FORMATIONS OF THE LARAMIE RANGE 407 



consisting of colonies of Osirea congesta. The thickness is about 350 

 feet. Exposures at the north end of the range are not complete, owing 

 to Tertiary overlap, but the total thickness probably is not great. The 

 material is a limy shale or impure chalk, with the characteristic light 

 straw color in weathered outcrops and the thin beds of hard limestone 

 filled with Ostrea congesta. 



PIERRE SHALE 



This shale outcrops continuously from the Bighorns to the north end 

 of the Laramie range, presenting its usual characteristics, a great mass 

 of dark gray shale with numerous concretions containing distinctive fos- 

 sils. In some localities local beds of sandstone are included. The for- 

 mation is exposed at the foot of the mountains, on Horse and Chugwater 

 creeks, where its thickness appears to be more than 2,500 feet. 



FOX HILLS FORMA TION 



Sandstones of the Fox hills approach close to the north end of the 

 Laramie range some distance east of Casper, and they are also exposed in 

 a small area at Horse Creek station. The rocks are soft, impure sand- 

 stones and sandy clays with more or less concretionary structure. Abun- 

 dant fossils were observed 2 miles south of Glenrock. 



Hartville Uplift 

 general characteristics and section 



The Rocky Mountain front range and the Black Hills uplift are con- 

 nected by an anticline which would have considerable prominence topo- 

 graphically were it not for the thick covering of Tertiary deposits. It 

 branches from the main Laramie uplift north of Iron mountain, is exten- 

 sively bared by erosion along the North Platte river and northward for 

 some distance in the Hartville region, and is marked by the prominent 

 peak of granite known as " Rawhide butte " and by numerous minor 

 outcrops of schists, limestone, and Cretaceous rocks in the eastern por- 

 tion of Converse county, Wyoming. 



This anticline, which may be designated the Hartville uplift, affords 

 extensive exposures, which throw much light on the stratigraphic vari- 

 ations between the Black hills and the Rocky mountains. These details 

 have been studied with special care in the area of the Hartville quad- 

 rangle in greater part by Dr W. S. Tangier Smith, the results of which 

 are published in the Hartville folio.* 



*U. S. Geol. Survey folio no. 91. 



