How the Mountains Around Canmore Were Built

Long before the Canadian Rockies became a skyline of limestone walls and glaciated peaks, the land that now forms them lay near the equator under a shallow tropical sea, where coral reefs, sand, and ocean mud slowly accumulated before being scraped off the continent and pushed eastward like snow in front of a plow.

That long geological history was the focus of a Jan. 18 presentation in Canmore by Dr. Eva Enkelmann, a professor in the Department of Earth, Energy and Environment at the University of Calgary. Speaking to a public audience hosted by the Alpine Club of Canada’s Rocky Mountain Section at Origin at Spring Creek, Enkelmann traced how the mountains surrounding the Bow Valley were assembled over hundreds of millions of years.

“The Rockies are really this easternmost part of a pile of sediments that were stacked on top of each other,” she said. “Think about a bulldozer. You scrape off the top layer, and in front of the blade you get sheets that are folded, broken, and pushed on top of each other. That pile in front of the bulldozer, that is the Rocky Mountains.”

The story begins more than 2.5 billion years ago with the formation of the Canadian Shield, the ancient core of the continent that still lies buried deep beneath Alberta. But the visible Rockies are much younger. Their building blocks were laid down when western North America sat far closer to the equator and was covered by warm, shallow seas.

Around 540 million years ago, the Canmore and Banff area lay along the shoreline of a newly opened ocean. Rivers carried sand from the interior of the continent and spread it along beaches. Over time, those sands were buried deep beneath other layers of rock and heated, squeezing the grains together until they turned into an extremely hard stone called quartzite. That ancient beach sand now forms the cliffs seen around Lake Louise and along the Continental Divide.

“It all started out as beach sand,” Enkelmann said. “Over time, another ten or twelve kilometres of sediment piled on top of it, and the weight and heat pressed the grains together into solid rock.”

As the ocean deepened slightly, the region became a vast tropical sea filled with coral reefs and other marine life. Corals, sponges, and shell-building organisms built hard skeletons from minerals in seawater, and when they died, their remains accumulated on the seafloor. Over millions of years, these growing piles of reef material were buried, compacted, and cemented into thick layers of limestone. Those layers eventually hardened into the massive rock formations that now form the vertical faces of peaks such as Mount Rundle, Cascade Mountain, and Castle Mountain.

“This is how Alberta looked for a really long time,” she said. “Warm, shallow water, building huge reef systems. That is what makes the big cliff-forming rocks of the Rockies.”

Between the reefs, deeper parts of the sea collected fine mud along with the remains of countless microscopic plants and animals. As this organic-rich mud was buried under more and more sediment, heat and pressure slowly transformed it into oil and natural gas. The newly formed oil and gas then migrated out of the mud layers and into the nearby reef rocks, which were full of tiny spaces left behind by corals and sponges and could trap and store the fluids.

“The source is the deeper shale basins,” Enkelmann explained. “The oil moves into the holes left by corals and sponges.”

The same ancient seas that built today’s limestone cliffs and mountain peaks also created the underground conditions that later made Alberta one of the world’s major oil and gas regions.

For much of the Paleozoic Era, from about 540 million to 250 million years ago, the future Rockies remained a stable marine environment. The major upheaval began much later, during the age of dinosaurs, starting around 200 million years ago, when the western edge of North America began colliding with offshore island chains and fragments of oceanic crust. These collisions triggered a long period of mountain building that would eventually raise the Rockies.

As tectonic plates converged, layers of sediment were scraped off the continental margin and pushed eastward. Huge slabs of rock, many kilometres thick and tens of kilometres long, were stacked on top of one another along thrust faults.

“We are literally looking at packages of rock that were transported tens of kilometres sideways and then lifted on top of much younger layers,” Enkelmann said.

One of the most prominent of these structures is the McConnell Thrust, a fault system that runs for hundreds of kilometres along the eastern front of the Rockies. At places such as Yamnuska, Cambrian limestones more than 500 million years old sit directly on top of Cretaceous sandstones that are less than 100 million years old.

“If you hike up Yamnuska, you start in young rocks and suddenly you are standing in rocks that are hundreds of millions of years older,” she said. “That only happens when entire slices of the crust are moved and stacked.”

The mountain building progressed gradually from west to east over more than 100 million years. As new ranges rose, they were immediately attacked by erosion. Rivers carried sediment off the growing mountains and deposited it in lowlands to the east, including the area around present-day Canmore. During the Late Cretaceous, roughly 100 to 66 million years ago, lush coastal plains and swamps covered the region, laying down organic-rich layers that later became coal.

“Canmore exists because of this part of the story,” Enkelmann noted. “The town grew around coal that formed in these ancient swamps.”

By about 50 million years ago, the major phase of mountain construction had ended. Since then, erosion has dominated. Rivers cut valleys, and during the past two and a half million years, repeated glaciations carved the peaks and troughs into their modern form. At the height of the last ice age, ice sheets hundreds of metres thick buried most of the Bow Valley, leaving only the highest ridges exposed.

“What we see today is the result of long-term decay,” she said. “The mountains are no longer growing. They are being taken apart, first by glaciers and now mostly by rivers.”

Even the distribution of forests and trails reflects the underlying geology. The rugged limestone ranges rise where resistant reef rocks remain. Gentler terrain such as the Canmore Nordic Centre and Spray Valley follows softer rocks laid down during the Jurassic and Cretaceous periods, roughly 200 to 66 million years ago, which erode more easily.

“The landscape, the valleys, where trees grow best, where we ski and bike, all of it is tied to the rocks underneath,” Enkelmann said.

From tropical seas to coral reefs, from continental collisions to ice sheets, the Canadian Rockies record a history that spans nearly half the age of the Earth. What now appears timeless and immovable is, in geological terms, the product of relentless motion.

“I hope next time you go into the mountains,” she told the audience, “you look at the rocks and realize you are walking through hundreds of millions of years of history that has been folded, pushed, and lifted into the peaks we call home.”

Learn more in Dr. Eva Enkelmann’s book Rocky Voices: The Memories of Minerals That Formed the Rocky Mountains.