Landforms of British Columbia 1976 - Department of Geography

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The extension of the trough through southeastern Alaska is marked by low-lying areas or areas underlain by Tertiary rocks. It terminates finally at Chatham Strait, which is the site of a major north-striking fault. The Coastal Trough has a width of 75 miles between Graham Island and Percher Island, is constricted to a width of 10 miles at Sayward in Johnstone Strait, and broadens to about 30 miles at Campbell River. It is 100 miles wide at the re- entrant of the Fraser River, where it extends eastward to a point just west of Hope. The constriction at Sayward, the Seymour Arch, * divides the Trough into a northern section, the Hecate Depression, and a southern section, the Georgia Depression. The line of the 2,000-foot contour is used arbitrarily as the boundary between the lowlands and the mountains. HECATE DEPRESSION The Hecate Depression between Johnstone Strait and Dixon Entrance is very largely beneath sea-level, being occupied by Queen Charlotte Strait, Queen Charlotte Sound, Hecate Strait, and Dixon Entrance. Between Graham Island and Percher Island the sea floor beneath Hecate Strait does not lie below a depth of 300 feet, but depths greater than 1,200 feet are reached farther south in Hecate Strait and to the north in Dixon Entrance, where ice-gouged troughs extend across the Depression. The parts of Hecate Depression above sea-level are the Queen Charlotte Lowland on Graham Island, the Nahwitti Lowland on northern Vancouver Island, and the Hecate Lowland along the mainland coast and on the offshore islands. Queen Charlotte Lowland The northeastern part of Graham Island, northeast of a line between Copper Bay and Beresford Bay, is low, with little relief, and constitutes part of the Hecate Depression. Northward from Lawn Hill (560 feet) and east of Masset Sound the terrain is very largely below 500 feet elevation, while west of Masset Sound the surface rises gradually but stays well below 1,000 feet. The boundary between the Skidegate Plateau on the southwest and the Queen Charlotte Lowland on the northeast is marked by the generalized line of the 500-foot contour between Copper Bay and Masset Inlet. Northwestward from Masset Inlet the generalized line of the l,OOO-foot contour is more suitable. For about 50 miles northwest of Copper Bay the boundary more or less coincides with the trace of a fault which has been postu- lated on both geological and geophysical grounds. Drainage on the lowland surface is poorly organized. A very large percentage of the area is muskeg and the timber cover is sparse. Except for the extreme northeastern tip of Graham Island, the area of the Argo- naut Plain, the lowland is underlain by early Tertiary basaltic lavas and late Tertiary sedimentary rocks covered with a thin veneer of till. Most of the lava flows do not exceed 100 feet in thickness, and many have well-developed columnar jointing; they dip seldom more than 20 degrees to the east. The gentle rise of the lowland surface westward from Masset Sound probably is the surface reflection of the gently east- dipping flows. The movement of glacial ice across the lowland is shown by drumlin-like forms and flutings, which at Juskatla Inlet and the head of Masset Inlet indicate a northeast diiection of travel. The direction changes progressively to more westerly, and striations at the mouth of Masset Inlet trend north 10 degrees west. The ice eventu- ally joined with ice from Alaskan and mainland glaciers which flowed through Dixon Entrance past and over Langara Island in a westerly direction. * Bostock, H. S., Gtd. Surv., Canada, Man. 241, p. 89. 33
The extreme northeastern tip of Graham Island is underlain largely by late Tertiary sedimentary rocks overlain by several hundred feet of Pleistocene and Re- cent unconsolidated sands and gravels. This is the Argonmt Pluin (see Plate VA), a glacial outwash plain of post-Pleistocene age whose surface lies at and below about 550 feet elevation and has only slight relief. The plain is younger than the last ice advance and, because it is not overridden, shows no glacial modification of the unconsolidated materials. The beach bordering the lowland between the entrance to Masset Sound and Dead Tree Point, a distance of more than 75 miles, is a remarkable feature (see Plate XLVB.) It is an almost continuous wide sand beach for the entire distance. The material has been derived from the erosion of unconsolidated Pleistocene materials along the east coast. Under the influence of the prevailing southeast wind and cur- rent, the material is transported northward along the beach to form Rose Spit. From there the material is transported westward, building the north coast beaches as far west as Masset Sound. Strong winds have produced sand dunes along the north and east coast beaches. The dunes may extend inland for several hundred yards beyond the back beach area. [References: MacKenzie, .l. D., “ Geology of Graham Island,” GeoZ. Surv., Canudu, Mem. 88,1916; Sutherland Brown, A., “ Physiography of the Queen Char- lotte Islands,” Gun. Geog. Jour., Vol. XLI, No. 1, 1960, pp. 30-37; Sutherland Brown, A., and Nasmith, H., “ Glaciation of the Queen Charlotte Islands,” Curt. Field Nuturulist, Vol. 76, No. 4, 1962, pp. 209-219.1 Nuhwitti Lowlund The northern tip of Vancouver Island, lying north of a line between Englewood and Quatsino Sound, is below 2,000 feet in elevation, except for a few isolated sum- mits. It is an area of low relief lying within the Hecate Depression. The high Vancouver Island Ranges end abruptly along the southern boundary of the Nahwitti Lowland, giving way to an area of low relief and of low rounded hills (,see Plate VB), in which remnants of the dissected Tertiary erosion surface slope northward to sea-level between Cape Scott and Cape Sutil. The area was overridden by ice, and it is evident from roches moutonnees on the ridge northeast of Rupert Inlet that ice from the mainland moved southwestward and westward across Queen Charlotte Strait, to discharge seaward along Quatsino Sound and Holberg Inlet. Within the lowland there is a roughly triangular area, bounded by lines extend- ing between Englewood, Quatsino, and Port Hardy, which has a gently rolling surface seldom exceeding 1,000 feet in elevation. This is tie Suquash Basin, which includes Malcolm and Cormorant Islands, and is underlain by gently dipping Upper Cretaceous coal measures. The southern margin of the basin is a westerly striking fault aIong the abrupt front of the Hankin and adjoining Bonanza and Franklin Ranges. Erosion of the soft Cretaceous sedimentary beds has produced the low-lying gentle topography within the Suquash Basin, whereas volcanic rocks of the older Vancouver Group, being harder and more resistant, underlie the somewhat higher ground within the Nahwitti Lowland. [Photographs: B.C. 664:52,57.] Hecute Lowlund A strip of low-lying country, including both mainIand coast and adjacent islands, extends along the eastern side of the Coastal Trough southward from Prince Rupert to Vancouver. A constriction of the trough at Sayward separates a northern 34
Page 1 and 2: Bulletin 48 Landforms of British Co
Page 3 and 4: The technical references are believ
Page 5 and 6: PAGE Chapter II.-Physiographic Subd
Page 7 and 8: FIQ. ILLUSTRATIONS 1. Landform map
Page 9 and 10: PLATE PAOE XVIII. (33) Stikine Plat
Page 11 and 12: PLATE XXXVIIB. (70) Rocky Mountain
Page 13 and 14: traveller. If he SO wishes, his enj
Page 15 and 16: pendants which, because of their se
Page 17 and 18: As Iava cools and consolidates, it
Page 19 and 20: are lithologic differences from pla
Page 21 and 22: mentary rocks are folded and faulte
Page 23 and 24: Field work by members of the Geolog
Page 25 and 26: BIBLIOGRAPHY Alden, W. C.: “ Phys
Page 27 and 28: The western boundary of the Eastern
Page 29 and 30: INSULAR MOUNTAINS The Insular Mount
Page 31: H. C., “ Geology and Mineral Depo
Page 35 and 36: the strait. It is suggested that th
Page 37 and 38: The lowland consists of many low, w
Page 39 and 40: anges farther west. The ranges sout
Page 41 and 42: Pacific Ranges The Pa&c Ranges (see
Page 43 and 44: Columbia. The mountains along the 4
Page 45 and 46: 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 and 98:
CHAPTER III.-SPECIAL ASPECTS OF THE
Page 99 and 100:
movement, and regional studies of t
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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