the Cassiar-Omineca section, the rocks are mainly Paleozoic
sediments, with granitic intrusive rocks of Mesozoic age.
One great batholith, from 10 to 30 miles wide, extends,
in the typical northwestward sweep, from latitude 55
‘degrees to well beyond the Yukon border.

THE YUKON PLATEAU

In their northerly aspects the Interior Plateaux,
generally describable as rough, elevated country, become
higher and are broken by various mountain groups to beyond
the British Columbia-Yukon border. Here begins what
must be regarded as a second plateau region known as the
Yukon Plateau. Bounded on the west by the Coast Range
and on the east by the Mackenzie Mountains, this plateau
extends along the valley of Yukon River and its great
tributaries to Alaska. This region may be more appro
priately termed “Plateau” than the great area to the south.
Although the terrain is broken by deep valleys sunk 1,000
to 3,000 feet below the general level, the uplands are
broad, gently sloping areas that appear to be remnants of a
once-continuous plainlike region. The plateau has a
general elevation of about 4,000 feet on the south and
slopes gently away to the north. In places this general
uniformity of elevation is interrupted by isolated mountain
groups, among which are the Dawson, McArthur, Glen-
lyon, Big Salmon, and Pelly Mountains, with peaks up to
7,000 feet.

The Yukon Plateau, lying between the granitic intrusions
of the Coast Range and the folded sedimentary rocks of
the Mackenzie Mountains, is made up of ancient meta-
morphic rocks, folded sedimentary and volcanic formations,
and scattered intrusive bodies.

THE ROCKY MOUNTAIN SYSTEM

The Rocky Mountain system and its counterpart in
the Mackenzie Mountains to the north are composed
almost entirely of folded and faulted sedimentary formations
(limestones predominating) chiefly of Paleozoic age, but
in places including great thicknesses of stratified late
Precambrian rocks. In the Rockies, infolded and infaulted
basins of Mesozoic strata are common. Since by com-
parison with the Coast Range, intrusions are of infrequent
occurrence, the possibility of deposits of metallic minerals
is not promising. The ranges of the Rockies in the North
Pacific Region have, however, been little explored.

The foothills of the Rocky Mountains, which in the
southern part of the North Pacific area have an average
width of about 40 miles, are composed mainly of a thick
series of alternating shales and sandstones, chiefly of
Mesozoic age, closely folded, and resembling strata in the
foothills farther south. Northward, the foothills dis-
appear, and north of the confluence of the Liard with Fort
Nelson River, mountains rise abruptly from an almost
level plain.

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Great CENTRAL PLAINS

In the North Pacific Region, the physiographic province,
known as the Great Central Plains, occupies the central
part of the Mackenzie River basin and includes all the area
between the Precambrian Canadian Shield on the east and
the Cordillera on the west, with a length of about 1,300
miles. Its width ranges from more than 400 miles, in
the latitude of Fort Vermilion, to 200 miles at latitude
63°. In the south, it is a northeasterly-sloping plateau
descending by a series of low escarpments to a lowland that
extends northward to the Arctic. West of this lowland, a
smaller plateau occupies the upper basin of Peel River.
Both the plateaux and the lowland are broken by low hills
or minor plateaux.

In the Central Plains province, the Precambrian rock
complex is overlain by horizontal or gently dipping beds
of unaltered sedimentary rocks of Paleozoic age which
extend eastward to the Canadian Shield. In the Mackenzie
Valley, the Paleozoic strata are, in turn, overlain uncon-
formably by Cretaceous and Tertiary sediments. Much
of the area is mantled by superficial deposits that conceal
the underlying stratified rocks which, in turn, rest on the
westward extension of the Precambrian basement.

THE ALBERTA PLAIN

The southerly part of the Central Plains, referred to as
the Alberta Plain, covers virtually the entire region south
and southwest of Great Slave Lake. It is intersected
by the valleys of the Peace, Smoky, and Athabaska Rivers,
and by the Hay, and other streams running into Great
Slave Lake. It ends to the northward in a north-facing
escarpment that extends from Fort Smith, on Slave River,
to the Liard River. The slope is gradual, and the drainage
immature, with nuMerous muskegs and lakes. There are
many areas of open prairie land, especially in the Peace |
Basin, where agricultural settlement is proceeding.

THE PEEL RIVER PLATEAU

The Peel River Plateau occupies the upper part of the
basin of the Peel River up to the base of the Richardson
Mountains. It forms a flanking platform to the Cordillera,
and slopes east and south toward the lower Mackenzie
Vailey. Its northern border is an abrupt escarpment that
rises 1,500 feet and overlooks the lowlands to the north.
Peel River cuts a valley in the plateau from 700 to 1,000 feet
deep. The plateau is everywhere covered with moss, is

wooded with small spruce and tamarack, and holds many
muskegs and lakes.

THE MACKENZIE LOWLANDS

The lowlands portion of the Great Central Plains,
referred to as the Mackenzie Lowlands, commences in the
lower part of Peace River and the extreme western end
of Lake Athabaska, whence it extends as a narrow belt
down the valley of the Slave River to Great Slave Lake.