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intervals since early Paleozoic times, and at other times has been the
locus of an abrupt change in lithological character and thickness of rocks
deposited to the east compared with those deposited to the west. So
far as known, it represents the northeast limit of the late Jurassic or
early Cretaceous orogeny, of all earlier (post-Silurian) periods of deforma-
tion, and of all post-Cambrian igneous activity in central and northern
British Columbia. The trench was an active structural feature during
the post-Paleocene deformation that affected most of northeastern
British Columbia west of the Interior Plains. No direct connection has yet
been established between the structures of the northern Rocky Mountains
and the contemporaneous, much milder structures produced by post-
Paleocene deformation west of the trench. The Rocky Mountain Trench
thus appears to have been a major depositional, lithological, and structural
boundary throughout much of the recorded history of the Cordillera.

That part of the Rocky Mountain Trench in and near Aiken Lake
map-area is apparently characterized by closely spaced, parallel, steeply
dipping faults. The movement on these faults has been partly, and
probably mainly, vertical, and has resulted in the lowering and preserving
of long narrow slices of the Sustut group (Sifton formation) in the
present valley floor. The southwest wall of the trench, as represented by
a 1,500-foot shear face on Mount Tsaydizkun and a line of lower cliffs
preserved at intervals for 15 miles to the southeast, may well be a fault
or a fault-line scarp.

EMPLACEMENT OF THE SIFTON FORMATION AND OLDER
ROCKS IN THE ROCKY MOUNTAIN TRENCH

Fault movements within the Rocky Mountain Trench appear to
have resulted in the down-dropping and preservation of long slices of the
Sifton formation, which now form narrow ridges striking parallel with
the axis of the trench. The beds on several of these ridges have a uniform
orientation, striking northwest, obliquely across the ridges, and dipping
northeast. It is difficult to conceive of movements by which nearly
flat-lying sediments laid down in the valley floor could have been tilted
to the northeast and obliquely cut through by north-northwest-striking
faults, and it appears more reasonable to suggest that the Sifton formation
is a part of the Sustut group rocks, which were deposited at a level
considerably above the present erosion surface. The formation is thought
to have been involved in the post-Paleocene deformation that affected
Sustut group rocks elsewhere, and then faulted into long thin blocks,
some of which were lowered to their present position with an unknown,
but probably small, amount of further tilting.

The amount of downfaulting suffered by the rocks in the floor of
the trench cannot be measured in Aiken Lake map-area, but 30 miles
to the northwest, relatively flat-lying, apparently Sustut group, strata cap
the massif known as Finlay Mountain, approximately 4,700 feet above
similar appearing sediments in the valley floor at the foot of the mountain}.

1 These rocks were examined, and their relations brought to the writer’s attention, by Mr. E.
Bronlund of The Consolidated Mining and Smelting Company of Canada, Limited. Finlay Mountain
may be seen from neighbouring peaks and from the air to possess the distinctive topography of
mountains capped by Sustut group strata; rocks of the Sifton formation type are reported by
McConnell (1896) to lie in the valley floor at this point.