Others may appear green, brown or black, or combinations of these colours. These icebergs have usually rolled over, exposing basal ice, or have emerged from below water level. The various colorations are caused by differences in density, air-bubble content and impurities. For example, black ice is of high density and bubble free; dark layers indicate the presence of rock materials derived from the base of the parent glacier. Occasionally, rocks may be found on the original upper surface of the iceberg. As the iceberg melts, these materials precipitate into marine or lake sediments.