Origins of parallel Harbours and Points around Salt Spring
I grew up in Lake Cowichan, but our family vacations always took us to sea. Some of my favorite memories are of traveling the Gulf Islands by sailboat and playing on sandstone shores. I was always fascinated by the beauty of our islands, but it wasn't until I got to university that I really understood the science that underlies that beauty. Along with the lovely sandstone sculptures, there's another feature in our the Gulf Islands that you may have noticed.
Like ripples in the sand, most of our bays, valleys, harbours and peninsulas run in the same direction. This image of Prevost Island, taken from Google Earth, shows the kind of parallel features that are common in the Gulf Islands.
The story of our island, starts 380 million years ago when underwater volcanoes started erupting near the equator. For millions years, this activity continued creating a series of underwater seamounts. After two more periods of volcanic activity, our seamounts became an island arc, a chain of islands stretching hundreds of kilometers. However, these islands weren't just sitting there, they were moving.
Riding the ocean plate, they headed north a on a collision course with North America at the break neck speed of a several centimeters a year. Their arrival 100 million years ago, was a like train wreck in slow motion, creating mountains on the Mainland and Vancouver Island. This is also the time when the Georgia Basin formed. For those of you who like scientific terms, this island arc is what we call a terrane. It is just one of many terranes that make up the majority of BC and the Yukon. Our particular terrane, is named Wrangellia, it stretches all the way from Victoria to Haida Gwaii.
What goes up must come down, so as soon as the mountains formed, they started eroding. Washed by rain and broken by ice, gradually the mountains started releasing sediments which were washed into the Georgia Basin. Layer by layer these sediments built up like a stack of pancakes, until they were several kilometers deep. The process of erosion happened in stages, sometimes producing sand and gravel, at other times producing clay and mud. The process of mountain building resumed 55 and 42 million years ago, when two smaller terranes crashed into southern Wrangellia. Wedging under Vancouver Island, they lifted some parts of the area as much as 10 kilometers! Layers that were once at the bottom of the sea, were now thrust skywards. The upper layers, composed of loose sediments, were washed away. At the bottom of the pile, some of the layers had become sedimentary rock.
These rock layers are what geologists call the Nanaimo Group. They are the dominant rocks in the Gulf Islands and make up the North End of Salt Spring. Our South End is composed mainly of 380 million year old volcanic rocks from the original series of eruptions. The Nanaimo Group consists of alternating layers of harder rocks like sandstone and softer rocks such as mud-stone. They range in age from 65 to 90 million years old. Of the eleven or so layers, all but the Gabriola Formation can be found on Salt Spring. When the layers were lifted, they were also folded and fractured. These folds run mostly in a northwest to southeast direction. The layers exist mostly underground. Where they reach the surface, they are usually seen on edge. Erosion from waves, rain, freezing and glacial activity, erodes the softer layers faster than the harder layers.
The diagram represents seven layers of rock that have been folded. The dark blue represents hard layers made of sandstone and conglomerate, while the soft layers, made of mudstone, are shown in light blue. It is the faster erosion of the soft layers that creates the parallel bays, harbours and valleys in the Gulf Islands.
Two notable geological features can be seen while traveling around our island.
If you travel from Moby's Pub to Walker Hook, you are crossing a huge fold in the Nanaimo Group. The middle of the fold is at the bottom of the big dip on Robinson Road. Each of the little valleys is made of mud-stone, while each of the ridges are made of sandstone. Both of the photos on the left are part of the same sandstone layer called the Protection Formation. The pictures were taken at Walker Hook and Ganges Harbour.
From Walker Hook, the Protection Formation dives more than a kilometer below Dunbabin Park before curving back towards the surface and reemerging near Ganges as the Chain Islands.
The ridge that connects Mt. Erskine to Mt Belcher (bottom of picture below) is composed of sandstone and conglomerate of the Extension Formation, while the Cranberry Valley (in the background) is composed of mudstone from the Haslam Formation. On the west side of Bader's Beach lies part of the Karmutsen Formation (top left), composed of volcanic rock that erupted underwater 380 million years ago.
The photo below, taken from Mt Erskine, shows seven formations of the Nanaimo Group. From bottom to top we have conglomerate of the Extension Formation, Booth Bay underlain by mud-stone of the Pender Formation, an un-named point made from sandstone of the Protection Formation, Vesuvius Bay underlain by mud-stone of Cedar District Formation, Dock Point composed of sandstone from the De Courcy Formation, Duck Bay underlain by mud-stone of the Northumberland Formation and Parminter Point made of sandstone of the Geoffrey Formation.
It's been forty years since those blissful days of playing on sandstone shores and I am now living on the Gulf Islands. I lead kayak tours in the summer and home-school our daughter in the winter. As I paddle, hike and cycle around Salt Spring, I still love the sandstone sculptures but now I keep a geology map at hand.
For more details, check out our page Salt Spring Rocks. Updates and additions will be made as I collect more photos.
Useful Links
Salt Spring Geology Map
BC and Yukon Terrane Map
http://www.arcgis.com/home/webmap/viewer.html?webmap=998ee7900e0b4d57af50c40700d4de80&extent=-164.1603,55.5602,-99.2531,71.5843
Nice Description of the Nanaimo Group
A little more info on the Nanaimo Group