Landforms of British Columbia 1976 - Department of Geography

which, by continued growth, expand into mountain and valley glaciers. Further expansion
of the ice produces a mountain ice-cap whose movement is controlled by the
underlying topography. Finally, at the maximum stage the land is occupied by a
regional ice-sheet whose movement and extent are controlled mainly by climatic
factors and are largely independent of the underlying topography. As the ice cover
diminishes, there is regression to the mountain ice-cap and then the valley glacier
stages, and the last ice to remain is in small cirque glaciers as in the beginning of the
cycle.
In each of the several stages of the glacial cycle the erosional and depositional
landforms are distinctly different. Mountain glaciers produce the numerous forms
so typical of high alpine scenery, particularly horn-shaped peaks, serrate ridges, highlevel
U-shaped divides (~01s) , and cirque basins with their steep head walls (see
Plates VIIIA, IX, XIIA, and XLIB) . Valley glaciers modify valley profiles by facetting
the spurs and producing the characteristic U-shaped cross-section of glaciated
valleys (see Plates XXVIIIc and XLIIB) . Hanging valleys result from the differential
erosion of main and lateral valleys (see Plate XLIIA). Through valleys are
eroded by outlet glaciers composed of ice moving outward from an area of accumulation.
Valley glaciers produce characteristic depositional forms, such as the terminal
and lateral moraines left upon disappearance of the ice. Features of glacial erosion
and deposition are common in the mountain ranges across the length and breadth of
British Columbia. They are such an integral part of the landscape that many residents
assume they are universal in mountainous regions and do not realize they are
unique to glaciated mountains.
When a mountain ice-cap is present, ice covers and moves over all but the highest
peaks and ridges (see Plate XLIA) . In so doing it rounds the ridge-tops, it covers
and produces dome-shaped summits (see Plate XA), and may destroy or greatly
modify the glacial forms of earlier stages. At the maximum stage, when a regional
ice-sheet covers the land, erosion is not great and the tendency is for the relief to be
reduced rather than increased. A widespread mantle of drift was deposited by the
Pleistocene ice-sheet in British Columbia. As the ice continued to flow from its
gathering grounds, it rode over ground moraine and produced in it drumlin-like
forms and flutings which are parallel to the direction of movement. The Interior
Plateau (see Plate XXV), Stikine Plateau (see Plate XVIIA) , and Liard Plain (see
Plate XXXVIIA) all display these features to a remarkable degree.
KEEWATIN ICE-SHEET
The Interior Plains region of northeastern British Columbia was occupied by ice
that accumulated in the Keewatin area west of Hudson Bay and, moving outward in
all directions, spread westward and southwestward to cover all the Fort Nelson
Lowland and Alberta Plateau (see Fig. 10). The western margin of the Keewatin
ice-sheet lay along the eastern front of the Rocky Mountain Foothills, although some
ice may have pushed westward into the foothills and mountains. The Keewatin
ice-sheet had reached its maximum extent and its front had receded before Cordil-
leran ice moved eastward from the Rocky Mountains and overlapped a narrow zone
formerly occupied by Keewatin ice., The zone of overlapping @as involved in a
complex glacial history as a result of fluctuations of the fronts of the two ice-sheets.
The detailed mapping of such a zone in southwestern Alberta* illustrates the sort
of glacial activity that took place all along the mountain front.
The Keewatin ice produced little erosion in British Columbia, and evidence of
its former extent lies in the varied deposits of drift containing boulders derived from
eastern sources. In some areas the drift is fluted parallel to the direction of ice
l Stalker, A. M., Geol. Surv., Canada. Map 31.1961.
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