ROCKY MOUNTAIN STRUCTURE IN IDAHO 465 
The ridge northeast of the graben is not clearly a horst though 
it is much broken by faults and has suffered extrusion of rhyolite. 
NOTES ON THE DEFORMATION OF SOUTHEASTERN IDAHO 
Epochs of deformation.—Although southeastern Idaho was 
profoundly affected by crustal disturbances at the close of the 
Jurassic, the observed mountain structures appear to be the result 
of two later epochs of mountain building. The earlier of these 
occurred after the deposition of the Wayan formation and before 
the deposition of the Wasatch formation. It probably corresponds 
Limerock 
Mt.Chorst) Meadow Creek 
(E ra en Yo 
~ 
~ 
ner, 
SECTION. E-E’ 
Pelican Ridge 
Khorst) Meadow Cr 
° (graben) 
90 Py == 2 
2 ; cw IRS 
> } 
"SECTION 6-G’ 
Fic. 10 B.—Sections along the lines E—E’ and G-G’ of Fig. 10A 
with the interval between the Adaville and Evanston formations of 
Veatch’ or the epoch which, according to Ransome,’ “‘appears to 
have begun at the close of the recognized Laramie or possibly even 
earlier, and to have attained its maximum between the Fort Union, 
which chiefly on the basis of its plant remains is generally classed 
as basal Eocene, and the mammal-bearing lower Eocene Wasatch.” 
The second mountain-building epoch occurred after the depo- 
sition of the Salt Lake formation which, on the basis of present 
rather unsatisfactory evidence, is tentatively assigned to the 
Pliocene. This formation locally has steep dips thought to-have 
been produced by deformation in late Pliocene or post-Pliocene 
time. . 
1 A.C. Veatch, ‘‘ Geography and Geology of a Portion of Southwestern Wyoming,” 
U.S. Geol. Survey Prof. Paper 56 (1907), p. 75- 
2F. L. Ransome, “The Tertiary Orogeny of the North American Cordillera and 
Its Problems,” Problems of American Geology, pp. 287-376, p. 322, New Haven, rots. 
