Landforms of British Columbia 1976 - Department of Geography

Plateau and the Nechako Plateau, and northward from the Nation River by the
Omineca and Cassiar MotWains. From Arctic Lake near the head of the Parsnip
River the eastern wall of the trench is formed by the unbroken western front of the
Rocky Mountains, and the trench is a continuous straight valley from 2 to 9 miles
wide occupied by the northward-flowing Parsnip River, the southward-flowing
Finlay River and Fox River, and the. northward-flowing Kechika River. Sifton
Pass (3,273 feet) is the highest divide along the entire length of the trench.
The trench is a struc~orally controlled erosional feature. The trench structure
is very largely unknown, but it is complex and unqoes~ionably differs from one
place to another. There is little doubt that the break in the trench at McGregor
River is the result of erosion controlled by two major structures. As a topographic
feature the trench existed early in the Tertiary, and Tertiary sediments were deposited
in it. It is known that during Tertiary Gme. the headwaters of the Peace and
L&d Rivers flowed on the site of the trench, as did the pre-glacial Fraser River.
The southern trench was probably drained by the south-flowing Columbia and
Kootenay Rivers. It developed as an erosional form during the Tertiary by streams
whose courses were antecedent to the building of the Rocky Mountains. They were
antecedent also to the uplift of the mountains on the western side of the trench.
The erosion of the trench was cornpl&ed by the end of the Pliocene. It was occopied
by ice and its form modified, but the main effect of glacial occupation was
the derangement of previously established drainage systems. The &earns became
established in their present courses with the waning of the ice, and during the postglacial
period have incised themselves to varying depths into the deeply drift-filled
floor of the trench.
[Reference: “ Rocky Mountain Trench Symposium: Can. Inst. Min. c? Met.,
Trans., Vol. LXII, 1959, pp. 152-172.1
[Photographs: B.C. 187:91; B.C. 762:llO; B.C. 766:37; B.C. 1949:24;
B.C. 1950: 23.1
SOUTHERN PLATEAU AND MOUNTAIN AREA
The Southern Plateau and Mountain Area includes most of the central and
southern interior of British Columbia lying south of latitude S5 degrees north. It
is the part of the Province best known to travellers because of its ready accessibility
by a nework of highways and railways. The area is divisible into two parts: one
of low to moderate relief comprising the basins, plahzus, highlands, and moo&&s
of the Interior Pla&m; and the second the mounGnous area of the Columbia
MounWins.
The Interior Plateau is one of the major physiographic divisions of the Prov-
ince, with a length of 560 miles and a maximum width of 235 miles. It is encircled
by mountains, being flanked on the west by the Coast and Cascade Mountains, on
the north by the Skeena and Omineca Mountains, and on the east and southeast by
the Rocky MooWains and Columbia Mountains. On the northeast the plateau
merges in the Rocky Mountain Trench, but elsewhere there is a transition between
plateau and flanking mountains (see Plate XXVIIB). On the east the transition
between the. partly dissected plateau and the Colombia MounGx is through r~ beIt
up to 50 miles or more wide. In it the plateau is more highly dissected, and the belt
is sufliciently large and distinct& to be separately designated as a highland. Else-
where the transition between plateau and rnounGns is fairly rapid, and the. boundary
is drawn where the plateau is completely dissected and the country is truly moon-
Gmus.
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