THE GEOLOGY OF NORTH DAKOTA ii 



corner of the state, contain numerous calcareous concretions varying 

 in size from a few inches to 6 and 8 feet in diameter. Many of 

 these are rich in marine shells, including the following: Scaphites 

 nodosus Owen vars. hrevis and plenus, Anisomyon patelUformis M. 

 and H., Inoceramus cripsi var. barahina Morton, Avicula lingucB- 

 formis E. and S., Ostrea pellucida M. and H., Chlamys nebrascensis 

 M. and H., Yoldia evansi M. and H., Nucula cancellata M. and H., 

 Lucina occidentalis Morton, Protocardia subquadrata E. and S., 

 Callista deweyi M. and H., and Nautilus dekayi Morton/ 



Below the upper beds of the Pierre, fossils are found only 

 sparingly, those which do occur being Baculites ovatus Say, Scaphites 

 nodosns Owen, and Inoceramus sagensis Owen. 



The position of the western boundary of the large Pierre area 

 is known only approximately, since the region is heavily drift- 

 covered and there are practically no outcrops. Not far west of the 

 boundary as represented on the map (p. 7) Fort Union lignite 

 beds are known to occur, so that the Pierre shale is believed to 

 disappear beneath the overlying Fort Union about as shown. It 

 will be noted that no areas of Fox Hills or Lance are represented 

 on the map along most of the margin of the Pierre shale. These 

 formations are not known to underlie the drift farther north than 

 indicated on the map, and in the absence of information regarding 

 their presence in the central and northern parts of the state they are 

 not mapped in that region. If they are actually absent from that 

 area it would of course mean an unconformity between the Pierre 

 and the overlying Fort Union. 



The Pierre is the thickest Cretaceous formation in North 

 Dakota, reaching 1,000 to 1,100 feet or over. It is not likely that 

 its entire thickness is represented throughout most of the large 

 eastern area, since the formation had undergone great erosion 

 before being covered by the glacial drift and hundreds of feet were 

 doubtless removed in many places. The aggregate thickness of the 

 Benton, Niobrara, and Pierre is considerably greater in North 

 Dakota than in northeastern South Dakota. In the latter region 

 the aggregate thickness of these formations ranges from 700 to 900 

 feet, but they become thicker to the north and northwest, where 



^ Identified by Dr. T. W. Stanton. 



