Landforms of British Columbia 1976 - Department of Geography

Both the Atlas of British Columbia Resources and the Glacial Map of Canada
show the known details of ice movement. Direction of movement across the
plateaus is well displayed on aerial photographs, but information from the
mountain belts is far from complete.
After the Cordilleran ice-sheet waned, it was succeeded by a stage of alpine
and cirque glaciation. The intensity of this glaciation and its localization was
largely controlled by a combination of altitude and climatic factors. Where
precipitation was heavy, as in the Fairweather, Boundary, and Pacific Ranges, this
stage was most intense (see Plates I, VIIIB, and IX). Where the precipitation
was less heavy, as in the Park (Main) Ranges of the southern Rocky Mountains
and in the Tagish and Tahltan Highlands, there is a noticeable difference in the
degree of alpine erosion (see Plates XIIIB and XVB). In areas such as the
Omineca, Cariboo, and Chilcotin Mountains cirque glaciation was active only on
the northern and northeastern .(protected from the sun) sides ‘of mountain summits
(see Plate XXA). In some areas in the southern and northern Rocky Mountains
and in the Okanagan, Shuswap, and Cariboo Highlands, low precipitation prevailed,
and there is little or no evidence of late-stage cirque glaciation (see Plates XXVIIA,
XXVIIIB, and XXXIB) .
Evidence has accumulated to indicate that there were at least two major
advances of the Cordilleran ice-sheet, separated by an interglacial stage* and that
in some peripheral areas there were three or more.?
Information of this sort is derived largely from a study of the glacial materials,
and it is only rarely that the landforms themselves reveal the great complexities of
glacial events. Actually, the landforms that we observe are very largely the
erosional or depositional products of the last ice advance (the Wisconsin stage).
One aspect of glacial history, that is not always evident from the observation
of present landforms, is that most of the major valleys of the Province were eroded
to considerable depths, and in some instances were greatly overdeepened by the
passage of valley glaciers through them. Subsequently the valleys have been partly
filled with unconsolidated sands and gravels, in many cases to considerable depths,
and it is only when a valley has been re-excavated, where drill-holes for dam
foundations, ground-water, or other purposes are put down, or where lake soundings
are recorded that the great depth to bedrock in the valley bottoms becomes known.
For example, information from an oil well at Commotion Creek discloses that
bedrock lies beneath 1,08 1 feet of Recent and Pleistocene unconsolidated sediments
below the Pine River; downstream from Hudson Hope, bedrock is at a depth of
320 feet below the Peace River, which flows over unconsolidated sands and gravels;
and in Okanagan Lake (elevation, 1 ,123 feet) the maximum sounding is 760
feet, while at Enderby about 1,300 feet of Pleistocene and Recent unconsolidated
sediments was encountered in a drill-hole. The greatest amounts of overdeepening
are in the coastal fiords. A sounding of 2,574 feet recorded from Finlayson Channel
and one of 2,898 feet near the south end of Chatham Strait indicate that local
overdeepening of more than 2,000 feet has taken place.
MELTING OF THE ICE
The Cordilleran ice-sheet disappeared at the end of the Pleistocene, and, as
the land emerged from beneath its cover of ice, drainage was re-established. The
ice-sheet in its waning stage became stagnant through lack of accumulation in the
source areas, and wasted away in place. As a consequence there are numerous
small-scale features related to the final stages of the disappearance of the ice.
* Armstrong, J. E., and Tipper, H. W.; Am. Jour. Sci., Vol. 246, 1948, p. 306.
t Halstead, E. C., Geol. Surv., Canada, Water Supply Paper No. 327, 1957, p. 8.
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