THE HEART MOUNTAIN OVERTHRUST, WYOMING 543 



15 miles where a maximum of about 2,000 feet of beds were here 

 eroded before the deposition of the lowest Fort Union beds. The 

 contact of these beds with the underlying Lance formation in a belt 

 50 miles southeast, however, yields no evidence of unconformity. 

 The unconformity indicates local warping and erosion of the Lance 

 and older beds before the deposition of the lowest Fort Union 

 sandstone. 



The top of the Fort Union formation in this region is a persistent 

 unconformity at the base of beds that yield a large mammalian 

 fauna until recently called Wasatch but now known to be charac- 

 teristic of the Wind River formation (8, 9). 



The unconformity is readily recognizable at every locahty 

 where dip cross-sections of the beds may be seen, but in strike sec- 

 tions it cati only be detected by close study of the lithological 

 features. 



Within these limits, the Fort Union formation attains a maxi- 

 mum thickness of more than 5,250 feet and is made up of many 

 beds of pale yellowish buff and white sandstone alternating with 

 gray, olive, and red shale. The sandstones of the lower 200 feet 

 commonly contain lenses of pebbles of many rock types. Black 

 and gray chert predominate but red and gray quartzite, pale pink 

 porphyry, gray sandstone, and siHcified wood are common. Pink 

 granite and coal pebbles are sparingly present but limestone has 

 never been found. The chert pebbles have yielded an interesting 

 collection of invertebrate fossils which are characteristic of the 

 Madison and Embar formations. At least three coal beds occur 

 in the lower 600 feet of the formation on the west side of the basin. 

 Thus far, in this region, the formation has yielded a single verte- 

 brate bone, but no invertebrate fossils. No bentonite or volcanic 

 ash have been recognized in it. 



There are good reasons for believing that the beds here con- 

 sidered as the Fort Union formation are part of an extensive sheet 

 of sediments spread over a large area of Wyoming and Montana, 

 at least as far west as the Rocky Mountains, and eastward into the 

 Dakotas. They probably covered the site of the present Bighorn, 

 Bridger, and Owl Creek Ranges. These beds are so involved in the 

 folds of the border belt of the basin that it is concluded that these 



