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 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 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 of Okotoks
