Jun 22, 2016· Sedimentary Rock. Sedimentary rock, one of the 3 major classes of rock comprising the Earth's crust (the others being igneous and metamorphic), is made up of loose, unconsolidated sediment that has been transformed into rock (ie, lithified) during geological history.. Only about 5% of the Earth's crust is composed of sedimentary rocks, but they cover 7075% of the exposed surface and contain ...