Erosion is the basic cause of sedimentation. Wind and water uproot materials and deposit them elsewhere, sometimes many miles from the original site. Strong winds have occasionally reduced visibility by raising huge quantities of dust from plowed fields or desert landscapes. When the wind dies down, the dust settles as sediments. Raging rivers often assume a muddy appearance as they transport eroded soil. Most of this uprooted material does not settle out as sediments until the water reaches the ocean. Ice also causes erosion. Glaciers, flowing slowly down a mountainside, may uproot soil and rocks in the process. When the glaciers eventually melt, this material is deposited as sediments.
Fragments of rock frequently form sediments. These rock sediments may be either sharp and angular, or rounded by the action of water and air. These sediments may become embedded in finer material and harden into sedimentary rocks. The rock containing the angular material is called breccia and the rock containing the rounded material is called conglomerate. Even smaller fragments called silt and clay regularly form sediments. Greek and Latin derivatives are used to describe rock fragments; they are called "clastic," which literally means "broken in pieces," and "detritus," which implies that the fragments have been worn away.
In lakes or seas, carbon dioxide regularly unites with water to form H2CO3, or carbonic acid. Since plenty of calcium ions exist in seawater, some calcium will unite with the carbonate radical (-CO3) to form CaCO3, or calcium carbonate. Calcium carbonate is insoluble in water, so it settles out and falls to the bottom as a precipitate. In this way, calcium carbonate becomes a mineral sediment. Magnesium carbonate forms sediments in the same way. Other minerals form sediments by a different process. When a body of water, such as a shallow desert lake, dries up completely, only the water evaporates, while the minerals that were dissolved in the water are deposited on the ground. Minerals such as salt and anhydrite form sediments in this way. Anhydrite is a dry form of copper sulfate.
Organic matter also forms sediments. This may happen after floods, when organic debris from dead plants and animals settle as sediments. These organic sediments might be quickly buried by other inorganic sediments deposited by the flood waters. They may also be subjected to such processes as anaerobic decomposition, heat and pressure. If so, these sediments turn into coal, oil or natural gas. Calcium carbonate and silica of organic origin also form biological sediments. For example, one-celled animals called foraminiferans have calcium carbonate shells, and a type of algae called diatoms and one-celled animals called radiolarians have silica shells. When such organisms die, their shells settle out of the water as sediments.