Landforms of British Columbia 1976 - Department of Geography

Plateau (see Plates XVIA, XVIB, and XVIIB) by flat and gently sloping upland sur-
faces at and above 5,000 feet. These upland surfaces are remnants of the late
Tertiary erosion surface which in the Stikine Plateau extended across the Tagish and
Tahltan Highlands (see Plates XIIIB and XVB) and probably across the Boundary
Ranges as well.*
During the Pliocene there was a general uplift of western North America, and
the late Tertiary erosion surface was uplifted differentially to elevations that range
from about 5,000 feet in the Taku and Kawdy Plateaus to 7,000 in the Spatsizi Pla-
teau and Tagish Highland. This uplift rejuvenated the erosive power of the streams,
and during late Pliocene but pre-Pleistocene time the surface was dissected by them.
The degree of dissection varies greatly-in regions such as the Spats%, Nahlin, and
Kawdy Plateaus there are wide areas of little or no dissection (see Plates XVIIB and
XIXA), whereas in regions such as the Taku Plateau and Tagish Highlands, in which
the degree of dissection is considerably greater, the Tertiary erosion surface is now
represented by rather small isolated fragments of gently sloping upland (see Plate
XVB). The depth of incision together with the amount of original relief of the Ter-
tiary erosion surface produce the present relief. This varies greatly, ranging from
small local relief in the Kawdy and Spatsizi Plateaus, where incision and dissection
are slight, to moderate relief in the Taku Plateau and Tagish Highland, where the
depth of incision is great.
A considerable number of centres of volcanic activity lie within the Stikine
Plateau. These during the late Tertiary, Pleistocene, and Recent were active, and
piles of volcanic material, both lavas and fragmentals, accumulated locally on the
late Tertiary erosion surface. They are represented by the shield volcanoes and
associated lava flows of Mount Edziza and Level Mountain, by the tuyas in the
Kawdy Plateau, and by volcanic cones rather widely distributed (see Fig. 1) .
During the Pleistocene the plateau was completely covered by glacial ice, which
eroded upland surfaces and deposited a veneer of drift over most of the country.
Late-stage cirque glaciation sculptured many of the peaks and ridges at higher levels.
The well-developed drainage systems established during the long period of erosion
in the Tertiary were disorganized by the effects of glaciation. Blockage by ice or
drift diverted the drainage of Dease Lake, which in pre-Pleistocene time was by Tan-
villa River into the Stikine River, northward into the Liard. Within the plateau, de-
pressions of glacial origin are occupied by numerous lakes, and drainage in many
instances is poorly established,
Since the Pleistocene the major rivers and their tributaries have resumed the
incision and headward erosion which were initiated by the Pliocene uplift, and the
drainage systems are recovering from their disorganization in the Pleistocene.
SKEENA MOUNTAINS
The Skeena Mountains (.see Plates XIXc and XXA) constitute the mountainous
area in the northern interior of British Columbia, extending from Telkwa on the
Bulkley River northward for 230 miles, almost to the Stikine River. They are 90
miles wide between Bowser Lake on the west and Tatlatui Lake on the east. The
mountains are bounded on the west by the Tahltan Highland, Boundary Ranges, and
Nass Basin, on the south by the Nechako Plateau, on the east by the Omineca Moun-
tains, and on the north by the Stikine Plateau.
The Skeena Mountains are a distinctive unit, being formed very largely of foIded
sedimentary rocks of Upper Jurassic and Lower Cretaceous age. The principal rocks
are black fine-grained argillite and shale, and dark greywacke. Limestone, or rocks
directly of volcanic origin, are absent; igneous intrusions are few in number and
* Kerr, F. A., Gad. Surv., Canada3 Mem. 246, 1948, p. 13.
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