FORMATIONS OF THE HARTVILLE UPLIFT 411 



uppermost sandstone there is usually more or less ironstone. On the 

 east side of Old Woman creek the Fuson formation is underlain by a 

 thin bed of limestone, which probably represents the Minnewaste lime- 

 stone of the southern Black hills. 



BENTON GROUP 



In the basin northwest of Guernsey the Graneros member of this group 

 is exposed to a thickness of about 120 feet, and the entire group is finely 

 exhibited in the anticline on the east side of Old Woman creek, dipping 

 steeply to the west. Here the lower formation, Graneros, consists of 

 dark shales 800 feet thick. A thin bed of hard sandstone occurs about 

 100 feet above the base, a feature which is also seen northwest of Guernsey, 

 where this sandstone is the uppermost member exposed. The Graneros 

 shales are capped by the Greenhorn limestone, consisting of about 50 feet 

 of thin bedded, impure limestone, weathering out in slabs of dirty buff 

 color and usually containing large numbers of Inoceramus labiatus. The 

 overlying Carlile formation has a thickness of about 425 feet, consisting 

 of shales of gray color, with occasional thin beds of sandstone, and 

 toward the top the characteristic horizon of biscuit-shaped concretions. 



NIOBRARA FORMATION 



This formation is exposed along the Old Woman Creek anticline, 

 having a thickness of 250 feet. It consists of impure chalk and limy 

 shale of lead gray color in fresh exposures, but weathers to a bright, 

 pale straw color, a highly characteristic feature which renders its out- 

 crops very conspicuous. The formation contains occasional thin layers 

 of limestone filled with the characteristic fossil, Ostrea congesta. 



PIERRE SHALE- 



This formation probably underlies the Arikaree formation along the 

 east side of the uplift from Iron Mountain station northward, and doubt- 

 less also the western side of the uplift north of Orin junction, but it is 

 exposed only in the lowlands north of the foot of Pine ridge. There it 

 is found on both sides of the anticline of Old Woman creek, extending 

 to Cheyenne river and thence along the slopes of the Black Hills uplift. 

 The beds dip steeply along the Old Woman Creek valley, where a cross- 

 section measurement gave a thickness of 1,200 feet. The rocks consist 

 of a monotonous series of dark gray shales, with occasional concretions, 

 and 200 feet below the top there is a zone of concretions consisting of 

 limestone filled with Lucina occidentalis. As in other regions, the lime- 



LIV— Bull. Geot,. Soc. Am., Vol. 15, 1900 



