THE GEOLOGY OF NORTH DAKOTA 1$ 



the heavy covering of drift the eastern boundary of the area is 

 probably only approximately correct. 



Not only is the Cannonball marine member well exposed along 

 the river from which it is named, but it is also well shown in many 

 outcrops along the Heart River in the vicinity of Mandan and in the 

 bluff of the Missouri near Bismarck. It is composed of dark gray to 

 black shale, arenaceous in part, with a subordinate amount of gray 

 and yellow sandstone. Some thin beds of limestone are also present. 



After the deposition of the Fox Hills sandstones the Cretaceous 

 sea withdrew from the region for a time and the beds constituting 

 the lower part of the Lance were formed. These and the Ludlow 

 lignitic member of the western area are of continental origin and 

 were probably formed in part by rivers, though much of the forma- 

 tion may be of lacustrine origin. As the lakes were filled with sedi- 

 ment, they were converted into swamps in which trees and other 

 vegetation grew and accumulated year after year to form the beds 

 of lignite which are characteristic of the upper non-marine portion 

 of the Lance. But while this Ludlow lignitic member was being 

 deposited to the west the sea again entered the region, per- 

 haps from the east, and in its waters were formed the Cannonball 

 marine member, with its abundant fauna of marine shells, which 

 Dr. Stanton characterizes as a modified Fox Hills fauna.' That 

 this sea may have extended westward almost to the Montana line 

 is suggested by the occurrence in the Little Missouri badlands of 

 western Slope County of a bed containing Ostrea glabra and Ostrea 

 suUrigonalis . Since these oysters are brackish- water forms the 

 open sea was probably not far distant, and the shale bed containing 

 them appears to be the westward extension of the Cannonball 

 marine member. The zone in which the shells are found lies 

 1 20 feet below the top of the Ludlow lignitic member. Following 

 the deposition of the Cannonball marine member the sea withdrew 

 from the region never to return again. 



The thickness of the Lance formation along the Little Missouri 

 is about 820 feet, the upper 300 feet representing the Ludlow lig- 

 nitic member. In the Cannonball River region the Lance is about 

 700 feet thick. 



' E. R. Lloyd and C. J. Hares, op. cit., p. 537. 



