ROCKY MOUNTAIN STRUCTURE IN IDAHO 449 
Mountains and a succeeding epoch of erosian long enough to remove 
all Eocene beds earlier than the Wasatch, if such were ever de- 
posited, the Cretaceous formations, and two of the upper Jurassic 
formations. 
A pronounced unconformity occurs between the Wayan forma- 
tion and the underlying Gannett group, both of supposed Lower 
Cretaceous age. Figure 2 shows one of the localities where this 
unconformity appears. A syncline composed of members of the 
Gannett group is overlapped on the northwest by the Wayan 
formation, composed of beds folded in a manner comparable to 
that of the Gannett group but not yet differentiated into members. 
Another exposure of the same unconformity, perhaps even more 
striking, occurs about 3 miles east of the locality shown on the 
map. 
“Swallowtail” folds ——These are folds in which the axes are 
nearly horizontal but are affected by cross folds in such manner 
that the outcropping formations as represented on the map resemble 
a swallow’s tail. A remarkable group of folds having this form 
occurs in the Slug Creek and Crow Creek quadrangles, as shown 
in Figure 3, and has great economic importance because of its 
contained beds of high-grade phosphate. The group lies in the 
arc of curvature of the trend lines previously mentioned. A trans- 
verse syncline near the northern part of the area shown on the map 
‘causes the widening of the two lateral synclines and depresses the 
axis of the intervening anticline. It also causes the widening of 
the canoe-shaped fold partly shown in the northwestern part of 
the area. Other transverse axes cause the ending of the canoe- 
shaped fold and the changes in width of the middle and southern 
portions of the ‘‘swallowtail’’ folds. The axes of the transverse 
folds are roughly radial to the curvature of trend of the ‘‘swallow- 
tail”’ folds. 
The Bannock overthrust—This great overthrust was_ first 
described in 1912." Since that date the writer has had opportunity 
to extend his observations along the fault line and to secure new 
TR, W. Richards arid G. R. Mansfield, ““The Bannock Overthrust: a Major 
Fault in Southeastern Ic'aho and Northeastern Utah,” Jour. Geol., Vol. XX (1912), 
pp. 681-707. 
