196

As it is probable that only the lower part of the Sustut group is exposed
on Finlay Mountain, and that the Sifton formation may be an upper part
of the group, the actual vertical displacement may be much in excess of
the above figure.

An island in Finlay River about 5 miles east of the map-area is reported
to expose altered green volcanic rock over an area of about 5 acres.1 If
these outcrops represent ‘a block of late Paleozoic or Takla group volcanic
rocks downfaulted to their present position, it follows that the Ingenika
group rocks now flanking the trench were overlain in this region by late
Paleozoic or Takla group strata that were in turn overlain by the Sustut
group. If such is the case, the amount of downfaulting suffered by the
rocks in the floor of the trench may be tens of thousands of feet.

TOMIAS LAKE SYNCLINORIUM AND PELLY CREEK THRUST:
THE ‘PELLY CREEK LINEAMENT?’

The overturned synclinorium whose axis passes through Tomias Lake
Valley (See Structure-section E-F), and is exposed on Swannell River,
becomes progressively more compressed toward the north, and develops
into a thrust fault in Pelly Creek Valley. North of the mouth of Tucha
Creek the plane of this thrust may be seen to dip about 40 degrees to the
northeast, passing under the overturned anticlinorium of the Russel Range
(See Structure-sections C-D and A-B). The comparatively more sym-
metrical series of folds of the Espee Range and Forres Mountain, them-
selves separated by a fault that is probably a northeast-dipping branch
of this thrust, have been relatively lowered, and possibly rotated so that
they strike northwesterly and abut against the more northerly striking
folds of the Russel Range.

The thrust along the line of Pelly Creek Valley appears to be the result
of horizontal shortening, caused by northeast-southwest compression, that
was greater in the north than in the south, and it must have formed during
the same period of deformation as the folds along the same structure to
the south. It, therefore, antedates all the known steeply dipping faults
in the area.

Pelly Creek and Tomias Lake Valleys form part of a straight, narrow,
topographic depression, more than 300 miles long?. This depression, which
might be termed the ‘Pelly Creek lineament’, diverges from the Rocky
Mountain Trench, in Parsnip River Valley, at the northern end of an
irregular area that separates the trench itself into distinct northern and
southern parts. It continues, as a valley occupied successively by Omineca
River, Mesilinka River, Carina Lake, Tomias Lake, Pelly Creek, Obo River
and Lake, Frog River, Dall River and Lake, to Deadwood Lake, west of the
north end of the Rocky Mountain Trench proper (Hedley and Holland,
1941). In part, it marks the division between Finlay Ranges and Swannell
Ranges (Bostock, 1948, p. 43). Throughout its entire length this depression
maintains a strike of about north 34 degrees west. The Rocky Mountain
Trench strikes north 31 degrees west at this latitude, and the two linea-
ments are probably genetically related. The evidence from the Aiken
Lake area suggests that, locally at least, the Pelly Creek lineament repre-
sents an east-dipping plane of weakness, which has formed the axial plane

1Bronlund, E.: personal communication, 1948.
2Well shown on National Topographic Series map, Sheet 94SW., Finlay River (8 miles to 1 inch).