TERTIARY PLAINS OF WYOMING. 69 



fashion, the softer rocks below, which liave been washed out by the driving 

 rains and storms. A peculiarity of this upper fine conglomerate is a ten- 

 dency to split up into blocks and slabs from 6 to 12 inches in thickness. 

 Near the mountains, these upper beds lie inclined at from 1J° to 2^°, form- 

 ing a most excellent railway-grade. Underlying the fine conglomerate 

 along the railway-bench, and in the valley of Crow Creek, occurs another 

 characteristic bed of the Pliocene basin, which consists of an exceedingly 

 fine, almost impalpable, arenaceous marl, of a light cream color, and free, 

 from all inclosed pebbles, which mark both the underlying and overlying 

 strata. Out upon the Plains, the upper stratum is usually somewhat finer 

 than near the mountains, but of essentially the same nature — light-gray and 

 ash-colored rocks, consisting of sands and Archsean pebbles. Below these 

 are alternating beds of marls, clays, and calcareous grits, with thin seams 

 of rtiud-rocks and belts of fine sand. 



South of the railroad and west of Chalk Bluffs, the country falls off 

 rapidly, and has undergone a very considerable amount of erosion, carrying 

 away a great part of the Tertiary strata, and leaving a broken, irregular 

 surface, with isolated hills and benches of Pliocene beds, which, where not 

 covered by Quaternary detritus, offer exposures of considerable vertical 

 thickness, but without any special geological interest.' Erosion has, in 

 several places, so worn away the overlying Tertiaries within the area colored 

 on the map as Pliocene, between Box Elder and Lone Tree Creeks, as to 

 expose, in a few localities, isolated patches of the underlying unconforma- 

 ble Cretaceous sandstones. About 12 miles south of Cheyenne, and 3 or 

 4 miles northwest from Carr's Station, the effect of erosion upon, these 

 horizontal strata is well shown at the "Natural Forts", where the reddish- 

 yellow sandstone is curiously worn away, leaving walls well arranged for 

 purposes of defence. About 5 miles south of Cheyenne occurs a develop- 

 ment of the light cream-colored limestone of the Pliocene basin. As it 

 weathers slowly, it stands above the friable sands that here form the top 

 of the plateau, and appears above the surface in a line of low, conical hills. 

 It has a hard, cherty fracture, and is traversed by thin seams of quartz, 

 which give the rock a somewhat more siliceous appearance than chemical 

 analysis would indicate. Attempts have been made to burn it for lime, 



