THE GEOLOGY OF NORTH DAKOTA 13 



more resistant masses project beyond the softer rock, and at the base 

 of slopes and scattered over the surface they are exceedingly abun- 

 dant. Many of the concretions are spherical in shape and of good 

 size, and it is these which have given the name to the Cannonball 

 River, since they occur abundantly along that stream. 



In the southwestern area in Bowman County the Fox Hills is 

 composed of 60 feet of massive gray sandstone, which weathers to 

 a yellow color, and below this is 25 feet of sandy clay formed of light 

 and dark laminae. 



Marine fossils are quite abundant in the Fox Hills sandstone, 

 and the following are among those found in the Missouri River area 

 and identified by Dr. T. W. Stanton: Tancredia americana M. and 

 H., Callista deweyi M. and H., Ostrea pellucida M. and H., Avicula 

 linguiformis E. and S., Avicula nebrascana E. and S., Protocardia 

 suhquadrata E. and S., Tillina scitula M. and H., Mactra warrenana 

 M. and H., Scaphitis cheyennensis (Owen) and Chemnitzia cerithi- 

 f or mis M. and H. 



The Fox Hills in North Dakota ranges in thickness from 85 to 

 about 200 feet, with an average of about 150 feet. 



CRETACEOUS OR TERTIARY SYSTEM 



Lance formation. — As shown on the geplogic map (p. 7) this 

 formation occupies a considerable area on either side of the Missouri 

 River in south-central North Dakota, and a smaller area in the 

 extreme southwestern corner of the state. The beds are well 

 exposed in the bluffs of the Missouri, particularly on the west side 

 of the river 15 to 20 miles below Bismarck, and in many places 

 along the Cannonball and Heart rivers. The bluffs and badlands 

 of the Little Missouri in Slope and Bowman counties also afford 

 many excellent outcrops. 



As shown by Lloyd,' the Lance formation in south-central North 

 Dakota consists of a lower portion of shale and sandstone beds of 

 continental origin, and an upper member of sandstone and shale 

 of marine origin which is known as the Cannonball marine member. 

 Farther west in Slope and Bowman counties the upper member of 



' E. R. Lloyd, "The Cannonball River Lignite Field, North Dakota," U.S. Geol. 

 Survey, Bull. No. 541, 1914, p. 249. 



