MEEK AM) IIAVDEN'S (JUETACEOIJS SECTION. 17lj 



The Fort Pierre is the most important of the (H\ isioiis of the Crettice- 

 ous in the Northwest, ahke in tlie extent of its devehjpment, in its influence 

 on the character of tlio country, and in the great abunthince and l)eauty of 

 its fossils. In the number of its fossils it surpasses all the other Cretaceous 

 groups collectivel}-. Its clays lose by wetting their Inminated structure 

 and become adhesive and pasty, and then by drying become hard and 

 crumbly, and split or crack from the surface to a depth of several inches. 

 There is something in its ])hysical or chemical constitution inimical to 

 vegetation, and over large aieas it weais a peculiar black and barren 

 appearance, scarcely relieved by a shred of vegetable life. In other 

 regions, however, the surface supports an excellent growth of grass. 



Black Hills. — The group is well exposed on Old Woman Fork near the 

 Cheyenne, and it exhibits there in its fossils a blending with No. 5. It is 

 in general plainly recognizable entirely around the Hills. It constitutes 

 the chief Cretaceous exposures on the Belle Fourche aiul South Cheyenne 

 from their junction upward to the vicinity of the uplift, and appears again 

 on the South Cheyenne near the mouth of Beaver Creek. 



The upper fossiliferous zone is well exposed along the South Cheyenne 

 from Rapid Creek downward, and near the month of that creek its charac- 

 teristic fossils occur in great profusion. They are found in calcareous con- 

 cretions or nodules, often of ver}' large size. Sage Creek and Bear Creek, 

 which enter the river nearly opposite the mouth of Rapid Creek, are 

 rendered classic by the collections of Meek and Hayden, 



Thickness, estimated, 150 to 250 feet. 



No. 3.— Niobrara group. 



Lead-gray calcareous marl, weathering- to a yellowish or whitish chalky ai)i)ear- 

 ance above. Containing large scales and other remains of fishes, and many specimens 

 of Ostrea congcsta atta(;he(l to fragments of Inocrramiis ; also .several species of Tcxtu- 

 laria. Passing down into light, yellowish, and whitish limestone, containing great num- 

 bers ot^ Inocerdinm probkmaticus, I. psendomytiloides, I. arieidoides, and Ostrea congesta, 

 fish scales, etc. 



Localities. — Blntt's along the Missouri below the Great Bend, to the vicinity of 

 Big Sioux Elver; aiso below there on the tops of the hills. 



Thickness, 200 feet. 



From this formation Professor Marsh and Professor Cope have de- 

 scribed nearly one hundred vertebrates, including birds, reptiles, and fishes. 



