Shale is a fine-grained sedimentary rock composed mainly of clay minerals (hydrous aluminum phyllosilicates) with minor proportions of quartz, feldspar, calcite, and iron oxides. Because of its small grain size, the constituent particles can settle only in water free of turbulence. This is only the case in nearly standing water bodies. Shale is thus deposited in closed basins, estuaries, lakes, and similar depositional settings protected from currents, waves, and wind and in deep water far from continents. Varieties rich in organic material, called black shales, provide valuable sources of information about the earliest forms of life and serve as petroleum source rocks. Shale is converted by metamorphism into mica schist. The occurrence of shale deposits in planetary surfaces would be a good indicator of the past presence of liquid water on their surfaces.