The west coast of Vancouver Island is awesome in so many ways, and not least of all because of it's huge variety of beaches. Picking a favorite beach along the southwest coast is not easy with strong contenders in French Beach, China Beach, Sandcut Beach, and Botanical Beach to name a few. But when it comes to geology, Sombrio Beach tops them all.
Sombrio is situated along the Juan de Fuca Trail and is a popular camping site for locals and tourists alike. On weekends in the summer, the top of the beach is usually packed with tents and campfires. What makes Sombrio so geologically fascinating is it's combination of exquisite beach ridges, beautiful outcrops and important faults (breaks in the rock where rocks on either side slide past each other). Really the geological party never stops when you're at Sombrio!
Leech River Fault
Probably the most understated and most important geological feature at Sombrio Beach is the Leech River Fault (LRF), which is part of a much larger fault system that extends across Vancouver Island, under the Strait of Georgia and east through northern Washington State, USA (1). Some even suggest that the LRF extends under the seafloor all the way to Alaska (2). The LRF at Sombrio Beach is a tectonically active, dextral strike slip fault meaning that if you placed your feet on either side of the fault line and waited several hundred to several 1000 years, your right foot would move backwards when the fault fails. That has happened at least twice in the past 9,000 years with the resulting earthquakes exceeding magnitude 6. However, I recommend not showing up expecting a lot of action because the average movement on the LRF is 0.25 mm per year (1,3). Not exactly nail-biting speeds, but still very significant to urban planners and to geologists! In fact, it was the possibility of movement on the LRF that prompted BC Hydro to buy all the houses in Jordan River in 2014 because of the risk of dam failure for a dam built directly on the LRF trace!
The rocks at Sombrio Beach
Any non-geologist (and probably most geologists) would be blissfully unaware that a major fault runs through the area and that is because there is only subtle physical evidence to mark its presence. The fault plane is essentially the northwest wall of Sombrio Point, which is the southeast limit of Sombrio Beach. South of the fault plain are 54- to 50-million-year-old volcanic rocks believe to be part of a string of volcanoes that was smushed onto the margin of North America (4,5). North of the fault plane you pass through 35-million-year-old sandstone that overlies ~130- to 160-million-year-old metamorphosed sedimentary rock (cooked and deformed sandstone and mudstone (4,6)). Yup, in geological jargon, as you walk northwest up the beach, you would be reading the story from back to front.
There is a really cool pseudo-secret waterfall carved into the young sandstone in that area.
There is a beautiful contact between the old metasedimentary rock and the young sandstone about 350 m northwest of the southern extend of the beach, and northwest of this there really are no more rock outcrops. However, the beach is backed by metamorphosed sedimentary rocks along much of its length, and these rocks dominate the pebbles and cobbles on the beach. The pebbles make absolutely incredible skipping stones, so don't miss out on the opportunity to test your stone-skipping skills!
When I go to Sombrio it’s like my kids going to the candy store, Stickys. There is just too much to see and it is easy to get distracted! Of course, my time there normally devolves into a rock skipping competition with my children but that is beside the point.
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Info Sources:
(1) Halchuk, S., Allen, T., Adams, J., and Onur, T., Contribution of the Leech River Valley - Devil's Mountain Fault System to Seismic Hazard in Victoria, B.C., in Proceedings 12th Canadian Conference on Earthquake Engineering, Quebec City, 2019, 8 p.
(2) Colpron, M., Crowley, J. L., Gehrels, G., Long, D. G. F., Murphy, D. C., Beranek, L., and Bickerton, L., 2015, Birth of the northern Cordilleran orogen, as recorded by detrital zircons in Jurassic synorogenic strata and regional exhumation in Yukon: Lithosphere, v. 7, no. 5, p. 541-562.
(3) Morell, K. D., Regalla, C., Amos, C., Bennett, S., Leonard, L., Graham, A., Reedy, T., Levson, V., and Telka, A., 2018, Holocene Surface Rupture History of an Active Forearc Fault Redefines Seismic Hazard in Southwestern British Columbia, Canada: Geophysical Research Letters, v. 45, no. 21.
(4) Fairchild, L. H. & Cowan, D. S. Structure, petrology, and tectonic history of the Leech River complex northwest of Victoria, Vancouver Island. Canadian Journal of Earth Sciences 19, 1817-1835 (1982).
(5) Massey, N. W. D. Metchosin Igneous Complex, southern Vancouver Island: Ophiolite stratigraphy developed in an emergent island setting. Geology 14, 602-605 (1986).
(6) Guthrie, R. H. Vancouver Island Bedrock Geology Map. (BC Ministry of Environment, Victoria, 2005).
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