand Island, on the Platte, to the margin of the first range. 

 As we pass southward from the railroad this lake-deposit soon thins 

 out and disappears, and the full series of the older sedimentary rocks 

 known in this region are exposed in their order of sequence. The 

 fresh-water Tertiary deposits not conforming with the older rocks and 

 jutting up against the sides of the front range almost to the summit, 

 necessarily conceal the latter over a very long distance. For one hun- 

 dred miles or more they are not visible, only as the former have been 

 washed away, and for a considerable distance north of the Laramie Eiver 

 the fresh- water deposits conceal all the older beds, and rest upon the gran- 

 ites direct. About four miles south of a locality known on the maps as 

 Spottswood Springs, the Lignitic beds begin to be revealed in the valleys 

 of the little streams, and very soon the fresh-water deposits entirely thin 

 out. So far as I have been able to ascertain, the older sedimentary strata 

 perfectly conform, and we have here a series of uplifted ridges, showing all 

 the sedimentary rocks of the region up to the Lignitic group inclusive, 

 with remarkable distinctness. I am not certain that we have well- 

 defined Carboniferous beds south of the railroad. Along the line of the 

 railroad, between Hazard Station and Granite, the limestones of Car- 

 boniferous age are exposed to a limited extent. So far as I have ob- 

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