A glacial erratic is a large boulder or rock fragment that has been transported and deposited by a glacier, often far from its original location. These rocks are typically different in composition from the bedrock on which they rest, serving as indicators of past glacial movement and ice sheet dynamics. Erratics are significant in studying the extent and direction of glaciation.
Reference: Benn, D. I., & Evans, D. J. A. (2010). “Glaciers and Glaciation.” Routledge.