Landforms of British Columbia 1976 - Department of Geography

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These valleys are at a fairly high level, and consequently the relief is considerably
less than it is in the adjoining Kitimat Ranges on the west, where the Kemano and
Kitlope Rivers flow at less than 1,000 feet elevation.
The lakes, which are prominent features of the Tahtsa Ranges, occupy ice-
modified valleys whose western ends are in the granitic mountains of the Kitimat
Ranges and whose eastern ends lie in the Nechako Plateau. Ice which accumulated
in the mountains to the west flowed eastward across the Tahtsa Ranges and through
these easterly trending valleys onto the Nechako Plateau and thence eastward toward
the Rocky Mountains.
[Reference: Duffell, S., “ Whitesail Lake Map-area,” Geol. Surv., Canada,
Mem. 299, 1959.1
CASSIAR MOUNTAINS
The Cassiar Mountains in the northern interior of British Columbia extend
southeastward from the Yukon Border for 230 miles to the bend of the Finlay River,
where, at latitude 57 degrees 40 minutes north, they adjoin the Omineca Mountains.
The mountains have their greatest width of 100 miles between the Three Sisters
Range at the head of the Tanzilla River and the Rocky Mountain Trench. The
mountains are bounded on the west and north by the Nisutlin Plateau, on the west
and southwest by the Stikine Plateau, on the northeast by the Liard Plain, and on
the east by the Rocky Mountain Trench. They are separated from the Omineca
Mountains by the through valley occupied by Chukachida River, Cushing Creek,
Thudaka Creek, and Finlay River. Mount Cushing (8,676 feet), the most promi-
nent peak in this vicinity, is in the Omineca Mountains just south of the transverse
valley.
The Cassiar Mountains are divisible into four major units: the Dease Plateau
on the northeast, adjacent to the Liard Plain; the Stikine Ranges, the extensive
mountains whose central part is composed of granitic rocks of the Cassiar batholith;
the Kechika Ranges flanking the Rocky Mountain Trench north of Sifton Pass; and
the Sifton Ranges flanking the Trench west of the Fox River.
Dease Plateau
The Dease Plateau consists of moderately high, flat-topped ridges and rounded
mountains separated by widely flaring valleys lying between the higher, more rugged
mountains of the Stikine Ranges and the low, gently rolling lowland of the Liard
Plain. A distinctive subdivision within it, the Horseranch Range, lies east of the
Dease River. The plateau, with a width of about 20 miles, extends from Looncry
Lake and Deadwood River valley along the northeast side of the Stikine Ranges for
75 miles to the Yukon Border, and thence into Yukon Territory. It is broken into
two segments by the valley of the Dease River, which allows a prong of the Liard
Plain to extend southward to the junction of Rapid River with the Dease.
,The plateau lies essentially between 3,000 feet, the level of the Liard Plain,
and 5,000 feet, above which elevation only three rounded summits project. The
western boundary of the plateau is drawn along the generalized line of the 5,000-
foot contour, separating the Stikine Ranges, which project well above 5,000 feet,
from the Dease Plateau, which, for the most part, lies below. Southeast from Black
Angus Creek the northeast boundary of the plateau is essentially an extension of the
western side of the Rocky Mountain Trench.
The plateau is underlain by folded Paheozoic sedimentary rocks. Pleistocene
ice rounded the summit peaks and ridges and covered the area with an extensive
mantle of drift.
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