Travels in Cascadia: Sitting Woman Falls. She's Sitting on Geological Pillows.

There are so many charming little corners to be found on Vancouver Island. We were well underway on our two-week field course on the geology and anthropology of British Columbia, and on this day we had already explored the Sooke Potholes and the base of the ocean crust at East Sooke Park (along with a couple of interesting petroglyphs). It was getting late in the afternoon and the crew was getting pretty tired, but I had heard that there was another site of interest at the end of a short trail, so we made one more stop. It was a place called Witty's Lagoon, which sounds like a theme park or something, but it actually is a geologically interesting section of coastline along the Salish Sea.

This area (and indeed all of Vancouver Island) was covered by glacial ice as recently as 13,000 years ago. When the ice melted, sea level rose to cover some of the previously exposed lands forming a series of bays and coves. Witty's Lagoon is a nice example of one of these, and it is largely unaffected by urban development. Metchosin Creek flows over a ledge of basalt to form Sitting Woman Falls at the upper end of the cove (above).

The basalt, part of what is called the Crescent Terrane, has an interesting story to tell. The basalt erupted on the floor of the Pacific Ocean around 50 million years ago at a divergent plate boundary. It originally melted because of the release of pressure at the mid-ocean ridge, and accumulated in plutons several miles beneath the ocean floor. As the crust spread apart, fractures formed and the basalt followed the breaks all the way up to the sea floor where it erupted out. When basalt erupts in water, it forms odd looking lumps about the size of old-fashioned down pillows, around two or three feet across. These pillows accumulated in layers hundreds of feet deep.

The sequence of gabbro plutons, sheet dikes (the filled fractures), and pillow basalts constitute an ophiolite sequence that in this area is called the Metchosin Igneous Complex. In our previous post we had a chance to see the gabbro as it was exposed on the shoreline of East Sooke and Becher Bay. We didn't get to see any good examples of the dikes, but the cliff at Sitting Woman Falls was composed of pillow basalts. I've zeroed in on the section of the cliff to the right of the falls in the picture above. The pillows are not really well exposed, so I've cheated by adding some pictures below of pillow basalts that we've seen elsewhere in Washington and California.

The picture above shows pillows exposed in the cliffs at Cape Disappointment at the mouth of the Columbia River in Washington. Below we can see some pillow basalts exposed near Nicasio on the Marin Headlands in California.

It's incredible to think of the forces involved in taking the oceanic crust from the bottom of the sea, and mashing it into the edge of the North American continent where it ended up being exposed at Witty's Lagoon, and indeed throughout the region, including the high peaks of the Olympic Peninsula across the Strait of Juan de Fuca.

Northern Convergence: Vancouver Island, the Plan That Was, Part II

Ah, the lost opportunities! I don't want to make it sound like we had a bad Northern Convergence trip; we actually had a great time taking our students through an exploration of the geology of Vancouver Island,  but the trip we made was different than the one we planned. In the last post we talked about the Sooke Potholes, and in today's post we are talking about the opportunity to understand why the basalt exists.

East Sooke Regional Park is a large wilderness park just 20 kilometers or so outside of Victoria at the south end of Vancouver Island. It has a nice network of trails that explore rocky coastlines and deep rainforests. I was really looking forward to exploring it, but time was conspiring against us. That opening picture? That's where the sun was in the western sky as we arrived in the park on our little scouting trip a few days before we were to meet the students. Sunsets take awhile at these northern latitudes, but there are still a finite number of hours of daylight, and we had used most of ours up.

I ran down the trail to Becher Bay because East Sooke Park is said to have marvelous exposures of the oceanic crust that had been pushed into the subduction complex along the western coast of North America, and lifted up above sea level. The ocean crust includes pillow basalts, which form as lava spills out on the ocean floor, sheet dikes, which were the conduits that fed the seafloor eruptions, and gabbro plutons, the magma chambers that fed the eruptions. These rock associations are called ophiolites, and they are often of great interest to geologists, not just for the knowledge they give us about oceanic processes, but because they often are a source of valuable ores, such as copper, mercury, chrome, and platinum.

I literally only had minutes, and didn't have time to walk to the petroglyphs, which would have offered exposures of all three kinds of rock, but I consoled myself by realizing I would be back in a few days with my students and more time. Which didn't happen, as it turned out (as I said, we saw great things, but we didn't make it to East Sooke). I looked at the beautiful small bay, snapped a few pictures of the Olympic Mountains across the Strait of Juan de Fuca, and headed back to the car. As mentioned in the previous post, much of the Olympic Mountain periphery rocks were composed of the same basaltic rocks.

In the dying light of the evening, I was going to try to make one more stop, Witty's Lagoon, to see what are said to be the best exposures of pillow basalts in the region. And I missed the turn. As a geologist and a life-long map-o-phile, I'm embarrassed even now to say that. And by the time I realized it, it was dark.  But we enjoyed a wonderful evening, with a late dinner courtesy of Red Robin in Victoria.

By the way, while I'm making confessions, about coming into Canada: If you want to not look like a complete doofus, learn how to pronounce the names of the places you are going to visit. At immigration, the polite officer asked why I was visiting Vancouver Island and I told him I was scouting out "Sookie" and "Na-na-emo". Eye-roll. It's "Sooke" with one syllable, and "Na-nymo" when you try to say Nanaimo. No wonder American tourists have a bad reputation....