Landforms of British Columbia 1976 - Department of Geography

tain Foothills. The foothilIs are lithologically and structurally different from the
Rocky Mountains, and in northeastern British Columbia the topographic separation
of the two mountain belts is distinct.
MACKENZIE MOUNTAIN AREA
The Liard Plateau (see Plate XXXIB) extends southward into British Columbia
from the Yukon, where it has a considerably greater extent. On the west the
plateau is bordered by the Liard Plain, the boundary being drawn east of the Smith
River along the general line of the 2,500.foot contour. On the east the plateau
merges in the Interior Plains, the boundary being drawn northward from the junc-
tion of the Toad and Liard Rivers along the general line of the 2,000-foot contour.
The Liard River forms the southern boundary of the Mackenzie Mountain Area. It
separates the lower heights of the Liard Plateau from the higher elevations of the
Rocky Mountains lying south of the river, It also marks a line ahzag which north-
trending bedrock structures north of the river are truncated by northwesterly trend-
ing Rocky Mountain structures south of the river. Geologically, the Liard Plateau
is at the southern termination of the Mackenzie Mountain structures.
The plateau is underlain by sedimentary rocks ranging in age from Devonian
to Triassic. The rocks are folded about southerly plunging axes which trend essen-
tially north, veering slightly east of north at the Yukon Border.
The Liard Plateau has its maximum height of 5,595 feet at a peak on the north
side of Grayling River, about 30 miles northwest of Hells Gate, For the most part,
the plateau consists of rounded and flat-topped summits and timbered ridges lying
below 5,000 feet elevation, gradually diminishing in height to the east. Northerly
and northeasterly trending summit levels are controlled by the trends of some of the
more res&mt underlying formations. The upland surface is incised to elevations
below 2,500 feet by the Grayling, Scatter, and Crow Rivers, which are tributary to
the Liard, whose elevation at the mouth of Smith River is 1,450 feet.
The area was covered by the Pleistocene ice-sheet, and the ice, as did the
glacial meltwater, escaped eastward down the Liard Valley (see Plate XXXVIIA)
from a gathering arca to the southwest. Ice which moved northeastward and
eastward across the surface of the plateau left drumlin-like forms east of the Smith
River Airport (see N.T.S. Sheet 94~).
[Reference: Kindle, E. D., “ Geological Reconnaissance along Fort Nelson,
Liard, and Beaver Rivers,” Geol. Sun., Canada, Paper 44-16.1
[Photographs: B.C. 952~6s; B.C. 957:59; R.C.A.F. T27Lzll9; RC.A,F.
T27R:lOl.l
ROCKY MOUNTAIN AEA
The Rocky Mountains extend in a northwesterly direction along the eastern
side of the Province for 850 miles between the 49th parallel and the Liard River.
Their western boundary is the Rocky Mountain Trench, which, except for the
interruption at the McGregor River, exists as a continuous valley for the entire
distance. On the east the Rocky Mountains are flanked by the Rocky Mountain
Foothills, which are not everywhere inferior in height to the main motmtains.
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