EOTLICH.J SOUTHEEN AREA FOX HILLS GROUP. 123 



to a settling of the strata along the lines of greatest pressure, or to an 

 exfoliation by partial hydration of those portions from which pressure 

 is removed, may remain an open question. Personally, I am inclined 

 to favor the latter proposition, although I am not able to prove it. East 

 of the ridge, the dip of the shales becomes very slight, amounting to 

 about 6°. A wide, flat valley is there excavated by erosion, the drift of 

 which effectually conceals all outcrops. Near the western base of a high 

 bluff east of the valley we obtain the data as to clip. On the western 

 side of the anticlinal we find the same group, but so arranged as to dip 

 in the opposite direction. At the northern end the arch connecting the 

 two is complete and well exposed. It is represented in Section XIII. 



Within the shales numerous fragments of Inoceramus and Ostrea con- 

 gesta were found wherever they were exposed. Toward the south the 

 group appears to develop considerably in vertical extent. Its maximum 

 thickness may be regarded at 900 feet. 



Directly north of Salt Wells we find the other occurrence of Colorado 

 shales in this southern area. It there owes its position to the formation 

 of an anticlinal fold extending to the southwest. We have within our 

 district but the extreme northern portion of this disturbance, which 

 manifests itself in the production of a partiversal arrangement of the 

 strata. At that locality, the oldest group reaching to the surface is that 

 of the Colorado shales. Dakota beds are not exposed until we travel 

 some distance southward. Covered by the Fox Hills series, the shales 

 occupy but a very restricted area, and do not appear again until north 

 of the Sweetwater divide. 



Fox Sills Group. 



In Whiskey Gap we find the northern termination of the area over 

 which this group is exposed. We there have occasion to observe very 

 clearly the effect of the anticlinal upheaval. The Fox Hills series is 

 broken along the line cut by section XIII ; but a short distance north- 

 ward it is continuous again. Forming a steep synclinal fold, the one 

 portion of the group branches off to the eastward, flanking the southern 

 slope of the Seminole Hflls. The other follows a line of outcrop about 

 southeast, dipping a little north of east. This latter owes its position 

 to the anticlinal uplift. We find that the beds of this group form a 

 prominent ridge, rising steeply from the valley directly east of that pro- 

 duced by the upturned Colorado shales. Dipping eastward at an angle 

 of 4° to G°, the Fox Hills strata gently slope toward the lower country. 

 As usual, the series is here composed of yellow, brown, and grayish, 

 highly arenaceous shales. Beds of sandstone and some of pure argfllites 

 occur within them. Toward the top, sandstones of comparatively fine 

 texture set in, forming a cap which protects the underlying shales from 

 total disintegration. Steep slopes characterize the exposed edges of the 

 strata, while the upper sandstones form even or gently-rounded slopes. 

 Some of the highest sandstone strata are very nearly white. Decom- 

 posing readily, the shales furnish an excellent supply for the production 

 of alkali. They contain, as Avell as the Colorado shales, small inclosures 

 and seams of pyrite, which aid rapid chemical changes. Within the beds 

 referable to this group we found numerous fragments of Inoceramus and 

 a Turbonella. Some small seams of coal occur near the top, but their 

 outcrops were so much obscured that the thickness could not be meas- 

 ured. Along this eastern ridge, the Fox Hills Group reaches a vertical 

 development of about 1,000 to 1,100 feet. A section (Section XV) taken 

 through the anticlinal ridge a short distance north of Eawlings Peak will 



