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area the ranges are separated by wide, rolling valleys, most of which drain
southeastward. The timbered higher benches of most of the valleys have
elevations of about 3,000 feet, and the valley floors from 1,500 to 2,500
feet above sea-level. The plateau is underlain by the same Palseozoic and
Mesozoic sedimentary formations as the Rocky Mountains and Foothills.
The separate ranges show a general tendency to parallelism, as in the
Rocky Mountains, but are more widely spaced. Folds trend northwest
for some miles north of Liard River, then swing northeasterly, and west of
Fort Liard they again strike uorth.

The east boundary of Liard Plateau trends northeastward and then
north on the west side of Liard River from opposite the mouth of Toad
River to South Nahanni River. Thus it continues the habit of the front
of the Foothills in following the first distinct folds west of the Interior
Plains. The west boundary of the plateau follows up Smith River through
Toobally Lakes to Caribou River.

LIARD PLAIN

Liard Plain includes the greater part of Liard River Valley west of
Smith River. It occupies a northwesterly trending area some 180 miles
long and up to 65 miles wide; the greater part is less than 3,000 feet high,
and much of the central part is at an elevation of only 2,200 feet (Bostock,
1948). The Plain is hemmed in by high plateaux and mountains in which
the tributaries of the Liard arise, and both Liard River and its main
tributaries are entrenched below its general level. Palwozoic sedimentary
rocks and Tertiary strata are exposed along the rivers and in the hilly
areas, but the greater part of the lower land is mantled by thick deposits
of glacial drift.

ROCKY MOUNTAIN TRENCH

The Rocky Mountain Trench forms the west boundary of the Rocky
Mountains. It has an overall length of more than 900 miles, from south
of the Forty-ninth Parallel to Liard Plain. Except in the central part,
between Téte Jaune Cache and Fort McLeod, the trench is bounded on
one or both sides by steep slopes. The floor of the trench is nearly flat, and
varies from 2 to 10 miles or more in width to where it is walled in by
mountains. For most of its length it lies between 2,000 and 3,000
feet above sea-level, the highest point being at Sifton Pass, elevation 3,273
feet. Peace River leaves the trench at a little below 2,000 feet. At the
north end of the Rocky Mountains the Cassiar Mountains recede from the
western side of the trench, which opens out into Liard Plain, and its
boundaries disappear. Some exceptionally fine views of the trench have
been published by Andrews (1942).

Dolmage (1928) describes the trench in the Finlay River district, as
follows:

“Finlay River occupies the Rocky Mountain Trench from Finlay
Forks as far north as Fox River, a distance of 125 miles. Beyond this
point the Fox occupies the trench for 41 miles to Sifton Pass, beyond
which the Trench is drained to the north by a tributary of the Liard.
The Finlay rises in Fishing Lakes 35 miles west of the Trench, and after
flowing north for 25 miles bends east and then southeast, and enters the
Trench at the mouth of the Fox.