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 left 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 (29.5 feet) high, 41 metres (134.5 feet) long and 18 metres (59 feet) wide. The rock has broken into pieces, but is still a large landmark on the flat prairie.