702 R. W. RICHARDS AND G. R. MANSFIELD 
of the great thrust fault to the north. This interpretation is 
tentatively shown on the map (Fig. 1). 
MAGNITUDE OF THE BANNOCK FAULT 
The trace of the Bannock fault.as above constituted with its 
major sinuosities as represented on the map (Fig. 1) extends 
approximately 270 miles from the vicinity of Woodruff, Utah, to 
the region north of John Grays Lake. The general trend of the 
fault trace is slightly west of north and the direction of movement 
must have been perpendicular to that trend and, as indicated in 
the above discussion, was probably from the west. 
The structure of the underlying block, as shown in the moun- 
tainous portion of the region, comprises a series of folds, for the 
most part close and overturned toward the east. Figs. 4 and 5 
(photo and cross-section) show folds in the overridden Twin Creek 
limestone in the upper waters of Montpelier Creek. The large 
alluvial area at the north end of Bear Lake Valley and extensive 
areas masked by Tertiary detrital deposits render the structure of 
the underlying rocks problematical. This structure must, however, 
be determined before the place of origin and the amount of 
displacement effected by the thrust can be satisfactorily deter- 
mined. It has been pointed out that in the Georgetown Canyon 
region the missing formations indicate a minimum vertical dis- 
placement in that locality of about 8,500 feet. On the west side 
of Bear Lake where Cambrian or Ordovician quartzites overlie 
Woodside and Thaynes formations the minimum throw on the same 
basis would probably be at least 12,000 feet. 
The structure section (Fig. 5) along the line A—B contains one 
of the best exposures of the underlying block and a minimum 
amount of cover. The section represents the supposed attitude 
of the deformed fault plane. 
Cambrian rocks nowhere rise to the level of the fault surface in 
the region traversed by the section. It therefore follows that the 
Cambrian portion of the overthrust block at the extreme west end 
of the section must have been derived from a fold lying to the 
west of this point, the structure of which is concealed by the 
overthrust block. 
A clue to the minimum horizontal displacement is suggested by 
the possibility that the Mississippian limestones of the western 
