THE GEOLOGY OF NORTH DAKOTA 17 



One of the most conspicuous features of the Fort Union is the 

 vast quantity of baked and fused rock, or cHnker, produced by the 

 heat of the burning hgnite beds. The overlying shales and sand- 

 stones have been burned to a red or pink color and in places com- 

 pletely fused to slaglike masses. This clinker caps many of the 

 ridges and buttes of the region, having protected them from erosion 

 by its superior hardness. The beds of clinker vary in thickness 

 from 5 or 6 to 40 feet and over, the thicker masses probably having 

 been produced by the burning of several lignite beds and the 

 baking of the intervening shales, all now forming a single bed. 



The Fort Union everywhere contains numerous beds of lignite. 

 These vary in thickness from an inch and less to 35 feet, beds 6, 8, 

 and 10 feet thick being common. Many of the lignite beds cover 

 large areas. On€ with a thickness of from 5 to 16 feet has a known 

 extent of 20 miles in one direction and 25 in another, thus covering 

 an area of at least 500 square miles. Another is known from its 

 outcrops to have an area of over 900 square miles, and its probable 

 extent is from 1,000 to 1,500 square miles. This bed ranges in 

 thickness from 9 to 1 5 feet and over. The lignite is found from top 

 to bottom of the Fort Union, and since it is present also in the upper 

 part of the Lance formation it has a vertical range of from 1,200 

 to 1,300 feet. 



The Fort Union, which is Lowest Eocene, or Paleocene, in age, 

 contains an abundant flora, about 300 species of plants having 

 been described from this formation. It has a fauna comprising 

 both fresh-water shells and vertebrates. Among the former are 

 Unio priscus M. and H., Viviparus trochiformis M. and H., Vivi- 

 parus leai M. and H., Viviparus retusus M. and H., Campeloma 

 multilineata M. and H., Campeloma producta White, and Corhula 

 mactriformis M. and H. Among the vertebrate remains are fishes, 

 turtles, the aquatic reptile Champsosaurus laramiensis, and mam- 

 malian teeth. 



A large part of the Fort Union formation has been removed by 

 erosion and only in the higher buttes and divides of western North 

 Dakota has the upper part been preserved. In Billings and Golden 

 Valley counties it has a thickness of 1,000 feet, while farther north, 

 in McKenzie County, the formation reaches 1,300 feet in thickness. 



