THE GEOLOGY OF NORTH DAKOTA 19 



of the lignite beds have a thickness of 20 feet and one of 35 feet. 

 That the coal swamps recurred repeatedly in many parts of the 

 region is proved by the presence in some vertical sections of as 

 many as 15 or 20 lignite beds, the majority of them, it is true, of no 

 great thickness. Among the lakes and swamps there grew many 

 varieties of trees, including, according to Knowlton,^ the poplar, oak, 

 walnut, fig, elm, maple, birch, alder, dogwood, hickory, box elder, 

 buckthorn, viburnum, witch-hazel, horse chestnut, and bittersweet. 

 Interspersed with these were scattered conifers and ginkgos. Thus 

 during Fort Union time North Dakota and adjoining areas were 

 covered with dense forests. Osborn thus describes the region as 

 it was at this period: "Vast stretches of subtropical and more 

 hardy trees were interspersed with swamps where the vegetation 

 was rank and accumulated rapidly enough to form great beds of 

 lignite. Here were bogs in which bog iron was formed. Amid the 

 glades of these forests there wandered swamp turtles, alligators, 

 and large lizards of the characteristic genus Champsosaurus.'^^ 



White River formation. — The deposition of the Fort Union sedi- 

 ments was followed by a long erosion interval during which hundreds 

 of feet of strata were removed. Erosion was going on during most 

 of the Eocene period, and a well-marked unconformity therefore 

 separates the Fort Union from the overlying White River formation 

 of the Oligocene. 



The White River beds occupy a number of small and widely 

 scattered areas west of the Missouri River. It is necessary to 

 exaggerate the size of these areas in order to represent them on the 

 geologic map (p. 7). The formation is especially well exposed in 

 White Butte, in Slope County, where it covers an area of 8 to 10 

 square miles, forming the highest portion of the divide at the head- 

 waters of the north fork of the Cannonball River and Sand and 

 Deep creeks. It is here composed of white clays at the bottom, 

 on which rests a coarse sandstone filled in places with large pebbles ; 

 this is overlain by about 100 feet of calcareous clays, which in turn 

 are overlain by more than 100 feet of fine-grained greenish sand- 

 stone. These deposits represent all three members of the White 



' F. H. Knowlton, U.S. Geol. Survey, Bull. No. 611, 1915, p. 59. 

 ^ H. F. Osborn, The Age of the Mammals, p. 100. 



