Glacial erratic

A rock transported by a glacier and deposited far from its source. Its composition or size can distinguish it from the surrounding bedrock and reveal past ice movement.

Glacial transport: The movement of rock, sediment, and other material by glacier ice. Erratics are durable evidence of material carried beyond its bedrock source.
Glacial geomorphology: The study of landforms created by glaciers and ice sheets. Erratics are one of its field clues to former glacier movement.
Big Rock (Alberta): A large quartzite glacial erratic near Okotoks, Alberta, transported from the Rocky Mountains. Its size and distant source make it a prominent example of an erratic.
Drumlin: An elongated hill of glacial sediment or bedrock formed beneath moving ice. Drumlins and erratics can record the direction of past ice movement at different scales.
Plucking (glaciation): A process in which glacier ice freezes onto bedrock and pulls pieces away as it moves. Plucking can detach the large blocks that later become erratics.
Lithology: The physical characteristics and composition of rocks. Comparing an erratic's lithology with local bedrock can identify its distant source.
Madison Boulder: A large glacial erratic in Madison, New Hampshire, composed of Conway granite. This well-known block illustrates how erratics can dwarf nearby landscape features.
Esker: A winding ridge of sand and gravel deposited by a stream flowing within or beneath a glacier. Like erratics, eskers preserve evidence of former glacial environments, but record meltwater pathways.
Glacial abrasion: The wearing down of bedrock by rock fragments carried at a glacier's base. Abrasion marks the same transport process that can leave large blocks intact.
Provenance (geology): The origin and source history of a rock or sediment. Erratic provenance links a deposited block to a source region.

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