Landforms of British Columbia 1976 - Department of Geography

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PHYSIOGRAPHIC HISTORY OF THE WESTERN SYSTEM
The early sequence of events in the area of the Western System is one of
marine sedimentation interrupted by periods of volcanic eruption during the Paheo-
zoic and early Mesozoic eras. This culminated in a period of mountain-building
and intrusion of granitic batholiths along the axes of the Insular Mountains and the
Coast Mountains, in Upper Jurassic and Lower Cretaceous time.
Sediments derived from the erosion of these mountains in Cretaceous time
were deposited in flanking basins under marine and brackish-water environments.
The granitic cores of the mountains were exposed, and the late Cretaceous land
surface resulting from the erosion was probably one of very low relief.
This ancient erosion surface was differentially uplifted at the beginning of the
Tertiary, along two axes of greatest uplift which corresponded with the earlier axes
of batholithic intrusion along the sites of the present Insular and Coast Mountains.
Separating them was a trough which lay along the site of the present Coastal Trough.
Sediments of Tertiary age were deposited within this ancestral Coastal Trough,
together with lava and the fragmental products of Tertiary volcanism in south-
eastern Alaska, the Queen Charlotte Islands, and southern Vancouver Island.
Tertiary sediments were also deposited on a shelf fringing the western side of the
Insular Mountains on Vancouver Island.
Continued erosion during the Tertiary resulted in further unroofing of the
Coast Intrusions on the mainland, and the reduction of the land to a surface of
generally low relief that was co-extensive with a similar surface in the interior of
the Province. Undoubtedly, however, there were areas of hard or more resistant
rock which withstood erosion, and monadnocks remained a thousand feet or more
above the general level. Flowing westward across the country were the ancestors
of all the present rivers that cross the Coast Mountains to the sea. These are the
Taku, Stikine and Iskut, Nass, Skeena, Bella Coola, Klinaklini, Homathko, and
Fraser Rivers.
The late Tertiary surface of low to moderate relief was again differentially
uplifted during the Pliocene along the same two longitudinal axes which correspond
essentially with the present axes of the Insular Mountains and the Coast Mountains.
Between the two was a low area which became the Coastal Trough. Eastward from
the Pacific the old surface rose from sea-level to the crest of the Insular Mountains,
descended to sea-level, rose to the crest of the Coast Mountains, and finally de-
scended to the level of the Interior Plateaus. A cross-axis connects the high massifs
centring around Mount Waddington with Victoria Peak on Vancouver Island, and is
represented by the Seymour Arch, which constricts the Coastal Trough in the
vicinity of Sayward. Northward there is a cross-sag, represented by the lower
summit levels of the Kitimat Ranges and the low area occupied by Queen Charlotte
Sound. Still farther north the old surface rises to culminate in the area of lO,OOO-
foot peaks around Mount Ratz north of the Stikine River.
The Pliocene uplift rejuvenated the erosive power of all the streams. Incision
of the streams began at their mouths and progressed upstream. It resulted in the
dissection and the partial to almost complete destruction of the late Tertiary surface,
and the production of a mountainous topography of mature relief before the onset
of Pleistocene ice.
The major rivers flowing westward to ‘the sea were able to maintain their
courses across the rising axis of the Coast Mountains. These valleys provide access
to the interior of British Columbia from the west.
Although glacial erosion during the Pleistocene greatly modified the pre-
existing topography by sharpening the high peaks, rounding and softening the lower
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