BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 34, pp. 263-284 JUNE 30, 1923 



STRUCTURE OF THE ROCKY MOUNTAINS IN IDAHO 



AND MONTANA^ 



BY GEORGE ROGEES MAXSFIELD 



{Presented before the Society Decemhei' 28, 1922) 



CONTENTS 



Page 



Introduction 263 



Fort Hall Indian Reservation 264 



Area of seven quadrangles 265 



Teton basin 266 



Idaho and the northern Rockies 266 



Thrust-faults of the northern Rockies 266 



Relative magnitude and importance of overthrusts 269 



Relative ages of overthrusts 269 



Possible relationships of overthrusts 270 



Deficiency of knovt^ledge 271 



Overthrusting versus underthrusting 272 



Restoration of structures in southeastern Idaho 273 



Isostatic features 278 



Hypotheses of mountain-building , 280 



Causes of mountain-building 281 



Tensional features in mountain structure 282 



Summary 283 



Introductioi^^ 



Most of the writer's experience v^itli Rocky Mountain structure has 

 been gained in eastern Idaho in connection with the United States Geo- 

 logical Survey's study of the western phosphate field, but some years ago 

 he conducted a series of field parties from the Harvard University Sum- 

 mer School in southwestern Montana. He therefore feels some measure 

 of acquaintance with the structural conditions in that State as well. The 



1 Published by permission of the Director U. S. Geological Survey. 

 This paper is one of a series composing a "Symposium on the structure and history 

 of mountains and the causes of their development." 



Manuscript received by the Secretary of the Society January 29, 1923. 



XVIII— Bull. Geol. Soc. Am., Vol. 34, 1922 (263) 



