Geology of James Bay — Gneiss and Ice
James Bay began its journey as a deep oceanic lava plain formed during the Devonian period 370 million years before present (BP) (Yorath and Nasmith, 1995). Subsequent episodes of volcanism, erosion, sediment accumulation and crustal movement (NRC, 2008) during the following 200 million years gave rise to the exotic terrane of Wrangellia that docked with North America 170 – 100 million years BP (Yorath and Nasmith, 1995). Much more recently, at the peak of the Vashon stade of the Fraser glaciation 15,000 BP, James Bay was entombed by at least 1500 metres of ice; and a "mere" 1,000 years later, it was submerged by marine transgression. After differential glacio-isostatic rebound and eustatic changes, modern sea level was reached approximately 5,000 BP (NRC, 2008).
Surveying the shoreline just west of Finlayson Point, evidence of an ice-dominated environment evokes awe not only of the scale, but also of the endless variety of features. The metamorphic Wark gneiss bedrock exposures exhibit many examples of the asymmetrical roche moutonnée, north-to-south-trending grooves and striations, and crag-and-tail features that attest to the immense power of ice and entrained debris. The sea cliffs in Beacon Hill Park along Dallas Road expose ice-contact gravels and Vashon till which accumulated adjacent to ice that had grounded in Juan de Fuca Strait about 12,000 BP (Yorath and Nasmith, 1995).
A reference to either the back end of a sheep or a barrister's wig, roche moutonnée is an example of stoss-and-lee terrain (Fettes College, 2008) that is rounded, smoothed, grooved, striated and polished by abrasion on the stoss, or upstream side, and steep, jagged and irregular on the opposing lee flank. Increased ice thickness and higher pressures on the stoss (north in this instance) side of obstacles decreases the melting point of ice (there is a decrease in the melting point of ice with increasing ice thickness of about 1°C per 1000 metres), creating a flux of basal meltwater which migrates to the zone of lower pressure on the lee side where it refreezes due to lower pressure, releasing latent heat of fusion (Summerfield, 1991). Depending upon the depth of joints in the bedrock, and facilitated by freeze-thaw cycles, blocks are mechanically pulled — or plucked — from the lowest to highest point in the rock floor when the basal ice freezes to them (Whittow, 1984).
At the peak of the Fraser glaciation, which began 29,000 BP, the land was depressed by approximately 250 metres, and sea level was 100 – 150 metres lower than it is today (Cannings, S. and Cannings R., 1999). When the ice retreated about 14,000 BP, the ocean invaded all but the highest points of the Saanich Peninsula. This marine transgression completely covered James Bay, allowing for the formation of the Victoria Clay.
The Vashon till, which accumulated beneath the base of the ice sheet, overlies most of the bedrock in James Bay and is generally less than a few metres thick (Yorath and Nasmith, 1995). Glaciomarine Victoria Clay and Wark gneiss outcrops dominate the surficial geology of James Bay, with an isolated example of Holocene peat overlying Victoria clay to be found on Niagara Street, between Rendall and Oswego. The clay, deposited at the close of the Fraser glaciation when the sea level was higher, is predominantly thick, soft, grey clay of more than 3 metres in depth. Of Jurassic age, the Wark gneiss outcrops have been potassium-argon dated from 131 – 182 million years of age (Ministry of Energy Mines and Petroleum Resources, 2007). Comprised of massive and gneissic metadiorite, metagabbro and amphibolite, the Wark gneiss may have originally been composed of rocks of the Sicker Group and Karmutsen and Quatsino formations (Vancouver Group) (Ministry of Energy Mines and Petroleum Resources, 2007). James Bay Square and the Royal British Columbia Museum, for example, are constructed upon Wark gneiss, while most of the area bounded by Menzies, Belleville, Government and Dallas Road is underlain by Victoria clay (Quaternary Geological Map of Greater Victoria, 2000).
References
Cannings, Sydney and Richard Cannings. Geology of British Columbia: A Journey Through Time. Vancouver: Greystone Books, 1999.
Fettes College. 2008.
Ministry of Energy Mines and Petroleum Resouces. 2007.
Natural Resources Canada (NRC). Geoscape. 2008.
Quaternary Geological Map of Greater Victoria. Map 2000-2. Victoria, BC: Ministry of Energy and Mines, 2000.
Summerfield, Michael A. Global Geomorpholgy. New York: Longman Scientific and Technical, 1991.
Whittow, John. Dictionary of Physical Geography. London: Penguin Books, 1984.
Yorath, C.J. and H.W. Nasmith. The Geology of Southern Vancouver Island: A Field Guide. Victoria: Orca Book Publishers, 1995.