THE HART MOUNTAIN OVERTHRUST 53 
overturned folds, in places carries the Pre-Cambrian granite out 
over the “Red Beds.”’ The fault planes could not actually be 
observed, but are undoubtedly much steeper than the plane of the 
Hart Mountain thrust. The amount of horizontal displacement 
was not determined. 
Mechanics of the faulting.—While no theoretical discussion of 
the mechanics of these great faults is proposed, it seems worth 
while to present some observations which may ultimately help to 
throw light on the problem: 
1. The fault contact is practically everywhere concealed by 
talus from the Madison cliffs, but at several places it could be 
located within a few feet, and the zone of crush breccia is notably 
thin at most points. 
2. The great limestone block above the fault plane is little 
folded. But while it presents the general aspect of a nearly flat- 
lying horizon, locally the dips are high, as a result of numerous 
small normal faults which appear to have been the result of the 
settling of the great block after the thrust ceased. 
3. The soft shales below the major thrust plane, while much 
crumpled at places, are nearly horizontal and almost undisturbed 
over considerable areas where exposed by erosion several hundred 
feet lower than the major thrust. 
HISTORY 
The history of the faulting in this region depends, for its correct 
solution, on the careful determination of the age of the beds herein 
called Fort Union(?) and on a knowledge of the relation of these 
beds to the faulted block of Madison limestone. The second part 
of the problem is comparatively simple, and will be discussed first. 
For long distances these beds occur at the foot of the Madison 
scarp, and there was little question from the very first that they 
passed beneath the Madison block. In view, however, of the 
statement of Hewett that these beds are probably of Wasatch age, 
it was thought possible that the sandstone might have been de- 
posited, after the faulting and erosion, against the foot of the 
limestone scarp, since it was nowhere possible to find an actual 
contact of the Madison resting directly on the Fort Union(?). 
