<P> Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock . A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering . Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits . </P> <P> Clastic sedimentary rocks are rocks composed predominantly of broken pieces or clasts of older weathered and eroded rocks . Clastic sediments or sedimentary rocks are classified based on grain size, clast and cementing material (matrix) composition, and texture . The classification factors are often useful in determining a sample's environment of deposition . An example clastic environment would be a river system in which the full range of grains being transported by the moving water consist of pieces eroded from solid rock upstream . </P> <P> Grain size varies from clay in shales and claystones; through silt in siltstones; sand in sandstones; and gravel, cobble, to boulder sized fragments in conglomerates and breccias . The Krumbein phi (φ) scale numerically orders these terms in a logarithmic size scale . </P> <P> Siliciclastic rocks are clastic noncarbonate rocks that are composed almost exclusively of silicon, either as forms of quartz or as silicates . </P>

Coarse-grained rock which is composed mainly of quartz and potassium feldspar