Landforms of British Columbia 1976 - Department of Geography
Landforms of British Columbia 1976 - Department of Geography
Landforms of British Columbia 1976 - Department of Geography
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Pacific Ranges<br />
The Pa&c Ranges (see Plates IX, XB, XIIA, XIIB, XLIB, XLIIA, and XLVIA)<br />
comprise the essentially granitic mountains extending southeastward from Burke<br />
Channel and Bella Coola River for about 300 miles to the Fraser River. The ranges<br />
have a width <strong>of</strong> 80 to 100 miles between their western boundary along the Coastal<br />
Trough and,their eastern boundary with the Interior System. On the western side<br />
the summit levels diminish to the west with the downward slope <strong>of</strong> the dissected late<br />
Tertiary erosion surface. The boundary between the Pacific Ranges and the Hecate<br />
and Georgia Lowlands is along the generalized line <strong>of</strong> the 2,000-foot contour. On<br />
the east the Pacific Ranges between Atnarko River and the head <strong>of</strong> the Yalakom<br />
River are flanked by the Fraser Plateau, from which the mountain front rises abruptly<br />
(.see Plate XIB) . From the head <strong>of</strong> the Yalakom River the eastern boundary is along<br />
the Yalakom and Fraser Rivers, which separate the Pacific Ranges from the Camelsfoot<br />
and Clear Ranges, and from the Cascade Mountains.<br />
The Pacific Ranges contain the highest peaks in the Coast Mountains. From<br />
Mount Saugstad (9,608 feet) just south <strong>of</strong> the Bella Coola River, peaks rise to 9,700<br />
feet at Silverthrone Mountain west <strong>of</strong> the Klinaklini River, to 13,177 feet at Mount<br />
Waddington (see Plate IX), and to 12,800 feet at Mount Tiedemann between the<br />
Klinaklini and Homathko Rivers. There are a number <strong>of</strong> 1 O,OOO- to 11 ,OOO-foot<br />
peaks between the Homathko River and Chilco Lake and south to the head <strong>of</strong> the<br />
Lillooet River. South and southeast from Wedge Mountain (9,484 feet) and Mount<br />
Garibaldi (8,787 feet) the summit elevations diminish as the edge <strong>of</strong> the mountains<br />
is approached. The mountain front rises from the Fraser Lowland on a long slope<br />
<strong>of</strong> about 15 degrees, which, in part, is an exhumed pre-Eocene erosion surface* on<br />
which Eocene and younger sediments had been deposited (see p. 37).<br />
Drainage in the Pacific Ranges is to the coast by way <strong>of</strong> the Bella Coola, Kingcome,<br />
Klinaklini, Homathko, Southgate, Toba, Squamish, and Lillooet Rivers and<br />
their tributaries. These have cut major low-level valleys through the mountains,<br />
dividing them into blocks.<br />
Bedrock structures have controlled the drainage pattern. In places within the<br />
granite a trellis-type pattern is parallel to regionally developed sets <strong>of</strong> joints or faults.<br />
Further structural control <strong>of</strong> drainage and <strong>of</strong> landforms is illustrated east <strong>of</strong> Cape<br />
Caution, where maps and aerial photographs show prominently developed easterly<br />
and northwesterly lineaments, represented by alignments <strong>of</strong> inlets, lakes, and stream<br />
valleys. The Owikeno lineament (,see Plate XLVIII) has a length <strong>of</strong> about 60 miles<br />
westerly from Mount Silverthrone. Another remarkable lineament can be seen on<br />
aerial photograph B.C. 566:55 to extend in a northwesterly direction for at least<br />
20 miles along Lillooet River between Pemberton Meadows and the junction <strong>of</strong><br />
Meager Creek.<br />
The high peaks are sculptured by cirque glaciers. Many projected as nunataks<br />
above the Pleistocene ice-cap, whose upper surface over the Pacific Ranges was from<br />
5,000 to 8,000 feet above sea-level. Lower summits were covered by the ice-sheet<br />
at its maximum, and many <strong>of</strong> these are rounded and domed even though they are<br />
scahoped by cirques on their northeastern sides. Evidence <strong>of</strong> tremendous ice erosion<br />
is everywhere to be seen.<br />
Extensive icefields remain in the areas around Mount Silverthrone, Mount<br />
Waddington, between the Homathko and Southgate Rivers, and at the head <strong>of</strong> the<br />
Lillooet River. These icefields are now receding and are small compared to their<br />
former extent, but they are an impressive indication <strong>of</strong> how much <strong>of</strong> <strong>British</strong> <strong>Columbia</strong><br />
looked 10,000 years or more ago.<br />
There is a noticeable difference between the heavy vegetation in the western<br />
ranges, where the rainfall is high, and in the eastern ranges, where the rainfall is<br />
* “ Guidebbok for Geological Field Trips,” prepared by GswIoghzZ Discussion Club, Vancouver, 1960, p. 12.<br />
42