Striated erratic, with a penknife (blue) provided for scale. An erratic is a boulder which has been transported from its point of origin by the moving ice of a glacier. Often the erratic is deposited in an area with a different geology, aiding identification of the erratic. Glacial striae are marks made on rocks by other rocks due to the weight of ice as a glacier passes overhead. Photographed in front of a glacier on Svalbard, an Arctic archipelago governed by Norway.