194

(3) A major orogeny between Upper Jurassic and late Lower
Cretaceous time resulted in severe rock deformation in response to regional
compression from the northeast and southwest. The deformation found
expression in large, compound, northwest-trending folds, with undulating
but more or less horizontal axes. The axial plane of each fold in the
series dips, in general, to the northeast, at a successively flatter angle
than that of the neighbouring fold to the southwest (See Structure-
sections C-D, E-F). Local deformation has been much more severe on
the shorter, steeper, southwest limb of each compound fold. The shortening
due to compression was greatest in the northern part of the area, and
developed thrust faults and large, recumbent, digitate, overturned
structures (See Structure-section A-B). The earlier folded structures
of the Tenakihi and Ingenika group beds were overwhelmed by these
asymmetrical anticlinoria and synclinoria. The late Paleozoic and Takla
group rocks in the southwestern half of the area form the northeast limb
of a large synclinal structure along whose axis the Hogem batholith was
intruded. The fracture and dyke pattern within the batholith suggest
that the younger, major part of the batholith was emplaced after the
rocks had been folded and at a time of little or no regional compressive
stress.

(4) A period of widespread flexure in response to compression from
the southwest followed the deposition of the Sustut group rocks. This
deformation is recorded in the synclinal structure of the Uslika formation
and the oblique tilting of the beds of the Sifton formation. Evidence
from the McConnell Creek map-area to the west, where the Sustut
group has been more widely preserved (Lord, 1948, p. 33), suggests that
this folding was, on the whole, comparatively gentle, with much variation
in local intensity. The effect of this post-Paleocene folding on the
previously folded Jurassic and older rocks was probably slight, and no
structures produced by it, superimposed upon earlier structures, have
been recognized.

(5) The major faults in the area probably formed at widely different
times, but the main displacements on all have post-dated the folding
of the rocks in which they occur. In general it seems that the most
important movements on nearly all the main faults took place in post-
Paleocene time. The contacts between the Tenakihi and Ingenika group
rocks east of Mount Lay and south of Jim May Creek may be, in part,
folded faults. As described on pages 189 and 192, the Uslika formation
and the small block of Sustut group rocks southwest of Uslika Lake
appear to have reached their present position among older rocks by two,
separate, post-Paleocene thrusts.

CHARACTER AND INFLUENCE OF THE ROCKY MOUNTAIN
TRENCH

The Rocky Mountain Trench is almost certainly underlain by great
faults that bring the formations of the Rocky Mountains on the northeast
into discordant contact with those of the Omineca and Cassiar Mountains
on the southwest. The line of the trench has marked the apparent
westward limit of sedimentation in the Rocky Mountain geosyncline at