Big Rock Erratic – Okotoks, Alberta - Atlas Obscura

Big Rock Erratic

Okotoks, Alberta

This massive and unusual rock formation near Okotoks is the world's largest known glacial erratic. 


An erratic is pretty much exactly what it sounds like. Large, hulking chunks of rock, jutting out of an otherwise flat and sparse landscape, catching the eye and mystifying any comprehension of its origin.

Properly called “glacial erratics,” these rock formations are actually relatively common. Formed by dislodged rocks that traveled on top of a glacier thousands of years ago, their decidedly out-of-place appearance comes from the result of that glacier-based piggyback ride, as the melting ice deposited the far-flung rocks where they didn’t belong.

In Alberta, Canada, there is a particularly large and far-reaching trail thousands of erratics known as the Foothills Erratics Train. The Big Rock erratic is its crown jewel.

Comprised of 16,500 tons of quartzite, the Big Rock, or Okotoks, erratic is the largest known glacial erratic on the planet. Located just over four miles from Okotoks, Alberta, from which it derives its name, the massive rock formation is an unmistakable monolith in an otherwise monotonous landscape.

Despite the interesting but simple origins of this slow-forming landmark, it has long attracted legends and myths. These date back thousands of years to the Blackfoot tribe, whose legends explained the rock’s peculiar location by detailing an argument between the rock and a young warrior that eventually gave way to a frantic chase and the eventual splitting of the rock down its center.

Today, mysticism-minded travelers still find themselves drawn to the Big Rock, usually avoiding the scientific explanation for the unusual formation, instead preferring to make up their own stories as time goes on. 

Know Before You Go

The Okotoks Erratic is located off Highway #7, with its own parking lot nearby. It is about 10 kilometres southwest of Okotoks, 15 kilometers northeast of Black Diamond, or an hour's drive from central Calgary.

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