64 DESCRIPTIVE GEOLOGY. 



them. Although the Laramie beds in the extreme northern portion of 

 Colorado have as yet furnished no coal deposits of gr6at economic value, 

 the region is of considerable geological interest from the occurrence of 

 Cretaceous types lying in such close relation with the coal strata. Beyond 

 the limit of the map, the Laramie beds were traced southward, and were 

 found underlying the plains at Denver, and to include the valuable coal 

 deposits at Erie, and the Marshall and Murphy mines north of Golden, 

 extending from within one-half mile of the base of the range far out upon 

 the Plains into Eastern Colorado. 



The Laramie beds form the uppermost members of the great series of 

 conformable strata that lie upturned against the Archaean mass of the 

 Rocky Mountains ; all overlying strata resting unconformably upon the 

 older rocks. South of Chalk Bluffs such beds cover very subordinate 

 areas, and are of but little geological importance. They are always found 

 lying either horizontally or upon some sloping bed of deposition, and fre- 

 quently occur filling depressions, the results of erosion in older formations. 



Along the immediate base of the range, usually just outside the 

 Dakota sandstone, occur isolated patches of irregular terraces and benches, 

 which consist of coarse gravel and smooth rounded boulders held together 

 by ferruginous sands. To the southward, they are much better developed 

 than in the extreme northern part of Colorado. In the region of the Big 

 Thompson, they form a local but well marked feature, appearing on both 

 sides of the valley, having been cut through by the stream. They reach a 

 development of about 200 feet in thickness, extending southward as far as 

 the Saint Vrain's Creek, There is no positive evidence as to age of these 

 terraces, and they may be either Pliocene-Tertiary or Quaternary; but, 

 from their close resemblance to beds east of the Laramie Hills and to others 

 westward on the Wyoming Plains, they have been referred provisionally 

 to the Wyoming Conglomerate, the latest Pliocene beds of the Rocky 

 Mountains. Out upon the Plains, away from the Colorado Range, the Lara- 

 mie sandstones are in places concealed by heavy accumulations of coarse 

 detrital material of Quaternary age, which consist of finer material than the 

 well-marked terraces, but would appear to be largely derived from their 

 decomposition. Such accumulations form quite a prominent feature in the 



