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“In Finlay River district, the Rocky Mountain Trench is from 4 to 8
miles wide and floored throughout by gravels and silts, usually well ter-
raced. Only one rock canyon occurs in this section of the Trench. This
is Deserters Canyon, situated 82 miles above the mouth of the Finlay and
30 miles above Fort Grahame. Here the river is a rushing torrent confined
within a width of less than 100 feet, whereas, elsewhere, throughout its
course through the Trench, it is from 100 to 300 yards wide and usually
divided into a number of channels. The current of the river is swift,
attaining a speed, during high water periods, of 12 miles an hour; it winds
from side to side of its broad valley, rapidly cutting away the soft sandy
or gravelly banks and felling into its stream large numbers of the trees
from the dense forests covering the entire bottom of the valley. The
meanders are rapidly cut through, forming new channels and many oxbow
lakes or sloughs, through which the river flows only during the periods
of high water. At such periods it is a maze of intricate channels very
confusing to the inexperienced navigator.”

OMINECA AND CASSIAR MOUNTAINS

Omineca and Cassiar Mountains form an almost continuous belt of
mountains along the west side of the Rocky Mountain Trench and Liard
Plain from the Interior Plateau to Yukon Plateau. The easterly trending
valley of upper Finlay River, west of the Rocky Mountain Trench, forms
a low, natural dividing line between Cassiar Mountains on the north and
Omineca Mountains on the south. Finlay Ranges lying along the west
side of the Rocky Mountain Trench constitute the most easterly division
of Omineca Mountains and include Russel, Butler, and Wolverine Ranges.
Finlay Ranges are described by Bostock (1948) as follows:

“Finlay Ranges are notable for the smooth profiles that characterize
their slopes and most of their summits in contrast with the ruggedness of
other parts of Omineca Mountains. The general plan of each of their
main ranges shows a single backbone ridge approximately parallel with
the Rocky Mountain Trench with spurs projecting normally from it.
Except for a small granitic stock between Mesilinka and Swannell Rivers
they are composed mainly of Precambrian and Paleozoic sedimentary
rocks, with areas of Mesozoic strata along their west flank. The only
glacier noted is near Swannell River, and as a whole the ranges show only
little evidence of glacial erosion.”

Cassiar Mountains comprise Stikine and Kechika Ranges and a
fringe of plateau country; Dease Plateau is included with them. The
Kechika Ranges are the northward extension of Finlay Ranges along the
west side of the Rocky Mountain Trench; they consist of more or less
parallel ridges, with moderately even summits, and are composed largely
of Paleozoic sedimentary rocks.

GLACIAL DEPOSITS

Glacial deposits are to be found almost everywhere in northeastern
British Columbia, and occupy all the larger river valleys. The wane of
the Ice Age and melting of the valley glaciers resulted in the deposition of
boulder clay, sand and gravel, and varved clays, and silts were laid down
in the numerous temporary lakes that formed near the melting ice fronts.