erratic
A boulder transported by a glacier and deposited far from its place of origin, often sitting conspicuously on terrain that doesn't match it — granite on limestone, basalt on sandstone. A geological stranger, sometimes the size of a house.
Etymology
From the Latin erraticus, wandering. The geological sense was established in the 19th century when scientists realized these out-of-place boulders had been carried by ice, not by floods.
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