468 GEORGE ROGERS MANSFIELD 
Later deformative epoch—rThe later epoch of deformation was 
marked by broad uplift rather than by intensive folding. There 
was, however, some folding, involving locally steep or overturned 
dips, but generally of an open character. The folding of the plane 
of the Bannock overthrust is of this type and is thus structurally 
more akin to the later than to the earlier deformative epoch. 
Relaxation and readjustment.—The vigorous compression of the 
earlier deformative epoch was succeeded by relaxational phases 
involving normal faulting and gradual readjustment to new con- 
ditions of equilibrium. Some normal faults along the Bannock 
fault zone represent with little doubt the fracture and jostling of 
blocks under light load near the margin of the fault block. Other 
normal faults partly concealed by Tertiary beds also may be 
referred to the interval of relaxation following the earlier defor- 
mative epoch. Many of the normal faults, however, including 
those that produced the horst and graben structure, are not asso- 
ciated with Tertiary beds. ‘There is a single doubtful exception to 
this statement. On the other hand there is definite evidence that 
Tertiary beds have been displaced by normal faults. Thus these 
faults have been considered as later than the Tertiary beds but 
earlier than the earliest Quaternary, and hence associated with 
the relaxational interval succeeding the later deformational epoch. 
The horst and graben structure is more or less intimately 
associated with extrusions of ryholite and basalt. The basalt in 
particular has flooded the valleys in the vicinity of these structures 
and has emerged along some of the fault lines. On the other hand, 
sufficient erosion had occurred after the faulting to produce prac- 
tically the present topography before the extrusion of the basalt. 
That event, therefore, probably accompanied a relatively late 
reopening of some of these faults, together with the development 
of new fissures. 
At present no definite evaluation of the parts to be assigned to 
the two relaxational intervals may be made. 
