12

Trench and Cassiar Mountains and to the north parts of the Liard Plain
and Liard Plateau (Bostock, 1948, Map 922A).

THE INTERIOR PLAINS

The Interior Plains of northeastern British Columbia comprise a
relatively high, rolling or slightly undulating country that is generally
underlain by flat-lying or gently folded beds of Upper Cretaceous age.
The Plains display a relief of some 3,000 feet and drainage is controlled
by a regional northeast slope. In the south, Peace River is incised below
the 1,600-foot contour east of Hudson Hope and occupies a valley some
800 feet deep and 2to 3 mileswide. Fort Nelson River, 200 miles north, is
incised below the 900-foot contour, and occupies a wide valley some 300
to 400 feet deep. The smaller streams have also cut deep valleys, but on a
lesser scale. Many of the headwater streams both east and west of the
Alaska Highway rise in hilly country above an altitude of 4,000 feet. On
the north side of Peace River, the Plains extend west as far as the Portage
Mountain, Bullhead Mountain, and Butler Ridge line of folding; south of the
river the Foothills belt swings more easterly to include the hills about
Moberly Lake. The Alaska Highway traverses the Plains for roughly
350 miles northwesterly from Dawson Creek, to enter the Foothills belt
57 miles west of Fort Nelson. Williams (1944) describes the Plains west of
Yort Nelson as follows:

“For about 57 miles west from Fort Nelson the Alaska Highway
traverses a gently undulating plain, below which Muskwa River and its
tributaries have carved valleys 200 to 400 feet deep. The plain varies in
elevation from about 1,800 feet in the west to about 1,500 feet in the east.
The Kledo branch of the Muskwahas a wide, flaring valley, whereas Muskwa
Valley itself is deep, with steep sides. Near its mouth the river is nearly
400 feet below the adjoining upland. Fort Nelson River Valley at this
locality is broad and well graded.”

Hage (1944) states that the Alaska Highway trends northwesterly from
Fort St. John for 97 miles, and follows the height of land between Cameron,
Blueberry, and Beatton Rivers. The divide is formed of gently sloping,
northerly trending ridges, and towards the north the elevation of the country
rises about 1,000 feet in the first 95 miles from Fort St. John.

The Highway assumes a more northerly route near mile 146! and
continues in this direction for 55 miles. For much of this distance it
follows a high, west-facing scarp that parallels the mountains 12 to 18 miles
to the west. The top of the scarp has a plateau-like appearance, and is
dissected by east-flowing streams, including Beatton, Sikanni Chief,
Buckinghorse, and Prophet Rivers. The height of land between the Peace
and Fort Nelson drainage systems lies between Beatton and Sikanni Chief
Rivers and near mile 157 on the road. The original army road in this area
follows the top of the scarp across the divide and attains an elevation of
nearly 4,200 feet a short distance north of mile-post 155, the highest point
along this section of the road. From there the elevation drops to 2,662
feet above sea-level at Sikanni Chief bridge. North of Buckinghorse River,
the scarp has an average clevation of 4,000 feet, and is about 1,000 feet
above Minaker River to the west. At mile 201 the Highway leaves the

1 Mile-posts referred to are numbered from Dawson Creek.