Landforms of British Columbia 1976 - Department of Geography

The Alberta Plateau in British Columbia is drained and incised by the Liard
and Peace Rivers and their tributaries. The elevation of the Liard River is less than
1,000 f&at the mouth of the Fort Nelson River and of the Peace River about 1,475
feet at the mouth of the Kiskatinaw, and incision has proceeded upstream on tributaries
ayay from the two main rivers. The upland surface of the Alberta Plateau has
been only partly dissected, and parts of it have been destroyed. Those parts (especially
in the drainage basin of the Fort Nelson River) where the surface is reduced
to below 2,000 feet elevation constitute the Fort Nelson Lowland, the boundary between
the two being arbitrarily drawn at the 2,OOO-foot contour, Commonly this
is along a scarp upheld by one of the Cretaceous sandstone members.
The upland surface is flat or gently rolling, It lies for the most part between
3,000 and 4,000 feet elevation, and rises to 5,000 feet between the Prophet and
Muskwa Rivers, whereas in northwestern Alberta a large proportion is between
2,000 and 3,000 feet.
Dminage over part of the upland surface is poorly organized; there are large
areas of mwkeg, and streams meander across the surface eventually to join one of
the trunk streams. North of the Peace River much of the drainage is controlled by
the Halfway, Beatton, Sikanni Chief, Prophet, Muskwa, Fort Nelson, and Petitot
Rivers. These have incised themselves below the upland surface into the soft Fort
St. John shale.
Between the Prophet and Muskwa Rivers and north of the Sikanni Chief River
there is an area of gentle folding in which the rivers are incised into shales that underlie
a capping of Sikanni sandstone. As a consequence a distinctive cuesta topography
has been developed. The gentle slopes of the cuestas generally face east and are
structurally controlled by the gently east-dipping sandstbne. A steep scarp slope
faces west and falls away into a lower area generally underlain by Fort St. John shale.
The Alaska Highway from Beatton River north for 50 miles is located along the crest
of a cuesta, west of which a low area about 10 miles wide lies between the cuesta
scarp and the edge of the foothills.
The @lateau for the most part is heavily wooded, but along the Peace River
considerable areas, generally below 2,500 feet elevation, are flat or gently rolling and
are only lightly wooded. Much of this riverside country has been cleared and is under
cultivation (see Plate XXXIXB)
The plateau was glaciated during the Pleistocene. Ice from the Keewatin centre
of accumulation moved southwestward across the Alberta Plateau to the foothills and
left a veneer of glacial till containing distinctive Keewatin boulders. After the maxinun
expansion of Keewatin ice and its retreat, Piedmont and valley glaciers flowing
eastward from the Rocky Mountains moved outward onto the plateau to leave
moraines in a narrow belt along the eastern edge of the foothills. Ice movements can
be interpreted from glacial fluting in the ground moraine and by the elongation of
drumlins. The minor relief on the plateau surface results from these features.
As the ice waned, channels discharged glacial melhvater into valleys, some of
which were still blocked by ice. The Peace River valley was occupied by a proglacial
lake which at one stage had its level at approximately 2,750 feet. This old
lake, c&xl Lake Peace, had shorelines now traceable mainly by old beach lines
and gravel occurrences. A lower and younger stage developed when the outlet was
at 2,260 feet. Sediments were deposited in this lake basin to a thickness of about
100 feet (see Plate XLIVB). Post-Pleistocene erosion has incised the trunk rivers
to their present elevations.
[References: Mathews, W. H., s Ground-water Possibilities of the Peace River
Block,” B.C. Dept. of Mines, Ground-water Paper No. 3, 1955; Mathews, W. H.,
“ Quaternary Stratigraphy and Geomorphology of the Fort St. John Area,~ B.C.
95
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