THE BANNOCK OVERTHRUST 607 
northeast of the village of Georgetown (see map). In ro11 this 
district was visited by the writers and the fault was studied and 
mapped in greater detail. From the north fork of Georgetown 
Canyon, where the fault emerges from beneath the late con- 
glomerates, its sinuous course was followed across the Preuss Range 
into Crow Creek as far as the mouth of Sage Creek (Fig. 2), a 
distance of approximately 30 miles, and here it appeared to continue 
northward. 
The sinuosity of the fault trace is due not only to erosion but 
to deformation as well. In the north fork of Georgetown Canyon 
an anticlinal axis has arched the thrust surface or plane so that it 
has been partly removed and the underlying Nugget and Twin 
Creek formations are exposed in the valley, while long strips of 
heavy Mississippian limestone reach down the spurs of the ridges 
like the fingers of some giant hand (see Figs. 1 and 2). A synclinal 
axis depresses the thrust surface where it passes beneath the Preuss 
Range in the headwaters of Montpelier Creek. In this region 
there is a marked contrast between the massive and castellated 
limestones of the Mississippian that constitute the upper part of 
the ridge and the chippy and shaly limestones of the Twin Creek 
formation that are exposed along the lower slopes of the valley side. 
In Georgetown Canyon and southeast across the Preuss Range 
(Fig. 3) the same relations obtain. The underlying Twin Creek 
beds are folded so that the stratigraphic throw cannot be obtained 
with accuracy, but the missing formations, including Pennsylvanian 
to Triassic (2) rocks (Nugget), represent a minimum vertical dis- 
placement of about 8,500 feet. Eastward the throw apparently 
diminishes partly by the agency of branching faults and partly on 
account of the folded structure of the rocks in the upper block. 
The general trend of the trace of the fault appears to be a few 
degrees to the west of north and the direction of thrust a little to 
the north of east. The dip of the fault surface gives little aid in 
determining the direction of movement because of its present 
deformed condition. The older rocks, however, lie to the west 
and the thickness of the thrust block appears to increase in that 
direction. 
The distance, perpendicular to the general trend, between the 
