ROCKY MOUNTAIN STRUCTURE IN IDAHO 447 
somewhat similar to those of the southern Appalachians. The 
Idaho folds, however, appear to be less regular in form than those 
of the Appalachian region. The intensity of the folding may be 
judged by the fact that within the region of the seven quadrangles 
mapped there are forty-two folds or groups of folds that have been 
considered of sufficient importance to receive names and to merit 
individual treatment in a detailed description of the region. 
The influence of the transverse folds is seen chiefly in the widen- 
ing or constriction of the longitudinal folds, in the production here 
and there of canoe- or cigar-shaped folds, and in the zigzag outcrop 
of certain formations, which cross the axes of the longitudinal folds. 
The principal faults of the region are reverse and are doubtless 
chiefly associated with the Bannock overthrust, which has a 
length probably greater than 270 miles and a horizontal displace- 
ment certainly not less than 12 miles and perhaps greater than 35 
miles. Normal faults are numerous and have produced a wide 
range of effects upon the pre-existing structures. Possibly some 
of the faults now regarded as reverse may prove to be normal. 
The intensity of the faulting is suggested by the fact that about 
sixty faults or groups of faults are sufficiently noteworthy to receive 
individual consideration in a detailed description of the region. 
About half of these are thrusts associated with the Bannock over- 
thrust. 
SPECIAL STRUCTURAL FEATURES 
The structures to which attention is especially directed in this 
paper are (1) noteworthy unconformities; (2) ‘‘swallowtail”’ folds; 
(3) the Bannock overthrust; (4) the Blackfoot fault; (5) drag 
folds; (6) fan folds; and (7) the Meadow Creek graben. ‘These 
will be described in the order named. 
Noteworthy unconformities—A very marked unconformity 
occurs in the southeastern part of the Montpelier quadrangle, 
where strongly folded Triassic and Jurassic beds pass beneath 
gently folded or nearly horizontal beds of the Wasatch formation 
(Eocene). This unconformity is the most striking of all the 
unconformities of the region. It represents at least the great 
post-Cretaceous mountain-building epoch of the northern Rocky 
