lO A. G. LEONARD 



about 900 feet at this point, and the Niobrara probably includes 

 300 to 400 feet of the section. 



Pierre shale. — The Pierre shale occupies most of the eastern 

 half of North Dakota west of the Red River Valley. It also out- 

 crops along the valley of the Missouri River for a distance of over 

 20 miles north of the South Dakota line, and is brought to the 

 surface in the southwestern corner of the state by the Cedar Creek 

 anticline, which lies mostly in Montana. Throughout the large 

 eastern area it is covered by glacial drift, except where the streams 

 have cut through this mantle and exposed the shale beneath. The 

 Pierre outcrops in many places along the James and Cheyenne rivers 

 and is finely exposed along the South Branch of Park River and 

 other tributaries of the Red River in the region of the Pembina 

 Mountains. The escarpment of the Pembina Mountains is com- 

 posed largely of this shale, which is well shown in the numerous 

 ravines and gorges, while in places the underlying Niobrara is also 

 seen. The base of the Pierre as found in this region is composed of 

 black, jointed, carbonaceous shales which contain many thin layers 

 of yellow or white non-plastic clay, which has much of the con- 

 sistency of cheese. The black and yellow strata present a striking 

 appearance and seem to be characteristic of the base of the Pierre 

 over extensive areas. They appear at frequent intervals for a 

 distance of 30 miles along the Pembina Mountain escarpment and 

 extend at least 250 miles northwestward in Canada, where they 

 have been noted in the Riding and Duck mountains. They are 

 also found 130 miles south of the Pembina Mountains at Valley 

 City. The yellow or white clay seams vary in thickness from i to 

 6 inches, and the interstratified black layers from 8 to 14 inches. 

 The uniformity and extent of some of the yellow seams are remark- 

 able because they have been traced continuously for a distance of 

 35 miles, and a single clay seam 2 inches thick for 25 miles. 



The typical Pierre overlying these basal beds is a bluish-gray 

 to dark gray jointed shale of remarkably uniform character, which 

 often weathers into small flaky fragments. The rock commonly 

 shows yellow spots or stains of iron oxide. Erosion has removed 

 the upper part of the formation over most of the large eastern 

 Pierre area, but the topmost beds, where exposed in the southwest 



