456 GEORGE ROGERS MANSFIELD 
quadrangle, about 5 miles east of the area illustrated. The rock 
formations cross this creek without apparent displacement by the 
fault, but they make a pronounced bend, which is favorably located ~ 
to mark the position of the unbroken portion of the axial zone 
of the anticline. 
The Blackfoot fault has a known length of about 13 miles west- 
ward from the point of origin above indicated. At this distance 
it disappears beneath basalt. The variations in dips on the 
flanks of the big anticline in the upper thrust block, some of the 
strata being locally overturned, and the presence of minor folds, 
make it difficult to determine the throw of the fault. The maxi- 
mum observed effect is produced where it cuts the big anticline. 
The fault is probably offset beneath cover by one of the normal 
faults shown in the southwestern part of the area. 
The upper fault block affords an unusually fine example of the 
manner in which the outcrops of a formation, such as the Phos- 
phoria, occurring on opposite limbs of an anticline, are spread by 
the uplift and erosion of a fault block, in which the anticline is 
included. In the lower block to the north the corresponding 
outcrops are much nearer together. 
The structure section along the line KK’ crosses the Blackfoot 
fault. It shows the anticlinorial and synclinorial character of the 
folds, their overturning toward the northeast, and the manner in | 
which some of the subsidiary thrusts are thought to have originated. 
The thrusts are presumed to pass into the Bannock overthrust, 
which underlies this district. 
Drag folds.—Folds of this type, usually sharp and unsymmetri- 
cal, occur at a number of places in the region. Several of them are 
shown in Figure 6 in connection with the big anticline in the upper 
fault block. One of them is crossed by the line of structure section 
MM’. Here upper beds of the Brazer limestone (Mississippian) 
in small sharp folds are locally exposed by the erosion of the over- 
lying Wells formation. 
Drag folds on a somewhat larger scale occur on the hills south 
of Montpelier Canyon about 3 miles east of Montpelier. In 
Figure 7A, a view south along the west flank of Waterloo hill 
shows an anticline overturned eastward in which curving beds of 
