Till is a mixed, unlayered, unconsolidated sedimentary deposit of clay, sand, pebbles, rocks, and boulders that was left behind my melting glaciers.
DEPOSIT, in geology, natural accumulation of mineral or rock material formed through deposition by waters, glaciers, or the wind (see GLACIER ). Alluvial deposits are formed by rivers and streams. Running water may carry suspended particles of sand, clay, or silt, which gradually settle to the bottom or at the sides of the stream, particularly in places where the stream widens and the current slows. The deltas of rivers are alluvial
deposits. Where swift mountain streams reach the floor of a flat valley, the sudden change in grade, and the consequent slowing of the stream, often results in the deposition of an alluvial fan, another form of alluvial deposit. The action of running water tends to sort out the different sizes and weights of particles that are carried in suspension. Thus, gravel and pebbles will be deposited farther upstream than the lighter sand, and fine silt will be carried farther downstream than sand. Deposits are also formed in the ocean, in lakes, and in other places by the settlement of sand, silt, and other materials. SEDIMENTARY ROCK (q.v.) is formed by the consolidation of such
deposits. Chemical precipitation from water results in the formation of salt, gypsum, and some limestone. Stalactites and stalagmites are typical of precipitation deposits (see
CAVE ; STALACTITE AND STALAGMITE ). Mineral material dissolved in underground water is commonly precipitated in rock fissures, forming veins.
The glaciers of the great ice ages, as well as modern glaciers, are responsible for several kinds of deposits. Glacial till consists of material that is picked up, transported, and deposited by glaciers. Such till commonly contains rock fragments of all sizes from clay to large boulders. Although till is not sorted according to size, the material deposited by meltwater streams derived from glaciers is graded in the same manner as are other alluvial deposits. A terminal moraine is a ridge of till that was deposited at the terminus of a glacier. An ESKER (q.v.) is a ridge of sand and gravel deposited by a meltwater stream. It marks the former position of an ice tunnel and a stream flowing beneath a glacier. See GEOLOGY.
The most common wind-formed deposit is the sand dune, found in deserts and on the shores of oceans and lakes. Dunes are formed by sand accumulating around any object that serves as an obstacle to the wind. As windblown sand is deposited in the lee of such an obstacle, the wind is slowed still more, and the dune continues to grow. Dunes in some localities may reach a height of 200 m (650 ft). When materials finer than sand are carried by the wind and are finally dropped to earth, the deposit that results is known as

loess. Thick layers of loess are found in Europe and the Americas, and loess deposits of
Asia reach a thickness of several hundred meters. They are the basis for some of the richest agricultural soils of the world.