CRETACEOUS AND TERTIARY FORMATIONS 509 



under the auspices of the North Dakota Geological Survey. The 

 work in Montana was confined mostly to Dawson and Custer 

 counties,' the Yellowstone River having been followed from its 

 mouth to Miles City, and a trip being taken north from Miles City 

 to the Hell Creek region and across the Missouri River to Glasgow. 



PIERRE SHALE 



The Pierre shale is exposed along the Missouri River for a dis- 

 tance of about twenty miles north of the South Dakota line, or 

 as far as the mouth of Big Beaver Creek in Emmons County; 

 in eastern Montana it appears along the Missouri Valley from a 

 point probably as far west as the Musselshell River to the station 

 of Brockton, on the Great Northern railroad, or a distance of 

 nearly 180 miles; it also occupies a small area on Little Beaver 

 Creek in northwestern Bowman County, North Dakota, which is 

 probably continuous with the Pierre outcrop on the Yellowstone 

 River, twelve miles above Glendive. 



The Pierre formation is a bluish gray to dark gray, sometimes 

 almost black, jointed shale, which often weathers into small, 

 flaky fragments. The rock commonly shows yellow spots or stains 

 of iron oxide. The topmost beds of the Pierre contain numerous 

 calcareous concretions varying in size from a few inches to six and 

 eight feet in diameter. Some of these concretions are rich in 

 invertebrates, which are characteristic of the upper forty or fifty 

 feet of the Pierre, while others are barren of fossils. Many are 

 cut by a network of calcite veins which are commonly lighter 

 colored than the matrix. The following species, identified by Dr. 

 T. W. Stanton, were collected in the Little Beaver Creek locality, 

 Bowman County, North Dakota: 



Ostrea pellucida M. and H. Lunatia. 



Avicula linguaeformis E. and S. Anisomyon patelliformis M. and H. 



Inoceramus cripsi var. barabini Mor- Margarita nebrascensis M. and H. 



ton. Fasciolaria? (Cryptorhytis) flexi- 



Chlamys nebrascensis M. and H. costata M. and H. 



Yoldia evansi M. and H. Pyrifusus. 



Nucula cancellata M. and H. Haminea? occidentalis M. and H. 



Lucina occidentalis Morton. Scaphites nodosus Owen vars. brevis 



Protocardia subquadrata E. and S. and plenus. 



Callista deweyi M. and H. Nautilus dekayi Morton. 



