ROCKY MOUNTAIN STRUCTURE IN IDAHO 461 
valley of Meadow Creek in the southern part of the Cranes Flat 
quadrangle is a fault trough or graben. This structure may be 
traced about 15 miles southeast into the Lanes Creek quadrangle, 
where it apparently dies out. The northward extension of the 
graben is concealed by basalt and Quaternary deposits. The 
bounding ridges are composed of Carboniferous rocks and are 
conspicuous topographic features. 
Two transverse normal faults intersect the graben, one near the 
south boundary of the Cranes Flat quadrangle and the other in 
the northwest corner of the Lanes Creek quadrangle, down-faulting 
the portion between them. The bounding ridges in the down- 
faulted area are farther apart than in the portions to the northwest 
or southeast. In the southeastern part of the graben beds of the 
Thaynes group (Lower Triassic) are exposed and in the widened, 
down-faulted portion both the Woodside shale (Lower Triassic) 
and Thaynes appear, though most of the area is underlain by 
basalt and Quaternary deposits. The structure of the rocks 
within the graben is probably synclinal, as shown in structure 
sections EE’ and GG’. It is with little doubt the continuation of 
synclinal structures observed farther southeast. 
The fault which lies along the northeast side of the graben 
is concealed for much of its length, but in the southeastern part of 
the area here shown is represented by two faults, separated by a 
narrow strip of the Phosphoria formation, but together bringing 
Lower Thaynes into proximity with the Wells. Northwest of the 
area illustrated the fault doubtless continues for some distance 
beneath the basalt. Its stratigraphic throw is not known but is 
estimated at 3,000 to 4,000 feet. 
The transverse normal fault that passes between Limerock 
Mountain and Pelican Ridge causes the mountain to stand nearly a 
mile northeast of the line of continuation of the ridge. Similar 
effects in reverse order are produced where the fault intersects 
Little Gray Ridge. Neither the amount of the downthrow nor 
the hade of the transverse fault is known, but from the effects and 
assumed values for the dips of the lateral faults that bound the 
graben it is thought that 5,200 feet may represent the order of 
magnitude of the vertical displacement. 
