gravel, particles of rock, i.e., stones and pebbles, usually round in form and intermediate in size between sand grains and boulders. Gravel is composed of various kinds of rock, the most common constituent being the mineral quartz. Deposits of gravel are formed as a result of the weathering of rocks and the erosive and concentrating action of rivers and waves. Sometimes gravel becomes consolidated into the sedimentary rock called conglomerate. Gravel is used extensively in building roads and in making concrete. For road building it is crushed into angular particles of uniform size. One or more layers of gravel underlie the road surface. A small percentage of clay must be present to act as a binder when gravel is used in macadam for road surfaces. When used as a coarse aggregate for concrete, gravel must be clean and free from clay and organic matter. Commercially, it is classified according to the size of the particles. In areas where natural deposits are inadequate, gravel is produced by quarrying and crushing durable rocks, such as sandstone, limestone, or basalt.