ROCKY MOUNTAIN STRUCTURE IN IDAHO 467 
the discussion of overthrusts to regard the lower block as passively 
overridden by the upper or thrust block, it is probable that both 
participate in the movement, the separated parts moving past 
each other, as suggested by Barrell. 
Horizontal thrusting.—The original nearly horizontal attitude 
of the Bannock thrust plane has been modified by subsequent 
compression and folding, but it indicates that the effective deforma- 
tive forces acted horizontally and were not the surface expression 
of obliquely emerging, deep-seated shear, such as was postulated 
by Willis? for the fault zone along the east side of the Sierra Nevada 
Mountains. 
Factors in deformation.—Chamberlin and Miller}? have shown 
from their own experiments and from the earlier work of Cadell, 
Willis, Adams, and others that many factors are involved in the 
production of low-angle faulting, such as is exemplified in great 
overthrusts. Among these may be mentioned: (1) rotational 
strain; (2) increase in resistance to deformation with depth; 
and (3) a relatively large ratio of thrust to weight. 
(1) Rotational strain as a factor in the deformation of south- 
eastern Idaho is clearly indicated by the frequency of inclined or 
overturned structures. 
(2) Although no data are available regarding conditions in 
depth, it is clear from the horizontality of the thrusting previously 
mentioned and from the locally fractured and generally unmeta- 
morphosed condition of the strata, that the deformation took 
place at no great depth. The visible structures at least were 
developed in the zone of fracture. No evidence of flowage has 
been found. 
(3) The great horizontal displacement produced by the Bannock 
overthrust shows that the thrust was enormous. The weight, 
on the other hand, could not have been very great because of the 
apparent shallowness of the deformation. 
t Joseph Barrell, ““The Upper Devonian Delta of the Appalachian Geosyncline,”’ 
Am. Jour. Sci. (4th ser.), Vol. XX XVII (1914), p. 107. 
2 Bailey Willis, “Structure of the Pacific Ranges, California,” Bull. Geol. Soc. 
America, Vol. XXX (1919), pp. 84-86. 
3R. T. Chamberlin and W. Z. Miller, ‘““Low-Angle Faulting,” Jour. Geol., Vol. 
XXVI, No. 1 (1918), pp. 1-44. 
