PIERRE SHALE 393 



colored shale weathering light brown. Its thickness is about 1,200 feet, 

 so far as can be ascertained, but, owing to the general low dips, the 

 amount rarely can be determined. No details of stratigraphy have been 

 established in the 'formation, excepting that at a horizon of about 1,000 

 feet above its base there frequently occur lenses of limestone containing 

 numerous shells of Lucina occidentalis. These lenses are often 15 to 20 

 feet in diameter and 6 to 8 feet thick, and, owing to their hardness, when 

 they are uncovered by erosion they give rise to low conical buttes re- 

 sembling in form a squat tepee. Accordingly these forms have been 

 designated tepee buttes, a term used for similar occurrences in the Pierre 

 shale in southeastern Colorado. 



Numerous concretions occur in the Pierre shales at various horizons, 

 which usually contain large numbers of very distinctive fossils, of 

 which the more abundant are the following species : Baculites compresus, 

 Inoceramus sagensis, Nautilus dekayi, Placenticeras placenta, Heteroceras 

 nebrascensis, and an occasional Lucina occidentalis. The most fossiliferous 

 horizon is in the upper part of the formation. The concretions are gener- 

 ally of small size, of a calcareous nature, and break into small pyramidal 

 fragments, which are more or less scattered all over the Pierre surfaces. 

 At the base of the formation overlying the Niobrara chalk there is always 

 a very distinctive series of black, splintery, fissile shales, containing beds 

 of concretions, which have been included in the Pierre shales, although 

 they have not yet been found to contain distinctive fossils. The thick- 

 ness of the series is about 150 feet, and it gives rise to a steep slope often 

 rising conspicuously above the* lowlands eroded in the Niobrara chalk. 



FOX HILLS AND LARAMIE FORMATIONS 



To the west and north of the Black hills there are extensive areas of 

 Laramie and overlying formations, separated from the Pierre shale by a 

 narrow band of outcrops of the Fox Hills sandstone. This sandstone 

 develops gradually from beds of passage with typical Fox Hills forms, 

 mainly Veniella, although in some districts Pierre fossils extend upward 

 for some distance into the sandy beds. The thickness of the Fox Hills 

 sandstones is from 250 to about 500 feet. They grade upward into soft 

 massive sandstones alternating with carbonaceous shales containing no 

 marine fossils, but often yielding plants of the Laramie flora. Not far 

 above the sandstones, which are believed to be at the top of the Fox 

 Hills formation, there occur the remains of Ceratopsidse, which have 

 been described by Hatcher, and these in turn are surmounted by beds 

 which Knowlton has found to contain a typical Fort Union flora. The 

 Ceratops beds have been described by Hatcher* and by Stanton and 



*Am. Jour. Sci., 3d series, vol. 45, pp. 135-144. 

 Am. Naturalist, vol. 30, pp. 112-120. 



