Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other planets. Regolith can vary from being essentially absent to hundreds of metres in thickness. Its age can vary from instananeous, for an ash fall or alluvium just deposited, to hundreds of millions of years old. Regolith of Precambrian age has been found in parts of Australia.
The origins of regolith on Earth are weathering and biological processes; if it contains a significant proportion of biological compounds it is more conventionally referred to as soil. People also call various types of earthly regolith by such names as dirt, dust, gravel, sand, and mud. The regolith is the zone through which aquifers are recharged and through which aquifer discharge occurs. Many aquifers, such as alluvial aquifers, occur entirely within regolith. The composition of the regolith can also strongly influence water composition through the presence of salts and acid-generating materials.