ASHLEY CEEEK BASIN. 291 



Ashley Creek Basin. — Under this general name are included the 

 deeply-eroded valleys of Ashley Creek, Brush Creek, and Lower Green River, 

 east of the Wonsits Ridge, the lowest portion of the region represented on 

 this map. Probably nowhere in the world is a more excellent opportunity 

 offered for studying descriptive geometry in its application to geology than 

 in this basin. In it, the sandstone ridges of the various Mesozoic formations 

 can be seen following the flanks of the higher hills in S-like curves; the 

 more yielding clayey and shaly beds being cut into narrow monoclinal 

 valleys, so that the system of three anticlinal folds, of which we have already 

 spoken, is impressed upon the mind of the observer at a glance. As the 

 observations made in this region formed the basis of the general grouping 

 of these formations throughout the whole region shown in this map, a 

 description of the various sections taken will be given somewhat more in 

 detail than has been done heretofore. 



The first line of section described will be that along the line of Ashley 

 Creek, from Geode Canon to the Quaternary valley of this creek. Geode 

 Canon is a narrow, picturesque gorge, in its deepest part about 2,000 feet 

 deep ; its upper portion being cut through horizontal beds of the light-colored 

 siliceous limestones and sandstones of the 'Upper Coal-Measures. Near the 

 mouth, it winds in and out between the beds, which here dip south at an angle 

 of 29°. Among the upper beds which form the steep southern slopes of this 

 bench country, the cherty fossiliferous seam, already mentioned, is prominent 

 at the mouth of the canon ; in it were found the usual siliceous casts of Ortho- 

 ceras, a smooth variety of Bellerophon, and the usnal Bellerophon carbonarius, 

 together with a Biscina or Orhiculoidea. This bed is here about 50 feet 

 thick, below which is found about 50 feet of yellow, compact cherty lime- 

 stone abounding in geodes, containing beautiful crystals of calcite and 

 small concretions of flint, some of which contain dendritic inclusions, and 

 resemble the moss-agates of the Bridger Basin, and at its base a thin seam 

 of green compacted sand and clay, which in places is very rich in oxide of 

 iron. Below these limestones are massive bodies of white compact sand- 

 stone, more or less calcareous, and passing into siliceous limestones, which 

 form the horizontal beds cut through in the upper canon, of which, however, 

 no detailed section was obtained. At the foot of the steep slopes on either 



