DAWSON ARKOSE 271 



The Castle Hock conglomerate is an outlying erosion remnant of a 

 former greater sheet of the White liiver group, which is typically de- 

 veloped in southwest South Dakota and west Nebraska. The source of 

 the components of the conglomerate undoubtedly was the rocks of the 

 Front Kange, the material apparently being laid down as outwash and 

 fluviatile deposits following a period of uplift of the mountains to the 

 west. 



The Dawson Arkose 



That part of Hayden's Monument Creek group which lies below the 

 unconformity at the base of the Castle Eock conglomerate is named 

 Dawson arkose, from Dawson Butte, situated 7 miles southwest of Castle 

 Eock. The Dawsosn arkose outcrops on the Platte- Arkansas divide, 

 between Denver and Colorado Springs, and extends eastward from near 

 the base of the mountains to the valleys of Kiowa and Bijou creeks, a 

 distance of 40 or 50 miles (see figure 1). 



Conforming with the general structure of the southern part of the 

 Denver Basin, the Dawson arkose constitutes part of a northward pitch- 

 ing unsymmetrical syncline. On the west the beds locally form part of 

 the foothills and dip eastward at an angle of about 45 degrees. This dip 

 rapidly flattens out, and the westward inclination at the opposite end 

 of the basin is very gentle. In the vicinity of Palmer Lake and for 

 several miles north and south of that place, where the arkose abuts di- 

 rectly against the granite of the mountains, the contact is faulted. 



The Dawson arkose has a maximum thickness of about 2,000 feet. It 

 is thicker on the west, toward its source in the mountains, and thins 

 out eastward. 



The formation is a complex aggregate of vari-colored and va'ri-textured 

 conglomerate, sandstone, shale, and clay, derived from the rocks of the 

 Front Eange and deposited under a variety of continental conditions. 

 Sandstones comprivse the greater part of the Dawson arkose. They are 

 medium to coarse-textured arkosic grits, composed chiefly of quartz and 

 feldspar derived from the Pikes Peak granite and associated rocks. Beds 

 and lenses of conglomerate occur throughout the formation, but they are 

 more common in the lower part and nearer the mountains, where a basal 

 bed is well developed, consisting of pebbles of granite, quartz, quartzite, 

 chert, and occasional fragments of limestone and sandstone, from the 

 foothills strata, in an arkose matrix. There are also local bodies of clay, 

 as at Calhan, where a deposit of commercial importance is being worked. 



There is pronounced irregularity in the arrangement and sequence of 

 the deposits which constitute this formation. Cross-bedding is common. 



