ROCKY MOUNTAIN STRUCTURE IN IDAHO 455 
The Blackfoot fault.—This fault, which is illustrated in part in 
Figure 6, takes its name from the Blackfoot River, which it crosses 
at the upper entrance to the Narrows. ‘The relations of the fault 
at this point indicate that its plane dips about 33° south. It is 
/acktoot Fiver 
Or "y Ridge 
C 
| 
| 
mm ee ae 
Overthrust 
Section MM 
Section KK 
Fic. 6B.—Sections along the lines K—-K’ and M-M’ of Fic. 64 
(eo) ) 
(2) {9) 
(oe) 
Ct 
regarded as a thrust fault and is supposed to have originated in a 
transverse anticline, located near the line along which occurred 
the maximum yielding of the rocks to the compressive earth 
stresses of the region. ‘The anticline broke and the southeast limb, 
which became the upper fault block, swung northward about a 
pivot located in the vicinity of Timothy Creek in the Freedom 
