clay,Β soilΒ particles the diameters of which are less than 0.005 millimetre; also aΒ rockΒ that is composed essentially of clay particles. Rock in this sense includes soils, ceramic clays, clay shales, mudstones, glacial clays (including great volumes of detrital and transported clays), and deep-sea clays (red clay, blue clay, and blue mud). These are all characterized by the presence of one or more clay minerals, together with varying amounts of organic and detrital materials, among which quartz is predominant. Clay materials are plastic when wet, andΒ coherentΒ when dry. Most clays are the result ofΒ weathering.