On a beautiful October day last fall, my partner Alex and I set out to hike Cowichan Bay’s iconic Mt. Tzouhalem. Winding our way through the forest trails we reached our near-panoramic viewpoint and settled down for a rest when I noticed the unusual texture of the rocks beneath our feet. Alex, like any normal human-being, marvelled at the greens of the farm fields in the distance and blues of the estuary, dotted with herons and geese, the waters’ braided channels presenting mesmerizing curves from the altitude of our rest point. My eyes, however, were narrowed to view a much closer subject, focused intently on the composition of the mountainous bedrock beneath my feet. “Conglomerates?” I said, observing the sub-rounded grains ranging from gravels to pebbles to small boulders, interlocked in a cement-like matrix, “How could there be conglomerates right next to the low energy environment of the Cowichan Estuary?”. Alex prompted me to come join him on his mossy perch, basking in the sun’s October warmth and golden tones of the maples that scattered the lowlands below. After snapping a few pictures of my mysterious bedrock observations, I did. 

Attached to this post is a photo of the beautiful rocky bulges defining the southwest cliff faces of Mt. Tzou, and without further ado, here’s some backstory on my conglomerate-confusion. Being a kind of sedimentary rock, the make-up of a conglomerate is defined by the energy of its formational environment. Sedimentary rock can form when a sediment-covered region is engulfed by shallow tropical seas, or in freshwater fluvial environments, where rivers transport and deposit sediment from highlands to lowlands. Unlike other sedimentary rocks such as sandstones and siltstones, conglomerates are characterized by their larger grain sizes and lithological variation. One key defining feature of conglomerates is that they often form in higher energy environments, typically in the highlands of mountainous regions. This is due to the fact that larger clast (rock grain) sizes, such as gravels and pebbles, take much more energy to transport than smaller grains do. The Cowichan River, for example, becomes a very low energy environment once it reaches the Cowichan Estuary, draining into the Strait of Georgia. At that point, it’s safe to say that, today, in the low energy stretch of the Cowichan River adjacent to Mt Tzouhalem, the waters aren’t transporting and depositing these larger grain sizes that make up the conglomerates that caught my attention on Mt. Tzou’s mountainous ridges. Pondering the thoughts of, How did the grains that make up these conglomerates get here? and At one point in time, these conglomerates formed as all sedimentary rocks do: from burial and resulting pressure binding the grains together  — so how are they so high above sea level today?, I began wondering about what the Cowichan River looked like in its youth, funnelling mountainous waters after the Fraser Glaciation 11,000 years ago, and subsequently, the age and formational history of Mt. Tzouhalem itself. 

After a few weeks of learning about the key geological groupings of sedimentary rocks within southern Vancouver Island, it really dawned on me how great of a time scale my questions crossed. Though I saw no similar pictures to my own to compare bedrock similarities, I identified these conglomerate cliffs to likely be that of the Benson formation: a basal conglomerate unit of the Nanaimo Group consisting of gravels, cobbles and boulders deposited by highly competent streams (White, 1983). Here I was, imagining the Cowichan River playing a role in the formation of the conglomerates I observed on the mountain, assuming they formed in less than ~10,000 years since the last glaciation. I quickly began to piece together how little of a roll the glaciation and subsequent formation of the Cowichan River had in the distribution of the island’s sedimentary bedrock, glaciation however a rather influential force in shaping the landscape as we see it today. 

Here’s what I’ve learnt about the sedimentary bedrock of mighty Mt. Tzou. Part of the Late Cretaceous Nanaimo Group, its sandstone and conglomerate cliffs form the highest slopes of the all  of the Nanaimo Group. What is the Nanaimo Group, you ask? Unlike much of Vancouver Island’s igneous and metamorphic terrane, the Nanaimo Group constitutes a grouping of siliclastic marginal-marine and marine sediments, together comprising a ~5000m thick sequence of conglomerate, sandstone and mudstone layers (Hamblin, 2012). Part of this Cretaceous sequence is present on Mt. Tzouhalem; Benson conglomerate cliffs overly Sicker Group metasediments deep within the mountain. One recurring piece of information I encountered in my search for answers was the remark that the Benson formation has yet to be studied in detail, however defined as a poorly to fairly sorted pebble to cobble conglomerates set in a arkosic sandstone matrix (White, 1983). Notably a division of the Comox sub group of the Nanaimo Group, it can thus be estimated that the conglomerate cliffs formed between 90 and 65 million years ago. Though relatively young on a geologic timescale, it’s hard to comprehend the evolution of Mt. Tzouhalem and the adjacent landscape since the conglomerates’ formation 90 to 65 million years ago. It’s even harder to find literature on the matter, besides that of which relates Mt. Tzouhalem to the Fraser Glaciation, which began only ~25,000 years ago (Landsharkz, 2014).

Fast forwarding to the end of the Fraser Glaciation ~11,000 years ago, known tectonic and post-glacial events such as subsidence-rebound tell us crucial details in the landscapes’ evolution, echoed in the appearance of Mt. Tzouhalem today. Two glaciers influenced the bulging appearance of the mountain and the shaping of the land that surrounds it, including the Cowichan Estuary. While the Cowichan ice tongue deepened valley lowlands, it was the Cordillerian ice sheet that covered the tops of local mountains, including the Benson conglomerates that capped Mt. Tzou (Landsharkz, 2014). Subjected to the immense pressures of glacial ice the conglomerates weakened, and with the retreat of the ice, some became dislodged and broke off from areas of higher elevation, creating some of the dramatic cliff faces that characterize the mountain today. With the absence of the glacier, areas of higher elevation began to rebound more quickly than the valley bottoms, still subjected to the weight of the ice. This imbalance of crustal rebound indeed created the dramatic shapes of Mt. Tzouhalem, as well as Mt. Maxwell and Mt. Tuam on Salt Spring Island. Moreover are the effects of notable megathrust earthquakes, such as that of the year of 1700. Hiking the landscape, Alex and I were in awe of boulders the size of small school buses littered 500m above sea level, casually perched on the edges of our mellow trail. At first appearing as glacial erratics — large boulders that fall on to glaciers, carried by ice sheets until they’re deposited in new locations due to the melting of ice. As discussed by several sources, including that of Landsharkz Geocachers (2014), these boulders appear to have fallen from the conglomerate caps of Mt. Tzouhalem’s higher elevations, thought to be dislodged due to the megathrust earthquake of 1700.

How fun it would be to revisit the bus-sized boulders and compare their conglomerate makeup to that of the cliffs that first sparked my interest. In all their glory, the weathered conglomerates of Mt. Tzouhalem silently echo the harsh climates of their past, telling a complex geologic story to those willing to listen.