Landforms of British Columbia 1976 - Department of Geography

H. C., “ Geology and Mineral Deposits of Quatsino-Nimpkish Area,” GeoZ. Surv.,
Canada, Sum. Rept., 1929, Pt. A, pp. 94-143.1
[Photographs: B.C. 664:20, 49; B.C. 666:20, 38, 58, 102; B.C. 673:17,
40; B.C. 1553:98; B.C. 1555:lO.l
The AZberni Basin (see Plate IIIB) extends northwestward from Port Alberni
and has a length of about 25 miles and a width of 5 to 8 miles. It is a low-lying
area of low relief drained by the Ash, Stamp, Sproat, and Somass Rivers into the
Alberni Canal. It is sharply bounded on its eastern side by an abrupt fault-line scarp
along the western front of the Beaufort Range. On the north, west, and south the
l,OOO-foot contour line marks for the most part its boundary with the surrounding
mountainous terrain.
The basin is underlain by Upper Cretaceous sedimentary rocks. These have
eroded more readily, and as a consequence lie at lower elevations than the older
volcanic and intrusive rocks which surround them.
[Reference: MacKenzie, J. D., “Alberni Area,” Geol. Surv., Canada, Sum.
Rept., 1922, Pt. A, pp. 51-68; Fyles, J. G., “ Surficial Geology of Horne Lake
and Parksville Map Areas,” GeoZ. Surv., Canada, Mem. 318, 1963, pp. 4-8.1
A narrow coastal plain, the Estevan Coastal Plain (see Plate IV), 1 to 2 miles
wide for the most part, extends along the west coast of Vancouver Island, from
Bunsby Islands east of Brooks Peninsula southeastward for almost 170 miles to
Cullite Cove, 8 miles southeast of Carmanah Point. Much of the coastal plain is
less than 150 feet above sea-level and is interrupted by irregular hills and isolated
knolls seldom more than 250 feet high. The plain is divided into sections by
various inlets, and reaches its greatest width of 6 to 8 miles at Hesquiat Peninsula.
Where the coastal plain, is underlain by relatively soft Tertiary or Pleistocene
and Recent deposits, its surface is flat and almost featureless, but where it is
underlain by harder rocks of the Vancouver Group, it is much more uneven, with
bluffs and protruding hummocks.
The Tertiary rocks of the coastal plain are gently dipping Oligocene and early
Miocene sandstones, more or less continuously exposed northwest of Cullite Cove.
These rocks were deposited on an early Tertiary erosion surface contemporaneously
with the dissection of the same surface where it was upwarped east of the coastline.
Between Cullite Cove and Sooke Inlet the early Tertiary erosion surface can be
clearly seen truncating the older rocks (see N.T.S. Sheets 92 C/9 and C/10), but
Tertiary sedimentary rocks, which at one time almost certainly existed, have been
stripped off almost completely, leaving a few remnants in protected areas.
The flat surface of the coastal pIain is the rest& of late Tertiary eros,ion of
the soft sediments, but insufficient time has elapsed for a comparable feature to
develop on the adjacent hard rocks.
[References: Jeletzky, J. A., “ Tertiary Rocks of the Hesquiat-Nootka Area,”
Geol. Surv., Canada, Paper 53-l 7; B.C. Dept. of Lands and Forests, N.T.S. Sheets
92 C/9 and C/l0 at 1 in. to 1 mi.; Dept. of Mines and Technical Surveys, N.T.S.
92 SW at 1 in. to 8 mi.; Hoadley, J. W., “ Geology and Mineral Deposits of the
Zeballos-Nimpkish Area?” GeoZ. Surv., Canada, Mem. 272, 1953.1
[Photographs: B.C. 677:98.]
COASTAL TROUGH
The Coastal Trough extends northwestward through British Columbia as a
structurally controlled topographically low belt from Puget Sound and the San Juan
Islands in Washington to Dixon Entrance, a distance of more than 500 miles. It lies
between the Insular Mountains on the west and the Coast Mountains on the east.
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