Foothills Erratics Train - the start of the Okotoks Erratics

Foothills Erratics Train

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Big Rock

The Foothills Erratics Train is a landscape feature of Alberta which is composed of thousands of boulders in a 644 kilometre chain ranging in size from pebbles to immense boulders like Big Rock.^[1] These boulders are composed of pink and purple quartzites that are not native to this region of Alberta. Their source has been identified as being near Mount Edith Cavell in Jasper National Park, most likely in the Tonquin Valley. It is thought that these boulders are debris from a landslide that fell onto a glacier during the late Pleistocene epoch. This glacier then carried the boulders north and east until it met the more massive continental glaciers, which deflected the glacial stream to the south-east. The advance of this glacier carried boulders well into the state of Montana. Once the glaciers began melting, the boulders were dropped where they are now found. The Big Rock, the most well known example, is located just west of the town of Okotoks on Highway 7.

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Foothills Erratics Train

In southern Alberta, along the eastern side of the Rocky Mountains, thousands of large boulders (erratics) form a train over 600 kilometres long. These rocks contain clues that have helped scientists to understand the movements of the ice sheets that covered Canada during the ice age.

When geologists examined the rocks, they discovered that they were all made of the same kind of rock, and they traced the source of that rock to an area around Mount Edith Cavell in Jasper Park. Thousands of years ago, these large stones fell onto the surface of the Cordilleran ice sheet during a landslide, and then were slowly carried by the ice sheet outward onto the Plains. When the ice melted, a long train of boulders was left behind.

The pathway of the erratics train reveals the direction in which the ice sheet travelled. The rocks take a sharp right-angle turn out on the Plains, changing from an easterly to a southerly direction. Scientists believe that the western Cordilleran ice sheet "bumped into" the eastern Laurentide ice sheet, and got deflected southward. This shows that the two ice sheets were joined together, and no "ice free corridor" could have existed between them until later, when the glaciers started to melt.

This example of a Foothills Erratic is located near Glenwoodville, Alberta.