Landforms of British Columbia 1976 - Department of Geography

The escaping meltwater eroded channels which were later abandoned as ice levels
were lowered and permanent drainage lines were established. These channels may
be seen at high elevations on valley walls or crossing plateau surfaces, and may
cut across rock spurs well above the present level of drainage (see Plate XXVIIIA) .
Large volumes of meltwater were released as the ice melted, and in many
places ice marginal lakes were formed about stagnant masses of ice lying in the
valleys. Sediments deposited in the lakes remained as terraces and abandoned
deltas when the water-level was lowered. Many such lakes were ephemeral, and
disappeared when the ice blocks finally melted. Other more permanent ones occupy
basins which were created during the Pleistocene. The major lakes, such as Atlin,
Babine, Whitesail, Quesnel, Okanagan, etc., occupy basins parallel to the direction
of ice flow, and in all instances the basins were overdeepened by valley glaciers.
Smaller lakes, many of them very beautiful and world famous, such as Lake Louise,
are tarns occupying rock basins excavated in the mountains by cirque glaciers.
There are in addition many hundreds of small lakes occupying shallow depressions
in areas of drumlinized till, or kettles in areas of glacial outwash.
One of the most widespread effects of the glacial occupation of the country
is the derangement of drainage, in some instances on a major scale. Derangement
of drainage was due largely to the blocking of pre-glacial drainage channels by
ice lobes and glacial moraines which forced the water to flow contrary to its pre-
glacial direction. When this took place on a regional scale, it resulted in major
shifting of the watershed between the Fraser and Peace River drainage basins,
disturbances of drainage in the area between Shuswap Lake and the north end of
Okanagan Lake, and creation of unusual stream patterns such as are followed
by the Kootenay, Nass, and Skeena Rivers (see pp. 56, 67, 110, 111).
POST-GLACIAL EFFECTS
At the maximum stage of ice occupation the ice reached an elevation of more
than 8,000 feet over the southern interior of the Province. The great weight of
ice overloaded the earth’s crust, and, as a consequence, the land surface was
depressed with respect to sea-level. Regionally, the amount of the depression
was proportional to the depth of ice cover. The amount of depression is considered
to have been at least 1,000 feet at the Fraser River delta.*
Since the ice-sheet disappeared the land has risen with respect to sea-level
and has essentially regained equilibrium. The rise in some places may be measured
by the occurrence of marine shells and marine sediments above present sea-level.
This rise accelerated erosion in the lower courses of post-glacial streams, and as a
consequence canyons in rock or in valley till have been incised. Because there was
unequal ice loading during the Pleistocene, there was an unequal rise of the land
when the ice melted. A differential uplift of as much as a few feet per mile has tilted
the old shorelines of glacial Lake Peace from their original (Pleistocene) horizontal
positions.
The length of time that has elapsed since the disappearance of glacial ice
varies from place to place. Some places are still covered by ice which might be
considered a last remnant of the Pleistocene ice cover, and some areas, such as Tarr
Inlet at the head of Glacier Bay, have only within the last several decades emerged
from beneath the ice. In general, however, the Pleistocene is considered to have
ended about 10,000 years ago. It is by means of radio-carbon age determinations
that precise time measurements are made. Using this method, wood from the
* Armstrong, J. E., and Brown, W. L., Geol. Sm. Am., Bull. Vol. 65, 1954, p. 362.
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