a rock that doesn't belong there, but was dragged there from miles away by a glacier
A rock or rock fragment transported by a glacier and deposited on bedrock of different composition. Glacial erratics range from a few millimeters to several yards in diameter.
large boulders and rocks carried across the landscape by ice. Some of this material could have it source hundreds of miles away.
a boulder swept from its place of origin by glacier advance or retreat and deposited elsewhere as the glacier melted; after glacial melt, the boulder might be stranded in a field or forest where no other rocks of its type or size exist. Erratic boulder, northeastern Manitoba, Canada. A sense of the size of the glacial erratic can be estimated by noting the person standing in front of the boulder, on the left side. This erratic, as well as neighboring ones, were carried by the Keewatian Ice Sheet. (Photo courtesy of Lynda Dredge, Natural Resources Canada. Copyright Terrain Sciences Division, Geological Survey of Canada.)
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. Erratics can range in size from pebbles to large boulders such as the Okotoks (16,500 tons) and Airdrie erratics found in Alberta, Canada.