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The Creation of Lake Kasshabog

By Lelia MacDonald

This Spring I decided to take a geology course online with Athabasca University. It turned out to be fascinating and, in the process, I discovered 15 interesting geological facts about our Lake. You may have followed my posts on the Facebook Group Lake Kasshabog’er. Here are a few of the things I learned. To discover the other 11, visit

Did you ever wonder why Kosh Lake looks like an upside down “T”?

Why do Portage Bay and Stony Lake Bay head off in right angles? It is not because of the rocks in the bay. Instead, it is because of the rocks 1km south. See photo. The two purple blobs are rocks that bubbled up from the mantle and melted their way through the crust. Like a lava lamp. They squished the bottom end of the lake flat. This happened billions of years ago back when the earth was like that lava planet (Mustafar) in Star Wars.

Have you ever noticed big boulders just lying around in the woods?

Geologists call those “erratics”. The most spectacular erratic on Kosh is hidden behind island #I8. It is a massive sphere that seems to float on the water. It is worth the kayak to see it. Erratics are boulders that were left behind as the glaciers retreated. Think about how heavy these erratics are and how powerful the glaciers must have been. The glaciers here were about 1km thick. As they moved along, they scraped up a lot of debris underneath them.

Have you ever wondered why Bill’s Bay is so straight?

There is a line heading NE that starts south of FR75, continues underneath Bill’s Bay through both narrows and then splits as it continues on north of MacDonald’s bay. (See straight dotted line in photo.) It is an ancient fault. Don’t worry, it is not active. Most likely the fault was created 1 billion years ago, when the lake was at the bottom of a massive mountain range (which has subsequently eroded). The fault made the rocks on either side more fragile, which allowed them to erode faster and form the river to drain MacDonald’s Bay.

Why are there white veins in rocks around Kosh?

If you look at granite close up, you will see flecks of pink, white, grey and black. Back in the day, when Kosh was under a huge mountain range, the temperature was hot and all 4 types of minerals were mixed together as a liquid. As the mountains eroded above us, the rock started to cool. At a temperature of 900C, the black and grey cooled enough to crystallize. At about 800C, the pink crystallized. Eventually the already hardened rock cooled so much that it shrank and cracked. The remaining liquid (white quartz) filled in the cracks and cooled.


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