Boulder

Migratory boulders, or coal stones, are larger than usual boulders that were detached from the bedrock by continental ice during the Ice Age and transported to their current locations. As the glacier flowed over the rock, a silo rock was formed on the opposite side of the glacier as a result of wear. The glacier melted from its base under high pressure, and the melt waters traveled with the glacier to the protective side of the rock, where the pressure was lower. As the pressure dropped, the melted water froze again in the rock crevices, breaking the rock and creating boulders. The boulders were transported with the flowing ice far away from their parent rock, eventually being deposited when the glacier melted or falling into the sea or the bottom of the glacial lake when the iceberg melted.