Sediments are loose grains and chemical residues of various Earth materials formed at the surface of the Earth by the process of weathering. Physical weathering involves the breakdown of larger rocks into smaller grains, as can occur when two rocks collide and one of them breaks into smaller particles. Chemical weathering involves chemical changes in minerals, often releasing ions into surface and ground water. A common example of this type of weathering would be the dissolution of salt (the mineral Halite) in water where the component of the mineral exist as ions of Sodium (Na) and Chlorine (Cl).

Sediments are transported from their place of origin by wind, water and ice, then deposited. These deposits of sediments undergo the process of lithification, or rock formation, to become sedimentary rocks. The process of lithification involves compaction (due to burial), cementation (the precipitation of chemical cements which bind the grains together), and recrystallization (the reforming of mineral crystals under low amounts of heat and pressure).

Sedimentary rocks are classified by the type of process that formed them. Those made up of particles (called clasts) are called clastic rocks. Those which are deposited from aqueous solutions (typically from the oceans) are called chemical rocks. There are very few other types of sedimentary rock, the only common one being coal (click on the link to Other).