Landforms of British Columbia 1976 - Department of Geography

a variety of schists and gneisses, which account for the physical character of the
ranges. Small granitic intrusions are known to occur in the vicinity of Spine1 Lake.
These ranges are more akin to the Finlay Ranges to the south than they are
to the Kechika Ranges to the north, from which they differ lithologically and
structurally.
The high peaks of the Sifton Ranges were intensely glaciated, but no cirque
glaciers remain.
OMINECA MOUNTAINS
The Omineca Mountains extend southward from the Cassiar Mountains in the
northern nart of the central interior of British Columbia. They are bounded on the
west by the Spatsizi Plateau between the Chukachida River and Thutade Lake, and
by the Skeena Mountains between Thutade Lake and Bear Lake, on the southwest
and south by the Nechako Plateau between Bear Lake and the Nation River, and
on the northeast by the Rocky Mountain Trench. On the north the Omineca Moun-
tains are arbjtrarily separated from the Cassiar Mountains by the valleys of Chuka-
ch’da River, Gushing Creek, Thudaka Creek, and Finlay River, although there is
complete topographic and geologic continuity between the two.
The mountains have a core of granitic rock of the Omineca Intrusions. This
is a composite batholith of Upper Jurassic or Lower Cretaceous age intruding
sedimentary, volcanic, and metamorphic rocks of Proterozoic to Jurassic age.
Post-batholithic sedimentary rocks of Cretaceous and Tertiary age occur in isolated
basins of deposition.
The Omineca Mountains are divided into three units: the greater part of the
mountains, between Mount Cushing on the north and Nation Lakes on the south,
have a central batholithic core and are named the Swannell Ranges; on their north-
east, and separated from them by the Pelly Creek lineament, are the Finlay Ranges;
on their southwest, and separated from them by the Pinchi Lake-Omineca fault, are
the Hogem Ranges.
Swannell Ranges
The Swannell Ranges extend southeastward from Mount Cushing (8,676 feet)
to the Nation Lakes, a distance of about 200 miles. Their greatest width, between
Bear Lake and the mouth of Ingenika River, is 85 miles. The lateral boundaries of
the ranges are structurally controlled hneaments. The Pelly Creek lineament* is an
extremely prominent topographic feature extending northwestward from the mouth
of the Omineca River for several hundred miles. Southeast of the Finlay River
along Pelly Creek, past Tomias Lake, and along the Mesilinka River to its mouth,
it separates the Swannell Ranges from the Finlay Ranges. Structurally controlled
topographic features provide the southwest boundary between the Swannell Ranges
and Hogem Ranges. The boundary follows a series of depressions and valleys
which coincide with the Pinchi Lake fault zone, and its extension northwestward is
along the Omineca fault, thence by Sustut Lake and Moose Valley to Thutade Lake.
The summit level gradually diminishes to the south, and the mountains merge in the
Nechako Plateau along a sinuous line between Bear Lake and the mouth of Nation
River.
The highest points in the ranges are the several granitic peaks at the northern
end-Mount Gushing (8,676 feet), Bronlund Peak (8,51 I feet), and Fleet Peak
(7,630 feet). Farther south the high points lie between 6,000 and 7,()00 feet
elevation.
* Roots, E. F.s Geol, Surv., Canada, Mem. 214, 1954, p. 196.
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