CRETACEOUS AND TERTIARY FORMATIONS 511 



In the southeastern corner of Custer County, Montana, as a 

 result of the Black Hills uplift, the Pierre shale outcrops over an 

 area of considerable extent, overlying the Benton and Niobrara 

 formations, which also appear at the surface. 



FOX HILLS SANDSTONE 



The Fox Hills sandstone is the most recent of the marine for- 

 mations of the Great Plains region. It is very variable in character 

 and undergoes considerable change in composition and appearance 

 from one locality to another. It is exposed on the Missouri River 

 as far north as old Fort Rice, about eight miles above the mouth 

 of the Cannon Ball River; it appears on Little Beaver Creek, a 

 tributary of the Little Missouri in southwestern North Dakota; 

 on the Yellowstone a few miles above Glendive, Montana; it 

 occurs in the Hell Creek region, and also on the Missouri River, 

 near the town of Brockton, Montana. 



On the lower Cannon Ball River, for a distance of ten or twelve 

 miles above its mouth, the Fox Hills formation is exceptionally 

 well shown. In many places it forms cliffs rising abruptly from 

 the water's edge, and the cuts made for the new branch line of the 

 Northern Pacific afford many excellent exposures. It rises eighty 

 to ninety feet above the Cannon Ball River, or approximately 

 1,680 feet above sea-level. 



The Fox Hills sandstone when unweathered is gray with yellow 

 patches, but in weathered outcrops it is yellow or brown in color. 

 The rock is rather fine-grained and, for the most part, so soft and 

 friable that it can be crumbled in the hand. Cross-bedding is 

 very common and the rock contains great numbers of large and 

 small ferruginous sandstone concretions or nodules, many of these 

 likewise exhibiting cross-bedding. The nodules are apparently 

 due to the segregation of the iron into irregular patches cementing 

 the sand into firm, hard masses, considerably harder than the 

 sandstone in which they are imbedded. In many places the iron 

 has impregnated certain layers and formed indurated ledges, 

 which resist weathering and project beyond the softer portions 

 (Fig. 1). The nodules vary in size from an inch and less to six 

 and eight feet. Small, irregular, twisted or stem-like forms are 



