CRETACEOUS AND TERTIARY FORMATIONS 543 



WESTERN BURLEIGH COUNTY, ABOUT 350 FEET ABOVE THE MISSOURI RIVER 



Viviparus retusus M. & H. Unio sp. fragments. 



Campeloma producta White. Corbula mactriformis M. & H. 



Campeloma multilineata M. &. H. Viviparus multilineata M. &. H. 



Vertebrate fossils are rare in the Fort Union formation. In 

 western North Dakota, near Medora, a few bones were collected 

 which were identified by Mr. J. W. Gidley as those of fishes, turtles, 

 and t!-c aquatic reptile, Champsosaurus laramiensis. The latter 

 has been found by Mr. Barnum Brown in the Lance beds of the 

 Hell Creek region, and also in the "lignite beds" just below the 

 typical Fort Union. 1 



WHITE RIVER BEDS 



The White River beds of the Oligocene occupy three small areas 

 in southwestern North Dakota, and several in the southeastern 

 corner of Custer County, Montana. The beds of this group are 

 found in White Butte, in southeastern Billings County, where 

 they cover an area from eight to ten square miles in extent, forming 

 the highest portion of the divide at the head waters of the North 

 Fork of the Cannon Ball River and Deep and Sand creeks. Ero- 

 sion has here left two ridges about two miles apart, with an eleva- 

 tion of 300 to 400 feet above the surrounding plain. Three miles 

 to the west, on the opposite side of the valley of Sand Creek, 

 Black Butte rises 450 feet above the creek, being capped by the 

 same sandstone as that forming the top of the other high buttes 

 of the region. But the beds of the White River group are wanting 

 on Black Butte, although occurring at a considerably lower level 

 only three miles to the east. In White Butte they are, however, 

 seen resting directly on this upper sandstone of the Fort Union, 

 which outcrops at several points near the base of the western slope 

 of the western ridge and also at its northern end. This sandstone 

 here dips strongly to the east so that within a distance of three 

 miles its dip carries it from the top of Black Butte to the base 

 of the ridge on the opposite side of the valley, where it is over 

 200 feet lower. 



1 Bull. Am. Mus. Nat. Hist., XXIII (1907), 835. 



