OVERTHRUSTING VERSUS UNDERTHRUSTING 273^^ 



These facts accord better with the idea of overthmsting and a westerly 

 source for the tangential pressures than with the reverse supposition. 



In Cadell's experiments, which were conducted with reference to con- 

 ditions in the Scottish Highlands/* it was found that the compressed 

 mass tends first to seek relief along a series of greatly inclined thrust- 

 planes which dip toward the side from which the pressure was exerted. 

 After a certain amount of piling up has taken place along these minor 

 thrust-planes, the whole heaped up mass tends to rise and ride forward 

 bodily upon the major thrust-planes. These results correspond with the 

 actual conditions in the Scottish Highlands. The Bannock overthrust, 

 in Idaho, is marked at several places along its course by a fault-zone of 

 heaped-up rock slices inclined westward and consisting of more or less 

 broken rock folds. 



Thus far the discussion has assumed that one or the other of the two 

 blocks in an overthrust remains passive. The probabilities are, since 

 gravity plays a predominant part among the forces which produce the 

 tangential pressure, that both blocks are subject to movement, and that 

 the motion of either block with respect to the other is purely relative. 

 This idea has been well expressed by Heim,^^ who considers that the 

 curvature of lines of folds affords the best means of determining the 

 direction of this relative movement. He states that the thrust is always 

 directed toward the outside of the arc, in the same manner as an arrow 

 is aimed with a bow. 



Eestoration" of Structures in southeaster^t Idaho 



In figure 2 an attempt has been made to restore the structure of a 

 characteristic portion of the mountains in southeastern Idaho for the 

 purpose of determining the amount of circumferential shortening and of 

 studying the conditions of deformation. The line selected cuts eastward 

 across the northern part of the Aspen Eange, in the southeastern part of 

 the Henry quadrangle, and then turns northeastward across the Lanes 

 Creek and Freedom quadrangles to Star Valley a short distance south of 

 the town of Freedom. The methods of restoration and measurem9nt were 

 similar to those employed by R. T. Chamberlin-^ for studies in the Appa- 



2* B. N. Peach, J, Home, W. Gunn, C. T. Clough, and L. W. Hinxman : The geological 

 structure of the northwest Highlands of Scotland. Geol. Survey Great Britain, Mem., 

 1907, p. 475. 



25 Albert Heim : Geologie der Schweiz, Bd. I, Leipzig. 1921, pp. 647-649. 



2" R. T. Chamberlin : The Appalachian folds of central Pennsylvania. Jour. Geol.. voL 

 18, 1910, pp. 228-251. 



: The building of the Colorado Rockies. Jour. Geol., vol. 27, 1919, pp. 145- 



164, 225-251. 



