Sedimentary rocks are primarily characterized by lithification. Lithification occurs when sediment has been deposited and accumulates at the earth's surface or in bodies of water to form a rock over time. Subgroups of sedimentary rocks are defined by their specific formation.
There are several types of sedimentary rocks, and they are generally differentiated by the types of sediment from which they are formed. Clastic rocks are generally formed by deposits left from moving bodies of water or liquids. This group includes sandstones and conglomerates, and they are further classified by the size of various grains in their composition. Biochemical sedimentary rocks are formed in part by calcified parts of animals. Chemical sedimentary rocks form when crystallization of inorganic compounds forms from a solution. Sedimentary rocks formed by other methods, such as volcanic activity, are classified into a miscellaneous category.