266 G. E. MAXSFIELD STRUCTURE OF ROCKY MOUNTAINS 



more important structural types of the area. These need not be repeated 

 here. Suffice it to say that they include swallowtail folds, drag folds^ 

 fan folds, both upright and inverted, a transverse fold passing into an 

 overthrust fault, an imbricated and folded ' overthrust fault, and horst 

 and graben structure. 



Teton Basin 



The Teton Basin area, which lies about iO miles north of the Freedom 

 quadrangle, along the Wyoming border, comprises an area of approxi- 

 mately 300 square miles in Madison. Teton, and Bonneville counties. 

 The eastern side of the basin is formed by the west flank of the Teton 

 Eange and the western side is bounded by the Big Hole Mountains, which 

 include a small coal-bearing area known as the Teton coal field. These 

 mountains, like those in the other two areas previously mentioned, have 

 a northwesterly trend and show unsymmetrical form with inclination or 

 overturning toward the northeast. A number of thrust-faults traverse 

 the area. Two of them, the Absaroka and Darby faults, have considerable 

 magnitude. An outlying mass of the Darby overthrust block, separated 

 from it by erosion, lies in front of the main block and testifies to the 

 gentle inclination of the fault-plane, which is elsewhere concealed hj 

 debris from the overlying block. A general account of this area, includ- 

 ing additional structural details, has already been published.* 



Idaho and the northern Rockies 



It has been shown that eastward inclined folds and eastward over- 

 thrusts are characteristic of the Eock}' Mountains in Idaho. Similar 

 structures have been observed by the writer in Montana, and it is believed 

 that they are fairly characteristic of the northern Rockies as a whole. 

 Certainly, remarkable eastward overthrust faults have been recognized 

 at a number of places from Canada as far southward as Utah and Colo- 

 rado. These may now be briefly reviewed (see figure 1). 



Thrust-faults of the northern Rockies 



The Lewis overthrust, first described by Willis^ and later mapped by 

 Campbell, has a minimum horizon t-al displacement of 15 miles. It has 

 a sinuous course for many miles along the Rocky Mountain front in 



* G. R. Mansfield : Coal in eastern Idaho. U. S. Geol. Survey Bull. 716-f, 1920. 



5 Bailey "Willis : Stratigraphy and structure, Lewis and Livingston ranges, Montana. 

 Bull. Geol. Soc. America, vol. 13, 1902, pp. 331-336. 



M. R. Campbell : The Glacier National Park. U. S. Geol. Survey Bull. 600, PI. 13, 

 1914. 



