Okotoks Erratic - "The Big Rock"
"The Big Rock" is an enormous glacial erratic – a rock transported far from its place of origin by glacial
ice. The Okotoks Erratic is the largest known rock in the Foothills Erratics Train, a group of rocks that were carried
by ice along the mountain front and let down as the glacier melted more than 10,000 years ago. The erratics lie in a
narrow band extending from Jasper National Park to northern Montana. The Okotoks Erratic weighs an estimated 16,500
tonnes. It measures about 9 metres high, 41 metres long and 18 metres wide. The rock has broken into pieces, but is
still a large landmark on the flat prairie.
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 between 600 and 520 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 kilometres deep
(the Gog Formation can be up to four kilometres thick in places). 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 between 80 and 50 million years ago, the Rocky Mountains were formed from beds
of sedimentary rock that were thrust up and eastwards. The source of the Big Rock was discovered by looking for
quartzite – the Rockies are predominantly limestone, and quartzite is less common in the Main Ranges. Big Rock was
originally part of a mountain in what is now Jasper National Park. During the last ice age, sometime after about 30,000
years ago, a large rockslide crashed debris onto the surface of a glacier that occupied the present day Athabasca River
valley, and this debris, including Big Rock, was carried out of the mountains on the surface of the glacier. The valley
glacier slowly moved eastwards to the mountain front, where it collided with a huge continental ice mass, the great
Laurentide ice sheet. The valley glacier was deflected to the southeast, parallel to the mountain front. The erratics
were later 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
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.
This site is of great spiritual significance to the Blackfoot people, and 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 indigenous 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 of Okotoks.
View Okotoks Erratic in a larger map
Last reviewed/revised: July 13, 2015