i6 A. G. LEONARD 



The relation of the Lance formation to the underlying Fox Hills 

 is of importance as bearing on the age of the former. In many 

 localities in Wyoming, Montana, and South Dakota, where the 

 contact has been observed, the Lance rests unconformably on the 

 Fox Hills . On Little B ea ver Creek in northwestern B o wman County 

 the Fox Hills sandstone had undergone erosion before the deposition 

 of the Lance beds, and although it has been questioned whether 

 this erosion represents a long time interval, the relationship else- 

 where suggests the probability that the unconformity here may 

 represent a considerable interval. 



Lying as it does on the border line between the Cretaceous and 

 Tertiary, with its Fort Union (Tertiary) flora and its vertebrate 

 fauna of Mesozoic types, the age of the Lance formation is still 

 undetermined. Some would place it in the Tertiary while others 

 would include it with the Cretaceous on account of its dinosaurs 

 and other Mesozoic forms. 



TERTIARY SYSTEM 



Fort Union formation. — Overlying the Lance and resting in some 

 places on the Cannonball marine member, in others on the Ludlow 

 lignitic member, is the Fort Union formation. Deposition went 

 on without interruption from the beginning of Lance time to the 

 close of Fort Union time, during which over i,8oo feet of sediments 

 accumulated, all of continental origin except those of the Cannon- 

 ball marine member. 



The Fort Union formation, which covers the greater part of 

 western North Dakota, contains most of the lignite deposits of the 

 state, the lignite beds of the Lance being relatively unimportant. 

 Outcrops are abundant in the Little Missouri badlands, as well as 

 along the Missouri and other streams of the region, and these 

 extensive outcrops afford exceptional opportunity for the study of 

 the formation. The Fort Union is remarkably uniform in color, 

 composition, and general appearance over many thousands of square 

 miles. It is composed of shales alternating with soft sandstone 

 and contains numerous beds of lignite. The prevailing colors are 

 light ash gray and yellow, but some layers are nearly white. 



