A glacial erratic is a piece of rock that deviates from the size and type of rock native to the area in which it rests; the name "erratic" is based on the errant location of these boulders. These rocks were carried to their current locations by glacial ice, often over hundreds of kilometres. Erratics can range in size from pebbles to large boulders such as Big Rock (16,500 tons) in Alberta.
[wikipedia]