VSHA Sunday Meeting Report – 10 AM, November 17, 2013
by Barrie Webster
Sunday’s guest speaker was Dr. Chris Yorath, retired Research Scientist from the Geological Survey Canada in Sidney, BC. Chris conducted petroleum exploration in the Middle East and Alberta before joining the Geological Survey of Canada in 1967. After working in the Arctic, he joined the Pacific Geoscience Centre at Patricia Bay in 1976.
His 1990 book, Where Terranes Collide, takes the reader through 17 million years of BC geological history from the perspective of the Trans-Canada Highway. He has also published a visitor’s guide to the geology of Banff and Yoho National Parks in How Old Is That Mountain? (Orca, 1997; Harbour 2006) and A Measure of Value (Horsdal & Schubart, 2000). His most recent book, an expanded edition of Geology of Southern Vancouver Island (Harbour, 2005), is for geologists and amateur naturalists alike.
A grasp of plate tectonics was first required to visualize the way in which the west coast of BC and, in particular, southern Vancouver Island was put together. The slow but steady movement of the surficial plates of the planet led to the accretion of several smallish segments of crust (or terranes), also termed volcanic island arcs (resulting from earlier subduction processes to the west and south of the BC coast), to the pre-existing granitic continental base of northwestern proto-North America. Much of BC west of the Rockies was derived this way. Much of Vancouver Island is part of the Wrangelia Terrane; however, two more recent accretions, the Crescent Terrane (Leech River Gneisses) and the Pacific Rim Terrane (Metchosin volcanics), are in clear evidence in Southern Vancouver Island (Figure).
Dr.Yorath’s presentation included many photographs of places in which the geological features of southern Vancouver Island were visible. Such physically visible features such as the Leech River Valley (the boundary between the Crescent and Pacific Rim Terranes) are dramatic evidence of different origins for these parts of the island. Further features that show how the region was formed include the Colwood gravel pit area, remnants of the outwash deposits from water flowing from the glacier that used to fill Finlayson Arm during the end of the last ice age towards the Strait of Juan de Fuca.
Many other details of the geology of southern Vancouver Island can be viewed and read about in Dr. Yorath’s book (2005).
Figure 1. The above figure was not part of Dr. Yorath’s actual presentation, but it is derived from BC Geological Survey material and shows the location and extent of the Crescent and Pacific Rim Terranes. Note the boundary (the Leech River Valley follows the western portion of this boundary) between the two terranes. The eastern-most end of the boundary emerges at the Esquimalt Lagoon.