CANNONBALL MARINE MEMBER OF LANCE FORMATION 527 



point it appears to the writers that the local unconformities that 

 have been observed are such as would be expected where a marine 

 formation is succeeded by one of continental origin. 



A significant feature of the Fox Hills sandstone and underlying 

 Pierre shale in the Standing Rock and Cheyenne River Indian 

 reservations is that the fauna found in the sandstone cannot be 

 distinguished from that found in the upper part of the shale, so 

 that the strata from which the Fox Hills fauna was collected include 

 not only the Fox Hills sandstone but also about 200 feet of the 

 underlying Pierre shale. 



TERTIARY? SYSTEM 

 LANCE FORMATION 



Lower part. — It has been previously stated that the Lance 

 formation in this region consists of three parts, a lower group of 

 shale and sandstone beds of continental origin and two contem- 

 poraneous upper members, one of sandstone and shale of marine 

 origin, the other of sandstone, shale, and lignite of non-marine 

 origin. The lower part of the formation outcrops in a wide belt 

 of country in Morton County, North Dakota. Northward it 

 passes below the flood plain of Missouri River a few miles below 

 Bismarck, and to the west it has been mapped to and beyond the 

 Montana-Dakota state line. It occupies a large area in Bowman 

 and Billings "counties in southwest North Dakota and in adjacent 

 parts of Montana. Throughout this whole region it is essentially 

 uniform in character, consisting predominantly of somber-colored 

 arenaceous shale intercalated with lenticular beds of brown or 

 buff sandstone. Beds of brown carbonaceous shale, bog iron ore, 

 and thin lignite are conspicuous in most outcrops. All the strata 

 of the lower part of the Lance are lenticular in character, and a 

 section exposed at one locality is different in most of its details 

 from one even a short distance away. Cross-bedding is common, 

 especially in the sandstone. Irregularity of deposition is char- 

 acteristic. Near Solen on Cannonball River, 9 miles above its 

 mouth, the lower part of the Lance has a thickness of approxi- 

 mately 400 feet and on Little Missouri a thickness of about 525 feet. 



