ROCKY MOUNTAIN STRUCTURE IN IDAHO 451 
data. The folding of the fault plane, previously announced, is 
well shown in Figure 3, both in plan and in section. Two aspects 
of this folding should be emphasized: (1) the folds of the fault 
plane are of a simpler and more open type than are those of the 
upper or lower fault blocks, as shown in the geologic structure 
section; (2) the fault plane previous to its deformation must have 
been nearly horizontal, else moderate folding would not have 
raised it to the level of erosion. 
The underlying block is in most places composed of Mesozoic 
rocks more or less intensely folded, whereas the upper block is 
more largely composed of Paleozoic rocks that in the main are more 
competent strata. Mesozoic rocks, however, form part of the 
upper block at many localities. The relation of upper to lower 
block is shown in Figure 4, which represents the faulted area on 
the boundary line between T. 11 S., R. 44 E., and T.11S., R.45E., 
Figure 3. The fault there shown is regarded as a branch of the 
Bannock overthrust and is so represented on the map, but it may 
be a window in the main fault plane, as is supposed in the case of 
the area surrounded by a fault in T. 9 S., R. 44 E. 
The Bannock overthrust is in some places a single fault plane, 
as in part of the area shown in Figure 3, but in other places it 
becomes complex and is really a fault zone composed of a number 
of rock slices separated by faults. Several branches are shown in 
Figure 3, and the branch that separates the Carboniferous from 
the Triassic rocks in T. 11 S., R. 45 E., is locally overturned and 
dips eastward. Three branches of the overthrust are shown in 
Figure 2. The East Stump branch, where crossed by the line of 
section KK’, is practically vertical, but farther northwest it is 
overturned and dips northeastward. The West Stump branch, 
where crossed by the lines of sections KK’ and LL’, is also prac- 
tically vertical, but at Boulder Creek it dips northeastward. 
Figure 5 shows a complex portion of the fault zone near St. 
Charles, west of Bear Lake. There are probably no less than six 
faults which divide the rocks into roughly parallel slices east of 
the belt of Brigham quartzite, which is the easternmost formation 
of the upper fault block. The trace of the west branch of the . 
fault in this district and southward lies east of a series of 
