542 D. F. HEWETT 



that include the beds from the Bighorn Hmestone (Ordovician) 

 to the Embar group (Permian and Pennsylvanian) , the sum of 

 whose average thicknesses in this region is about 1,280 feet. Most 

 of this thickness is beds of dense homogeneous gray hmestone that 

 range from 5 to 50 feet thick. This group is to be regarded as the 

 most competent unit in the entire section. 



The second group includes several distinct sandstone formations, 

 such as the Chugwater (Triassic) about 750 feet thick, and Mesa- 

 verde formation (Upper Cretaceous), which is largely sandstone 

 and about 1,200 feet thick. Both of these formations include some 

 shale. As the Chugwater formation overlies the Paleozoic lime- 

 stones and sandstones, it would tend to increase the strength of 

 those beds. 



The third and weakest group includes the remaining formations. 

 The Deadwood formation (Cambrian) made up of shale and sand- 

 stone is 700 feet thick. The Mesozoic and Tertiary formations 

 (12, 14) that are made up of thin soft sandstones, locally con- 

 glomerate, sandy shale, and shale include the Sundance, Morrison, 

 Cloverly, Thermopolis, Mowry, Frontier, Cody, Meeteetse, Lance, 

 Fort Union, and Wasatch formations, 13,600="= feet thick. 



It will be noted that the most competent unit is that which 

 includes beds that range from the Bighorn limestone (Ordovician) 

 to the Chugwater sandstones (Triassic) about 2,030 feet thick. 

 If this section be compared with others of regions in which large 

 overthrust faults have occurred, the thinness of the competent 

 part of the section is impressive. 



The characteristics and structural relations of those beds only 

 that may bear upon the period of overthrusting will be presented 

 here. The recognition of the Fort Union formation on the west 

 side of Bighorn Basin is based upon numerous collections of leaves 

 that have been studied and identified by F. H. Knowlton. The 

 base is considered to be a conglomeratic sandstone, which is locally 

 unconformable on underlying beds that range from the middle 

 part of the Meeteetse formation (roughly equivalent to the Judith 

 River formation of the Montana group) to the top of the Lance 

 formation ("Ceratops beds"). This unconformity has only been 

 recognized in one locality east of Oregon Basin for a distance of 



