Boulder

a loose, waterworn, or rounded rock lying on the surface or imbedded in clay and other soils. The name is given to loose rocks lying in water courses, but especially to the rounded blocks of granite and other durable stone found in the paths of former glaciers. The theory is that they were torn from ledges of rock by streams of ice and were transported sometimes for thousands of miles. They vary greatly in size from mere pebbles to huge specimens fifteen feet in diameter. See Glacier; Agassiz