Landforms of British Columbia 1976 - Department of Geography

The extension of the trough through southeastern Alaska is marked by low-lying
areas or areas underlain by Tertiary rocks. It terminates finally at Chatham Strait,
which is the site of a major north-striking fault.
The Coastal Trough has a width of 75 miles between Graham Island and
Percher Island, is constricted to a width of 10 miles at Sayward in Johnstone Strait,
and broadens to about 30 miles at Campbell River. It is 100 miles wide at the re-
entrant of the Fraser River, where it extends eastward to a point just west of Hope.
The constriction at Sayward, the Seymour Arch, * divides the Trough into a north-
ern section, the Hecate Depression, and a southern section, the Georgia Depression.
The line of the 2,000-foot contour is used arbitrarily as the boundary between the
lowlands and the mountains.
HECATE DEPRESSION
The Hecate Depression between Johnstone Strait and Dixon Entrance is very
largely beneath sea-level, being occupied by Queen Charlotte Strait, Queen Charlotte
Sound, Hecate Strait, and Dixon Entrance. Between Graham Island and Percher
Island the sea floor beneath Hecate Strait does not lie below a depth of 300 feet, but
depths greater than 1,200 feet are reached farther south in Hecate Strait and to the
north in Dixon Entrance, where ice-gouged troughs extend across the Depression.
The parts of Hecate Depression above sea-level are the Queen Charlotte Lowland
on Graham Island, the Nahwitti Lowland on northern Vancouver Island, and the
Hecate Lowland along the mainland coast and on the offshore islands.
Queen Charlotte Lowland
The northeastern part of Graham Island, northeast of a line between Copper
Bay and Beresford Bay, is low, with little relief, and constitutes part of the Hecate
Depression. Northward from Lawn Hill (560 feet) and east of Masset Sound the
terrain is very largely below 500 feet elevation, while west of Masset Sound the
surface rises gradually but stays well below 1,000 feet. The boundary between the
Skidegate Plateau on the southwest and the Queen Charlotte Lowland on the north-
east is marked by the generalized line of the 500-foot contour between Copper Bay
and Masset Inlet. Northwestward from Masset Inlet the generalized line of the
l,OOO-foot contour is more suitable. For about 50 miles northwest of Copper Bay
the boundary more or less coincides with the trace of a fault which has been postu-
lated on both geological and geophysical grounds. Drainage on the lowland surface
is poorly organized. A very large percentage of the area is muskeg and the timber
cover is sparse.
Except for the extreme northeastern tip of Graham Island, the area of the Argo-
naut Plain, the lowland is underlain by early Tertiary basaltic lavas and late Tertiary
sedimentary rocks covered with a thin veneer of till. Most of the lava flows do not
exceed 100 feet in thickness, and many have well-developed columnar jointing; they
dip seldom more than 20 degrees to the east. The gentle rise of the lowland surface
westward from Masset Sound probably is the surface reflection of the gently east-
dipping flows.
The movement of glacial ice across the lowland is shown by drumlin-like forms
and flutings, which at Juskatla Inlet and the head of Masset Inlet indicate a north-
east diiection of travel. The direction changes progressively to more westerly, and
striations at the mouth of Masset Inlet trend north 10 degrees west. The ice eventu-
ally joined with ice from Alaskan and mainland glaciers which flowed through Dixon
Entrance past and over Langara Island in a westerly direction.
* Bostock, H. S., Gtd. Surv., Canada, Man. 241, p. 89.
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