THE BUILDING OF THE COLORADO ROCKIES 155 



Denver formation in Kthological character and in its fossil plants.^ 

 Along the Grand River, in the Hne of the section which was 

 measured as the basis for this present study of the Rocky Moun- 

 tain deformation, the Middle Park formation outcrops continously 

 for a distance of 4! miles. Its structure, as was first noted in the 

 very excellent description by Marvine,^ is basin-Uke. On the 

 eastern limb of the syncline the steeply incKned, westerly dipping 

 beds of the basal portion of this formation, form the conspicuous 

 mountain ridge through which the Grand River flows at Windy 

 Gap, 3^ miles northwest of Granby. Westward toward the 

 middle of the basin the dip rapidly lessens, and for several miles 

 is not far from horizontal, but farther west the lower beds of the 

 volcanic series emerge again about 2 miles east of Hot Sulphur 

 Springs. 



At Windy Gap the Middle Park formation consists of a series 

 of breccias, tuffs, conglomerates, and sandstones dominantly of 

 andesitic derivation. In the lower portion there are several 

 hundred feet of coarse breccia containing many cobbles and angular 

 fragments of andesite up to six inches in diameter. Higher up in 

 the series the material becomes finer, and rounding by water action 

 becomes more prominent. Within the first five hundred feet of 

 the formation no evidence of granitic materials was observed. The 

 pre-Laramide formations at this stage supphed very little detritus 

 to be mixed in with the andesitic derivatives. But in time the 

 lithological character of the accumulation underwent a change as 

 new sources of material became available. About 1,000 feet 

 stratigraphically above the base of the formation some very 

 striking conglomerates appear in the hills immediately west of the 

 Windy Gap Ridge. Well-rounded cobbles of four or five inches 

 represent the average coarseness, though bowlders of a foot or more 

 in diameter are not difficult to find. These cobbles are of granitic 

 rocks and various fine-grained porphyries, particularly grayish- 

 green andesite. The only available source for the granites is the 



^Whitman Cross, "The Post-Laramie Beds of Middle Park, Colorado," Proc. 

 Colo. Sci. Sac, IV (1891-93), 192-214. 



^ A. R. Marvine, "Report on Middle Park," U.S. Geol. and Geog. Survey of 

 Colorado (Hayden), 1873, pp. 154-92. 



