Colwood Creek Watershed
The upper Colwood Creek watershed, from Humpback Reservoir to Glen Lake, is situated to the south of the Leech River Fault. This fault was formed around 42 million years ago, when a large piece of the earth’s crust called the Crescent Terrane collided with the rest of Vancouver Island, including the adjacent Pacific Rim Terrane and the much larger Wrangellia Terrane (see image below). On land, this fault extends for 153 km from Sombrio Point, west of Sooke, to Esquimalt Lagoon.(1) This area is formed of various types of volcanic rock known as the Metchosin Igneous Complex, on top of which glacial till has been deposited since the Fraser Glaciation (see also the A Brief Geological History of Vancouver Island).
The Leech River Fault marks the zone of impact between the Crescent and Pacific Rim Terranes; Colwood Creek is located at the southernmost intersection of the two terranes (image from Natural Resources Canada, www.geoscape.nrcan.gc.ca, used with permission)
The rest of the Colwood Creek watershed lies to the north and east of the Leech River Fault, on the Pacific Rim Terrane. The underlying bedrock in this area is composed of metamorphic rock, including deposits of sedimentary and volcanic rock that were altered (metamorphosed) by heat and pressure in the earth’s crust.(1,2) Over top of the bedrock lies a large deposit of sand and gravel called the Colwood Delta, which was formed at the end of the Fraser Glaciation around 13,000 years ago. Rivers flowing from the melting ice sheets carried sediment that was previously tied up in the ice and deposited it where they met the ocean; sea level was as much as 100 metres higher at that time, consequently the Colwood Delta today is now located on dry land. Material from this deposit is mined today in the Metchosin gravel pit.
The western (upper) portion of the watershed is quite hilly, and includes the summits and northern slopes of Mt. Macdonald (439 m) and Mt. Wells (352 m). The middle portion of the watershed, including Glen Lake and the surrounding area as far east as Sooke Rd., is located on the Colwood Delta and is fairly flat. At the eastern edge of the delta, on Royal Roads University property, the land slopes steeply down to Esquimalt Lagoon in the form of an escarpment. Groundwater seeps from the base of this escarpment in many places.