Okotoks Erratic - "The Big Rock"
"The Big Rock" is the world's largest known glacial erratic--rock
transported far from its place of origin by glacial ice. Big Rock,
also known as the Okotoks Erratic, is the largest rock in the Foothills
Erratics Train, a group of rocks that were carried by ice along the
mountain front and dropped as the glacier melted some 10,000 years
ago. The erratics lie in a narrow band extending from Jasper National
Park to northern Montana. The Okotoks Erratic weighs 16,500 tons. It
measures 9 metres high, 41 metres long and 18 metres wide. The rock
has been eroded into pieces, but is still a large landmark on the flat
If you look closely at the rock, you can see hardened layers of sand,
silt and small pebbles. It is a piece of the Gog formation, layers
of sediment deposited some 570 to 540 million years ago in a shallow
sea long before the uplift of the Rocky Mountains. As time passed,
the sediment was buried as layer upon layer built up thousands of feet
thick. The heat and pressure generated by the weight of the overlying
sediments compacted the sand grains and cemented them into an extremely
hard, durable rock called quartzite.
During a period of mountain building 150 to 50 million years ago,
the Rocky Mountains were formed from beds of sediments that were thrust
up and eastwards. Quartzite is common in the Main Ranges of the Rockies.
Big Rock was originally part of a mountain (likely Mount Edith Cavell)
in what is now Jasper National Park. About 18,000 years ago, a rockslide
crashed material to the surface of a glacier in the present day Athabasca
River valley, and Big Rock was carried out of the mountains on the
glacier's back. The valley glacier slowly moved eastwards to the plains,
where it collided with a continental glacier, the great Laurentide
ice sheet. The valley glacier was redirected to the southeast, parallel
to the mountain front. The erratics were deposited as the ice melted.
One interesting feature of Big Rock is the large split down the middle.
A Blackfoot story describes how this may have happened:
One hot summer day, Napi, the supernatural trickster of the Blackfoot
peoples, rested on the rock because the day was warm and he was tired.
He spread his robe on the rock, telling the rock to keep the robe in
return for letting Napi rest there. Suddenly, the weather changed and
Napi became cold as the wind whistled and the rain fell. Napi asked
the rock to return his robe, but the rock refused. Napi got mad and
just took the clothing. As he strolled away, he heard a loud noise
and turning, he saw the rock was rolling after him. Napi ran for his
life. The deer, the bison and the pronghorn were Napi's friends, and
they tried to stop the rock by running in front of it. The rock rolled
over them. Napi's last chance was to call on the bats for help. Fortunately,
they did better than their hoofed neighbours, and by diving at the
rock and colliding with it, one of them finally hit the rock just right
and it broke into two pieces.
Not only does this story explain why the rock is in two pieces, but
also why bats have squashed-looking faces. The tale provides helpful
caution against taking back what you have given away.
The name of the erratic was derived from the Blackfoot word for rock, "okatok."
Quartzite is slippery to climb and although it is hard, pieces can
break off in climbers' hands. Please do not climb the rock, as tempting
as it looks. Also, there are aboriginal pictographs on the rock, and
these could easily be damaged by climbers. Enjoy the beautiful colours,
textures and feel of the rock, but stay on the ground. Please help
us protect this Provincial Historical Resource for others to enjoy.
The Okotoks Erratic is located off Highway #7, 10 kilometres southwest
View Okotoks Erratic in a larger map
Last reviewed/revised: May 22, 2012