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Physiography and Geology *

The North Pacific Region is a complex of mountain,
plateau, and plain, but the presence of a number of broadly
developed structural elements permits the drawing of a
generalized picture.

From the view-point of topography and_ geological
structure, the Region may be thought of as occupying parts
of two great physiographic provinces that are also distinct
geologically, namely, the Cordillera in the western section,
and the Great Central Plains in the east. Each of these,
in turn, may be subdivided variously into lesser physio-
graphic units, of which the following may serve for purposes
of the present outline:

( Coastal Mountain System
| Interior Plateaux and Mountains
of British Columbia
Yukon Plateau
Rocky Mountain System

Alberta Plain
Great Central Plains.. { Peel River Plateau

Mackenzie Lowlands

Cordillera

Although the eastern limit of the North Pacific Region
may be regarded, generally, as marked by the contact be-
tween the Precambrian rocks of the Canadian Shield and
the sedimentary rocks of the Great Central Plains, the
influence of the former in the future economy of the

Region is sufficient to warrant some reference to its features
and potentialities.

CorDILLERA
THE COASTAL MOUNTAIN SYSTEM

The Coastal Mountain System of the Cordillera com-
ptises the Coast Range in British Columbia and the Coast
and St. Elias Ranges in Yukon. The Coast Range is a
sharply-defined, deeply ‘dissected granitic ridge that rises
directly from the sea to heights of from 4,000 to 5,000 feet
and gradually increases in elevation toward the axis of
the range to from 7,000 to 8,000 feet. Deep, steep-sided,
ice-eroded valleys, many holding living glaciers, penetrate
the range in all directions and, with its fiord system, form
its most characteristic features. The fiords not only
penetrate the Coast Range for distances, in some instances,
of more than 100 miles, but form similar deep, narrow
depressions that trend in different directions and separate
the numerous islands fringing the coast. In places the
Range is cut by the deep, transverse valleys of rivers
draining the interior of British Columbia and Yukon. The
mountains of the Queen Charlotte Islands may be regarded

as part of an outer chain separated from the main range by
a submerged valley.

* Contributed by the Geological Survey of Canada.

This Coastal System is characterized by a huge granitic
batholsh, with an average width of 90 miles, and is probably
a composite of intrusions separated in age by considerable
time intervals. Following the great curve of the Pacific
Coast for 1,200 miles from the lower Fraser Valley to
Alaska, this batholith is the dominating geological feature
of the coastal section of the North Pacific Re

In Yukon, the Coast Range develops an irregular
series of peaks and ridges with little pattern other than a
rough alignment parallel to the northwesterly-trending axis.
Here the Range presents a precipitous and jagged aspect,
with knife-edged crests, rugged and sometimes needle-like
summits, and sharply incised valleys.

The great peaks of the St. Elias Range form the north-
western part of the Coastal System. They are composed
of sedimentary and volcanic rocks ranging in age from
Precambrian to Cretaceous, broken through by intrusions,
and overlain by sediments and volcanic rocks of early
Tertiary and later ages. These mountains have
heavily glaciated, and are even now largely ice-covered.

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INTERIOR PLATEAUX AND MOUNTAINS OF BRITISH COLUMBIA

Bordering the- Coastal mountain system along its
inland edge and extending thence east, northeast, and north
to the Rocky Mountain system lies the great region of
interior plateaux and mountains of British Columbia which,
with its northerly member, the Yukon Plateau, has a width
of 250 to 300 miles. Although over" large areas it is
mountainous, relief being expressed in long, even ridges or
isolated groups of peaks rising to 5,000 or 6,000 feet, its
general physiographic expression is in marked contrast
with the lofty bordering ranges. The terrain, with a
general average elevation of perhaps 3,500 feet, is traversed
by great valleys whose bottoms do not lie more than 1,000
feet above sea-level. The area is underlain by Paleozoic
and early Mesozoic formations, in many places largely of
volcanic origin, folded and faulted, and invaded by granitic
bodies, some of great size. Over large areas, these strata
are concealed by later Mesozoic and Tertiary beds.

The dominating topographic feature of the eastern
border of this system is the Rocky Mountain Trench, one
of the most extraordinary valleys of its kind in the world.
It is a straight, narrow trough extending from south of the
International Boundary in Montana to
the Turnagain and Kechika Rivers in northern British
Columbia, a distance of over 1,000 miles. It is occupied
at varying elevations, by the headwaters of the Columbia,
Fraser, Parsnip, Finlay, and Kechika Rivers. The east-
flowing streams leave the trench by transverse gorges cut
through the Rockies. Northwestward, toward the
Parsnip-Finlay part of the trench, in what is referred to as

the confluence of

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