Landforms of British Columbia 1976 - Department of Geography

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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. 100
movement, and regional studies of the movement may be made from aerial photo- graphs, which are readily available. Keewatin ice melted from the land when its source of supply failed. The ice- sheet lay stagnant and wasted away in place, producing great volumes of meltwater which accumulated into streams and flowed northward across the Fort Nelson Low- land to discharge into the Liard and Mackenzie Rivers. These meltwater channels were a prominent though temporary phase of the ice disappearance. After the meltwater diminished, the channels remained (see Plate XLA), and they are occupied now by underfit streams that carry only the present normal run-off and conse- quently are quite out of proportion in size to the channels they occupy. g@) ICC dammed lake -3 Ice movement ---Main ice divide -I-I Eastern limit Cordilloran ice . . .,. -. . . . . . . ^ “. . . . rigure lu. uiagram snowmg me alrections 01 movement or glacial ice and location of post-Pleistocene pro-glacial lakes. CORDILLERAN ICE-SHEET The Cordilleran Region in British Columbia was entirely covered by the Cor- dilleran ice-sheet. The ice reached its maximum extent later than did the Keewatin ice-sheet. The Pleistocene began with the accumulation of ice at several major centres and numerous subordinate ones. The distribution of present-day glaciers to a degree indicates where the major centres were located. These were in the Coast Mountains north of Portland Canal, in the Pacific Ranges between Bella Coola and Garibaldi, in the northern Selkirk and Purcell Mountains, and in the Rocky Moun- tains north of Banff. Others were on the Queen Charlotte Islands, Vancouver Island, northern Cassiar Ranges, northern ranges of the Rocky Mountains, centring about Mount Churchill, and in the Premier Group of the Cariboo Mountains. An irnpor- tant centre, not indicated by present-day glaciers, was in the Kitimat Ranges. The 101
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Bulletin 48 Landforms of British Co
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The technical references are believ
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PAGE Chapter II.-Physiographic Subd
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FIQ. ILLUSTRATIONS 1. Landform map
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PLATE PAOE XVIII. (33) Stikine Plat
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PLATE XXXVIIB. (70) Rocky Mountain
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traveller. If he SO wishes, his enj
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pendants which, because of their se
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As Iava cools and consolidates, it
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are lithologic differences from pla
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mentary rocks are folded and faulte
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Field work by members of the Geolog
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BIBLIOGRAPHY Alden, W. C.: “ Phys
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The western boundary of the Eastern
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INSULAR MOUNTAINS The Insular Mount
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H. C., “ Geology and Mineral Depo
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The extreme northeastern tip of Gra
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the strait. It is suggested that th
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The lowland consists of many low, w
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anges farther west. The ranges sout
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Pacific Ranges The Pa&c Ranges (see
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Columbia. The mountains along the 4
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ones, and widening and deepening th
Page 47 and 48: south end of Teslin Lake, which sep
Page 49 and 50: Between the Taku &nd Sheslay Rivers
Page 51 and 52: The tuyas consist of nearly horizon
Page 53 and 54: sloping areas of upland remain undi
Page 55 and 56: mostly in the Babine Range. The roc
Page 57 and 58: to Eutsuk Lake on the southern side
Page 59 and 60: I These valleys are at a fairly hig
Page 61 and 62: Detailed studies made around severa
Page 63 and 64: The area is drained by the Finlay,
Page 65 and 66: Plateau and the Nechako Plateau, an
Page 67 and 68: iver in pre-Pl&ocene time or whethe
Page 69 and 70: dissected upland surface of the pla
Page 71 and 72: Thompson and North Thompson River v
Page 73 and 74: Adams, and Shuswap Rivers and their
Page 75 and 76: highlands, but the main effects of
Page 77 and 78: eda Rivers on the west and the Colu
Page 79 and 80: The amount of relief is governed by
Page 81 and 82: “ Striae, roches moutonn&s, and l
Page 83 and 84: The boundary between the Rocky Moun
Page 85 and 86: [References: Price, R. A., “ Fern
Page 87 and 88: 8,820 feet, and the lowest point in
Page 89 and 90: margin of the Rocky Mountains% it s
Page 91 and 92: Devonian limestones that are thrust
Page 93 and 94: locally, as a rule only on the nort
Page 95 and 96: Dept. of Mines and Petroleum Remmx?
Page 97: CHAPTER III.-SPECIAL ASPECTS OF THE
Page 101 and 102: Both the Atlas of British Columbia
Page 103 and 104: ase of the Sumas till in the Fraser
Page 105 and 106: British Columbia includes the drain
Page 107 and 108: The Parsnip and Finlay Rivers were
Page 109 and 110: upper Fraser River is the result of
Page 111 and 112: Its pattern of flow within the moun
Page 113 and 114: materials. Along the west coast of
Page 115 and 116: , Pleistocene probably has been bet
Page 117 and 118: A small volcanic cone with associat
Page 119 and 120: ‘I , titularly for those interest
Page 121 and 122: The Pinchi fault and its northweste
Page 123 and 124: component of the discordant pattern
Page 125 and 126: APPENDIX B.-GLOSSARY * Aggrade-to b
Page 127 and 128: Lineation-in rock the parallel orie
Page 129 and 130: INDEX The place names listed are th
Page 131: 85 126 :: :: 116
Page 134 and 135: Quam Quartz ~ountam Queen Bess, Mou
Page 136: 138
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Landforms of British Columbia 1976 - Department of Geography

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