Canadian Rockhound: Geology of the Fraser Valley

During the late Cretaceous and the early Tertiary time, the area encompassed by the Lower Fraser Valley was a low lying coastal-plain surrounded by low hills, enjoying a sub-tropical climate. Much later, gradual changes in environmental and geologic conditions led to the development of a glaciated river valley hemmed in by mountains.

The valley is funnel shaped and is about 120 kilometers (75 miles) long and about 90 kilometers (56 miles) at its widest at the Strait of Georgia. To the north is the Coast Mountain Range and to the east and southeast are the Cascades. The mountains surrounding the valley rise to some 1650 meters (5000 feet) and are characterised by deep U-shaped valleys whose floors range in elevation from near sea-level to 100 meters above it. Hatzic Valley, Stave River and Harrison River and Lake occupy some of the main glaciated troughs. In the valley itself, few areas are higher than 130 meters (400 feet) above the current sea level.

During the last 100 million years, the geological history of the valley has been that of the infilling of a gradually subsiding basin. Throughout the preceding Triassic and Jurassic periods, the Coast and Cascade Mountain regions had been the site of volcanic island arcs. After the volcanic activity to Cretaceous times, the volcanic and deep-sea sediments had been uplifted, folded, sheared and intruded by plutonic rocks. By the late Cretaceous times, these mountains had been intensely weathered to a saprolite (a rock weathered in place). The weathering reduced the mountains to low rolling hills with a mean elevation of 500 feet above sea-level. From this time (about 65 million years ago), to about mid-Miocene, the area had received a layer of sediments at least 4600 meters (14,000 feet) deep. These sediments and the preceding volcanic rocks have been thoroughly metamorphosed by the granitic intrusions of late Mesozoic and early Cenozoic age. There are many separate granitic intrusions of various sizes, shapes and areas.

In Stanely Park can be seen a volcanic dike. It extends from Siwash Rock in a northeasterly direction to Prospect Point. Siwash Rock itself is volcanic material surrounded by "baked" sedimentary rocks. At Prospect Point you can see the columnar jointing typical of rapidly cooling basaltic lava.

The present Coast Mountains and the Cascade Mountains were raised in the late Cenozoic and are probably still rising today. Young volcanoes came into being in the Coast Mountains after the main rise of the mountain chain. To the near north is a good example of one of these young volcanoes in Mount Garibaldi.

The Cascades to the south and southeast consist of an axial core of gneiss and granitic rock flanked on the east and west by belts of folded and faulted, but not highly metamorphosed, sedimentary and volcanic rocks of late Paleozoic to mid-Cretaceous in age. Young volcanoes also exist in the Cascades of which Mount Baker, just south of the Vancouver area, is a prime example. It has been speculated that the Cascade volcanoes are a result of the movement that may produce hotspots beneath the surface which may rise and eventually break through the crust at weak points.

In the Fraser Valley, surface deposits cover the whole area. These deposits range from 100 to 1100 meters. In the main, these are the remnants of the glacial activity of the last 25,000 years.

The mountains and valleys around the upper part of the valley contain many ore deposits. However, the deposits are too low grade to warrant mining, so there are no working veins in the area today. In 1868, at Silver Peak (9.6 kilometers south of Hope), silver was mined for a short while. In 1895, placer gold was discovered in the gravel bars of the Fraser. Copper and gold in place has been found in the Hope area. Garnet, a metamorphic mineral, is found at several locations in and about the valley, such as at Ruby Creek, east of Agassiz and along the east side of Harrison Lake. Abbotsford has large deposits of sand and gravel which are used for building material, cement mix and road surfacing.

Clay has been produced from the Eocene strata at Clayburn since the early 1900's and later from Kilgard. Lime is produced from the marl deposits near Rosedale.

I've always thought of the geology of the Vancouver area as pretty dull, i.e., granite mountains surrounding a river plain. But, on closer inspection, the geology records a fascinating story of mountain building and volcanic processes that have occurred over many millions of years.

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The preceding article was first published in the December 1990 issue (p. 2-3) of the British Columbia Newsletter, the official publication of the Lapidary Rock & Mineral Society of B.C. Updated December 1999. Reprinted in the Canadian Rockhound with permission from the author.