22

Lake. These small streams became Thane Creek, Vega Creek, and the
south branches of Tenakihi Creek. By the onset of the last period of
glaciation the drainage pattern was apparently not greatly different from
that at present.

The last advance of the Cordilleran Ice-sheet had little large-scale
effect on the drainage pattern. The trunk glaciers followed the large
troughs, as before, although a proportionately larger amount of the ice
drainage from the upper Osilinka Valley appears to have been distributed
through Wasi Lake Valley. The south-trending through valley west of
Uslika Lake, which had been segmented by Thane, Vega, and Tenakihi
Creeks, was again filled by a south-flowing glacier, which deposited till in
the gorges of the pirate streams. The newly completed valley northeast of
Uslika Lake appears to have been too tortuous, and in too sheltered a
position, to carry a vigorously flowing glacier, and here, unique among
the larger valleys in the map-area, evidence of glaciation is limited to a
slight rounding of the interlocking spurs, and a series of connected and
en échelon channels cut in the valley walls by streams flowing along the
margin of the glacier. Till was spread on the remnant shorelines of the
inter-Glacial Uslika Lake. The recessional moraine in the upper Osilinka-
Discovery Creek trough was preserved and perhaps added to; near Chude-
latsa Lake it now stands 450 feet above the slightly entrenched Osilinka
River. But wherever the deposits of the last ice advance have been
identified they are thin; and all the evidence is in accord with the conclu-
sion, reached elsewhere (Armstrong, 1949, p. 14), that the last ice-sheet in
north-central British Columbia, and the valley glaciers of its dying stages,
carried a comparatively light load of debris.

Post-Glacial stream erosion has as yet caused little change in the
drainage pattern: a pirate stream from Tutizika River has captured what
was formerly the head of the Tenakihi Creek system; down-cutting in the
narrow valley north of Uslika Lake has entrenched Osilinka River in a
rocky gorge; the level of the outlet of Uslika Lake is being maintained by
an alluvial fan at the mouth of Thane Creek; and Wasi Creek appears to
be resuming its status as a pirate stream, and is cutting a new channel,
in bedrock and moraine, into the upper Osilinka-Discovery Creek trough.

GLACIO-FLUVIAL DEposits

Glacio-fluvial material is mixed with moraine in almost all the valleys
of the map-area, and presents no special features. Most is an interlayered,
highly variable assemblage of silts, sands, and gravels. In the larger
valleys this outwash has accumulated to an unknown depth, forming a flat
floor that has been cut into by the present streams. The original level of
glacio-fluvial fill is preserved in a line of extensive terraces on each side of
the more open parts of Ingenika, Mesilinka, and Osilinka River Valleys,
below which the present river meanders over a flat floor as much as 150 feet
lower (PlatesI, IIA). The abundance of smaller terraces on the mountain-
sides, made at the edge of valley glaciers, has been noted above (Plate
IIIB). Ina few places no terraces are recognizable, but a cover of boulder
clay and sand blankets the valleys slopes to a depth of as much as 50 feet
up to 1,500 feet above the river level (Plate V A). At other places, boulder