Geologic Units of British Columbia

The Insular Belt forms the offshore islands (including Vancouver Island).

The farthest west mainland topographic unit is the granitic Coast Belt, an uplifted plutonic complex possibly formed by chains of younger volcanoes built on top of older ones and former island and continental arcs accreted onto the continent 95 million years ago. The western mountains (Coast and Cascade Mountains and the Mount Saint Elias range) are rugged areas of high relief, with steep glacial-cut valleys and fjords, and heavy rains.

Traveling east, the next major unit is the Intermontane Belt, lightly metamorphosed former island arcs and ocean basins accreted onto the continent 180 to 175 million years ago, and intruded by plateau basalts 10 million years ago. The interior plateaus (Shuswap and Okanagan Highlands) and low mountain ranges (Skeena Mountains) are generally low-relief, although rivers have cut deep, rugged canyons (including the Fraser and Thompson rivers). In a rain shadow from the coastal mountains, the interior is semiarid.

The Omineca Belt, and farther east, the Foreland Belt, cross the southern border into Alberta. The Omineca Belt is dominated by highly metamorphosed terranes altered by mountain building 180 to 60 million years ago, and crustal stretching 55 million years ago. The Foreland Belt is composed sedimentary materials originally deposited along the continental margin, then folded and faulted as the entire unit was thrust over the edge of the ancient continent 100 to 60 million years ago. The Columbia and Rocky Mountains dominate the eastern ranges, divided by the Rocky Mountain Trench (a fault). The Monashee, Selkirk, and Purcell ranges follow the north-south trend, divided by long, narrow lakes.

Further Reading
Mathews, B., & Monger, J. (2005). Roadside Geology of Southern British Columbia. Mountain Press Publishing Company.

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