Sedimentary rocks are classified (depending on their mode of formation) into three major groups. Those which are formed from solid particles (clasts) derived from other rocks, are called clastic or detrital sedimentary rocks. Those which are made up of the dissolved weathering materials accumulated as a result of chemical processes, are called chemical sedimentary rocks. Finally, those which were formed by living things, be it plants or animals are called organic (or biogenic) sedimentary rocks. 

Clastic sedimentary rocks are those which formed from the lithification of solid weathering products, rock fragments, sands, and clays. As these materials are picked up and transported away from their source, they are subjected to mechanical and chemical action. Generally, they become more rounded (less angular), more spherical, and smaller. Chemical processes change unstable minerals, such as feldspars, into more stable ones such as clays. Transport and deposit processes separate the sediments into distinct size classes, a process called sorting. The extent to which sediments have changed as a result of these processes is called maturity. Maturity, sediment size and mineral composition provide the basic framework for the classification of clastic sedimentary rocks.