118 DESCRIPTIVE GEOLOGY. 



are soon lost beneath the later Tertiary strata of the basin. These con- 

 formable beds dip to the southward at angles varying from 22° to 25°. 



Just where the road from the Laramie Plains enters the North Park, 

 the Dakota Cretaceous occurs, apparently resting next the Archaean founda- 

 tion, and standing in bold characteristic ridges, with a mural face toward 

 the mountains, and with a gradual slope in the direction of the plain. They 

 occupy a slight re-entering ba}'^, or curve, in the Archaean mass, and dip with 

 an angle of 18° to 22°. A point of some interest in these ridges consists in 

 the development of conglomerate, which forms so marked a feature over wide 

 areas, at the base of the Dakota group ; it is only to be observed in most locali- 

 ties in cross-sections, but here the ridge-summits consist of conglomerate 

 beds offering very excellent exposures. This conglomerate is formed of 

 small fragments of worn and rounded quartz, associated Avith pebbles of 

 dark chert, the mass held together by a cementing material of fine sand and 

 ferruginous earth. A peculiar feature of the conglomerate is, that many 

 layers are so extremely hard that under the hammer the rock fractures in 

 lines across the chert, as well as through the binding cement. The beds 

 dip 18° to 20°. Still farther eastward, on Retreat Creek, just above the 

 camp station, the probable line of junction between the Dakota and Colo- 

 rado groups is well shown along the banks of the stream. Plere the dark 

 black clays lie directly upon the lower sandstones, both formations dipping 

 with an angle of 17° to 19°. The former rises abruptly in a high ridge, 

 while the latter dies away in low, rolling benches. The low bench just 

 south of the camp station probably represents the Colorado marls overlaid by 

 the upper sandstones. 



From Retreat Creek southeastward to Elk Camp, some 16 or 18 miles, 

 the sedimentary ridges present a nearly uniform appearance that calls for but 

 little mention, rising to about the same height above the plain, and with an 

 average inclination of 20° to 30°. Unlike the Mesozoic strata, however, of 

 Northern Colorado, instead of forming long unbroken lines of upheaval, the 

 beds show a constantly changing strike, which produces short broken ridges, 

 between which small streams have found their way from the main range to 

 the forks of the Platte. 



From 1 to 2 miles east of Retreat Creek occur a number of prominent 



