A Glacial erratic is a piece of rock carried by glacial ice some distance from the rock outcrop from which it came. Erratics can range in size from pebbles to massive pieces such as the Okotoks (16,500 tons) and Airdrie erratics found in Alberta, Canada. Geologists identify erratics by studing the rocks surrounding the position of the erratic and the rock of the erratic itself.
The Foothills Erratics Train is a deposit of rocks of many sizes. These deposits stretch in a narrow belt for about 600 km (400 miles) from the Athabasca River Valley in Alberta to southwestern Alberta.
Geologists have suggested that landslides or rockfalls initially dropped the rocks on top of glacial ice. The glaciers continue to move, carrying the rocks with it. When the ice melted, the erratics were left in their present locations.
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