Gazing up in awe at the majestic lofty peaks of Mt. Rundle, The Three Sisters and others surrounding Canmore, who would imagine let alone believe these summits were once the bottom of ancient tropical seas! Indeed, if one were to climb to the top, one would find the fossils of sea creatures 350 million years old! What tumultuous forces and how much time would be required to pull off such a feat?
To begin, understand that the age of the rocks and that of the mountains are not the same. The Rocky Mountains are divided up into three sections from west to east – the Western Main Range extending from Golden, BC to just west of Emerald Lake in Yoho National Park. The Eastern Main Range continuing from there to Castle Mountain in Banff National Park, and the Front Range extending from Castle Mountain to the eastern edge of the mountains including Banff and Canmore. The even younger foothills extend east from the mountains to about Cochrane. The Main Range mountains are composed of very hard 500 million year old flat-lying quartzite rock layers, while the Front Range mountains are severely tilted and largely composed of softer limestone and shale.
Mount Rundle and The Three Sisters are classic tilted Front Range mountains. If you look closely, the mountains are comprised of three distinct layers, stacked one atop the other like a sandwich (see diagram below). The lowest and oldest layer is the Palliser Formation formed as tropical sea sediments 366-363 million years ago. This massive, thick layer is composed of Devonian age fossil-rich limestone and dolomite. Being erosion resistant, it forms the steep cliffs that mountain climbers love to tackle. Back then, these rocks were coral reefs like those in the Caribbean we see today. Above this layer is The 360 to 350 million year old Mississippian – age Banff Formation of dark grey shale. Because they are softer and weaker, this layer forms a more jagged stepped-back appearance. The top and youngest layer is the 350 – 300 million year old Mississippian-age Rundle Formation of fossil-rich (mainly corals) limestone and dolomite.
Far to the West at the Pacific Ocean, new land was added to North America. With continued pushing and rippling, the Front Ranges were thrust up over a period from 85 to 75 million years ago, 10 million years before the great sudden mass extinction of the dinosaurs. Originally the Rocky Mountain chain extended over 500km wide but were “telescoped” together to less than half that width! The rock had to go somewhere and was thrust up in a series of buckled and fractured earth shaking events.
At the top of our summits you will experience the youngest layer. You can actually walk among, explore and touch an ancient 380 – 365 million year old Devonian age coral reef if you visit the Grassi Lakes above Canmore on the Spray Lakes Road. This massive dark grey limestone dolomite complex known as the Fairholme Formation lies below the Palliser Formation and was formed in ancient clear warm tropical sea when this area was near the equator and at the edge of a continent. These cliffs that surround the Grassi Lakes are over 300m thick and were subsequently buried by younger sediments that eventually turned to rock. Note the unusual porous nature of the rock that resembles Swiss Cheese or a giant sponge. These bubble cavities used to hold the oil and natural gas that is still present in the same rock layer under the prairies today. The petroleum here at the Grassi Lakes is now long gone so you won’t strike it rich unfortunately. But if you look closely, you will discover other rewards such brachiopod, coral, snail and sponge fossils and First Nations pictographs can be seen. Be sure you take only photos of these home with you.
This is one of the best areas in the Rockies to observe such dramatic layering and subsequent differing erosion and is the reason visitors from all over the world come to Canmore to experience the majesty of the mountains. Whether you are here to experience the incredible scenery or up to deeper explorations of why it became what it did, those who take time to look will find the geological clues that explain the story of how seas became summits!