Landforms of British Columbia 1976 - Department of Geography

The Coast Mountains comprise sedimentary and volcanic rocks of middle
Jurassic and older age that have been intruded by a composite batholith comprising
multiple intrusions of rocks that are essentially granodiorite and quartz diorite in
composition-the Coast Intrusions. The eastern contact of the Coast Intrusions is
a somewhat irregular line extending southeastward from Taku Arm of Tagish Lake,
passing west of Tulsequah, through Stewart, the west ends of Whitesail and Eutsuk
Lakes, and the south ends of Chilko and Taseko Lakes to Lytton. Within the Coast
Mountains there are ranges, particularly on the west, formed very largely of granitic
rocks as well as ranges which are largely composed of sedimentary and volcanic
rocks but whose cores may be stocks or batholiths of granitic rock. To the south of
the Nass River the eastern contact of the Coast Intrusions provides a satisfactory
boundary between the Hazelton Mountains and the granitic mountains of the
Kitimat Ranges on the west. It also provides a boundary between the Chilcotin
Ranges and the rest of the Pacific Ranges on the west.
Division of the mountains into three natural units is made along the Nass River
valley and along the Burke Channel-Bella Coola River valley. The northern section,
comprising the Boundary Ranges, has a high proportion that is glacier-covered and
culminates in the high peaks of Mount Ratz ( 10,290 feet) and Kates Needle ( 10,002
feet) north of the Stikine River; the central section, comprising the Kitimat Ranges,
has a few peaks between 8,000 and 9,000 feet, but most summits are below 8,000
feet; and the southern section, comprising the Pacific Ranges, has numerous peaks
higher than 9,000 feet. The Pacific Ranges culminate in Mount Waddington
(13,177 feet) and the surrounding mountain masses centring around Mount Silver-
throne (9,700 feet), Mount Queen Bess ( 10,700 feet), and Monmouth Mountain
(10,470 feet). Summit elevations in the Coast Mountains on the whole decrease
northward from a culmination just north of the Stikine River and decrease south-
ward from the culmination of Mount Waddington. Summit elevations are low in
the area between the Nass River and the Bella Coola River.
Further subdivision of the Coast Mountains may be made by using as bound-
aries the valleys of a number of major rivers which rise east of the axis of the moun-
tains and flow across them westward to the sea. In addition to the Nass and Bella
Coola, there are the Taku, Whiting, Stikine, Unuk, Bear, Skeena, Dean, Klinaklini,
and Homathko Rivers, whose valleys break the mountain units into smaller blocks.
Boundary Ranges
The Boundary Ranges (see Plates VIIIA, VIIIB, and XLIA) comprise the
dominantly granitic mountains along the Alaska-British Columbia Boundary, extending
northwestward from the Nass River.
In the northern section, in particular north of the Iskut River, the Boundary
Ranges are bordered by the Tagish and Tahltan Highlands. The highlands form
mountainous transition belts lying between the high rugged granitic mountains along
the Alaska Boundary and the essentially 5,000-foot upland surface of the Yukon
and Stikine Plateaus respectively.
The ranges have a core of intrusive granitic rocks which are flanked along the
eastern margin by sedimentary and volcanic rocks of Paheozoic and Mesozoic age.
Granite is extensively exposed along the axis of the ranges. Mixed assemblages of
Triassic and Jurassic greywackes and volcanic rocks predominate along the eastern
contact of the batholith.
The high peaks are serrate. The summit level is somewhat lower in the ranges
east of the granitic contact. There are noticeable topographic differences between
the erosion forms in sedimentary rocks and in granitic rocks, the sedimentary rocks
tending to produce a sharp topography that is more irregular than that of the granitic
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