Landforms of British Columbia 1976 - Department of Geography

Pacific Ranges
The Pa&c Ranges (see Plates IX, XB, XIIA, XIIB, XLIB, XLIIA, and XLVIA)
comprise the essentially granitic mountains extending southeastward from Burke
Channel and Bella Coola River for about 300 miles to the Fraser River. The ranges
have a width of 80 to 100 miles between their western boundary along the Coastal
Trough and,their eastern boundary with the Interior System. On the western side
the summit levels diminish to the west with the downward slope of the dissected late
Tertiary erosion surface. The boundary between the Pacific Ranges and the Hecate
and Georgia Lowlands is along the generalized line of the 2,000-foot contour. On
the east the Pacific Ranges between Atnarko River and the head of the Yalakom
River are flanked by the Fraser Plateau, from which the mountain front rises abruptly
(.see Plate XIB) . From the head of the Yalakom River the eastern boundary is along
the Yalakom and Fraser Rivers, which separate the Pacific Ranges from the Camelsfoot
and Clear Ranges, and from the Cascade Mountains.
The Pacific Ranges contain the highest peaks in the Coast Mountains. From
Mount Saugstad (9,608 feet) just south of the Bella Coola River, peaks rise to 9,700
feet at Silverthrone Mountain west of the Klinaklini River, to 13,177 feet at Mount
Waddington (see Plate IX), and to 12,800 feet at Mount Tiedemann between the
Klinaklini and Homathko Rivers. There are a number of 1 O,OOO- to 11 ,OOO-foot
peaks between the Homathko River and Chilco Lake and south to the head of the
Lillooet River. South and southeast from Wedge Mountain (9,484 feet) and Mount
Garibaldi (8,787 feet) the summit elevations diminish as the edge of the mountains
is approached. The mountain front rises from the Fraser Lowland on a long slope
of about 15 degrees, which, in part, is an exhumed pre-Eocene erosion surface* on
which Eocene and younger sediments had been deposited (see p. 37).
Drainage in the Pacific Ranges is to the coast by way of the Bella Coola, Kingcome,
Klinaklini, Homathko, Southgate, Toba, Squamish, and Lillooet Rivers and
their tributaries. These have cut major low-level valleys through the mountains,
dividing them into blocks.
Bedrock structures have controlled the drainage pattern. In places within the
granite a trellis-type pattern is parallel to regionally developed sets of joints or faults.
Further structural control of drainage and of landforms is illustrated east of Cape
Caution, where maps and aerial photographs show prominently developed easterly
and northwesterly lineaments, represented by alignments of inlets, lakes, and stream
valleys. The Owikeno lineament (,see Plate XLVIII) has a length of about 60 miles
westerly from Mount Silverthrone. Another remarkable lineament can be seen on
aerial photograph B.C. 566:55 to extend in a northwesterly direction for at least
20 miles along Lillooet River between Pemberton Meadows and the junction of
Meager Creek.
The high peaks are sculptured by cirque glaciers. Many projected as nunataks
above the Pleistocene ice-cap, whose upper surface over the Pacific Ranges was from
5,000 to 8,000 feet above sea-level. Lower summits were covered by the ice-sheet
at its maximum, and many of these are rounded and domed even though they are
scahoped by cirques on their northeastern sides. Evidence of tremendous ice erosion
is everywhere to be seen.
Extensive icefields remain in the areas around Mount Silverthrone, Mount
Waddington, between the Homathko and Southgate Rivers, and at the head of the
Lillooet River. These icefields are now receding and are small compared to their
former extent, but they are an impressive indication of how much of British Columbia
looked 10,000 years or more ago.
There is a noticeable difference between the heavy vegetation in the western
ranges, where the rainfall is high, and in the eastern ranges, where the rainfall is
* “ Guidebbok for Geological Field Trips,” prepared by GswIoghzZ Discussion Club, Vancouver, 1960, p. 12.
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