Comox Valley Geology

The bedrock beneath most of the Comox Valley is sedimentary rock laid down on the floor of warm, shallow seas or swampy lagoons between 90 and 65 million years ago while dinosaurs roamed the land in the late Cretaceous Period. It consists of layers of shale, sandstone, and conglomerate, partly overlying thick coal beds near Cumberland. Shale is consolidated clay, and in some places it contains numerous fossils . But fossils are not found in sandstone (cemented sand) or conglomerate (cemented pebbles and cobbles). Before these comparatively big rock fragments became cemented, they swirled around in rough seas smashing any bones and shells among them.

Examples of all three kinds of rock are easy to find: shale along the banks of the Puntledge River, and on both shores of Baynes Sound; sandstone beside the Inland Island Highway, at Nymph Falls and along the backbone of Denman Island; conglomerate is less common but forms the highest part of Hornby Island and much of Mt Geoffrey.

To see older rocks, go to Strathcona Park. There, the most abundant rock is 200 million-year old (Triassic Period) basalt, solidified from lava that flowed from fissures in the seafloor and hardened quickly in the cold water. The rock is a dark greenish, greyish black and is fine-grained and smooth. (Sometimes it’s flecked with white where quartzite has crystallized inside tiny bubbles in the lava, in which case it’s amygdaloid basalt. )

The lava erupted when the land destined to become Vancouver Island was thousands of kilometres away, on the floor of the South Pacific Ocean, on a drifting tectonic plate. The plate drifted northeast and became attached to Canada’s western margin about 100 million years ago, while still submerged below the sea and covered with a layer of Cretaceous sediments. As the plates converged, the newly arrived Vancouver Island rose above sea level, becoming buckled and folded into mountains. Much of the sedimentary rock that was once on their surfaces has been eroded away, leaving the basalt as the exposed rock in most of Strathcona Park, including Paradise Meadows, Forbidden Plateau and the mountains beneath Comox Glacier. Much of it is “pillow” basalt: the hardening lava coiled as it flowed and congealed into pillow shapes. Pillows are clearly visible near the southern end of Comox Glacier.

Here and there lava forced itself some way up through the overlying rock, but cooled and hardened gradually without ever reaching the surface. It formed imbedded masses known as plutons. Many are now exposed to the open air because the rock that covered them has since been eroded away. The summit of Mount Washington is an example. Unlike basalt which cooled fast and has microscopically small mineral crystals, the plutonic rock crystallized slowly, forming big, interlocking crystals which give the rock a coarse granite-like texture, with a black and white pepper-and-salt pattern. It is mostly diorite – a mixture of silicate minerals.

Some of the Cretaceous sediments which had covered the igneous rocks escaped erosion and still remain in the mountains, for example Strata Mountain. Another example, easier to get to, is the sandy slope immediately above Mt. Washington’s Raven Lodge parking lot.

Another place to see ancient igneous rocks is on our beaches, where pebbles of basalt and diorite are abundant. Pebbles on a beach arouse questions: what kinds of rock are they, where have they come from, and how did they get there? The pebbles on Seal Bay Beach, at Point Holmes, and below Willemar Bluffs come from both Strathcona Park and the Coast Range on the other side of Georgia Strait, where the rocks are like ours. They were carried here frozen into the ice of the last ice age.

Perhaps they should now count as “young” rocks — no rock is ever new. Pebbles, with sand and clay, are simply the raw materials of which future solid rock-masses will consist. And these loose, raw materials were carried here in abundance by the glaciers and ice sheets of the last ice age.

The ice age created a third landscape, this one along our shores. Of our three landscapes, it is the one nearest to the sea, lowest in elevation, and youngest in age. It was created in the Quaternary Period, the period we’re now living in, which began only two million years ago. The ice age landscape has three components. The oldest is the Quadra Sand, which makes up most of the high sand cliffs from Comox Bay to Black Creek between the shoreline and the Tsolum River. The sand was produced by the weathering of granite and diorite plutons in the Coast Range and was carried and pushed into the lowland between the Coast Mountains and Vancouver Island by the ice sheets on the mainland as they spread southeastward. The first deposits were laid down about 29,000 years ago, at the northern end of the Strait of Georgia, and the “sand front” took about 12,000 years to advance to the southeast end of the strait. A good place to examine the sands is from the beach below Willemar Bluffs. The lower parts of the cliffs are layer upon layer of Quadra Sand.

Until about 15,000 years ago, all of BC, including Vancouver Island and the Strait of Georgia, was covered by ice sheets up to 2 km thick. The weight of the ice depressed the earth’s crust and sea-floor making sea level about 80 metres higher relative to the land than it is now. Then the ice began to melt, its weight lessened, and the dented crust gradually rebounded. Sea level started to fall and continued to fall until roughly 7,000 years ago when it was about 5 metres below its present level – this in spite of the fact that, worldwide, the volume of the oceans was increasing as the ice sheets melted and the water warmed and expanded. Finally, the swelling ocean dominated the rebounding crust and sea level rose, very slowly, to its current level. Before global warming began, it was rising at about 1 or 2 mm per year. Possibly sea level is also being affected by the drifting tectonic plates of the Pacific forcing their way under our west coast.

Returning to Willemar Bluffs, overlying the Quadra Sand are two contrasting layers:first, a band of gray clay, and above that a thin strip of horizontally-bedded gravel. The gray layer is “till”, material frozen to the bottom of glacier ice and plastered onto the surface it spreads over. The till was overlaid on the sand by a late, local, short-lived re-advance of the big ice sheets, and is known as the Vashon stade (substage). The thin layer of gravel beds between the till and the surface soil is the Marine Veneer, and is the seafloor sediment deposited on the till while the seafloor was still 70 or 80 metres above its present level.

Here and there rocks can be found scratched and grooved by sharp rocks frozen to the bottom of a glacier that was dragged over them, when the ice sheets were advancing. A good example is to be found on the seaward side of the path along the south fence in front of Filberg Heritage Lodge & Park, opposite the Lodge garages.

© E.C. Pielou, Comox Valley Naturalists Society

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