264 G. R. MAXSFIELD STRUCTURE OF ROCKY MOUNTAINS 



remarks that follow are based chiefly on work in Idaho, l^nt some mention 

 is made of Montana and of other regions for comparison. 



Detailed or semi-detailed study has been made of three separate dis- 

 tricts in eastern Idaho, namelv : (1) the Fort Hall Indian Eeservation, 

 (2) an area comprising 7 quadrangles lying about 15 miles east of the 

 Fort Hall Indian Eeservation and extending to the southeastern corner 

 of the State, and (3) an area lying immediately west and south of the 

 Teton basin. Since some of the structural features of these areas have 

 already been described elsewhere, only brief reference need be made to 

 them here, and the paper will be devoted chiefly to broader aspects of 

 mountain structure suggested by the detailed studies. 



Fort Hall Ixdiax Reservatiox - 



The Fort Hall Indian Reservation comprises an area of about 800 

 square miles in Bingham, Bannock, and Power counties and includes 

 jDarts of several mountain ranges. The older sediment-ary rocks, of 

 Paleozoic and Mesozoic age, which make up these ranges are overlapped 

 by Tertiary and Quaternary sediments and by igneous rocks as the ranges 

 descend northward to the Snake River plain. 



The chief mountain-building activity in the reservation occurred after 

 the deposition of the Jurassic beds, the latest Mesozoic rocks there ex- 

 posed, and before the deposition of the Tertiary rocks, which are believed 

 to be of Pliocene age, but these last have been affected to a certain extent 

 by subsequent movements. The major deformation was undoubtedly a 

 part of the great Laramide Revolution. The Paleozoic and Mesozoic 

 strata have been folded, locally with considerable intensity, and the folds, 

 which have in o^eneral a northwesterly trend, have been inclined or even 



c 



overturned northeastward. Transverse folding at right angles to the 

 general trend is also present. Many of the folds are broken by thrust- 

 faults and one overthrust-fault of considerable magnitude, the Putnam 

 overthrust, has been recognized and described. Rock slices or cappings 

 in the Portneuf Range, in the southeastern part of the reservation, which 

 represent parts of an overthrust block now detached Idv erosion, are be- 

 lieved to be associated in some way with this great overthrust. Similar 

 rock slices in the northern part of the Bannock Range, farther west, and 

 in the unnamed range east of Bannock Valley, in the southwestern part 

 of the reservation, may also be connected with this overthrust, but con- 



- G. R. Mansfield : Geography, geology, and mineral resources of the Fort Hall Indian 

 Reservation, Idaho, with a chapter on water resources by W. B. Heroy. U. S. GeoL 

 Survey Bull. 713, 1920. 



