Sedimentary rocks are formed on the surface of the Earth. They often appear as layered deposits forming a strata or bed structures. Sedimentary rocks can be classified into different categories depending on their origin or composition.
Clastic sedimentary rocks are fragments of rocks and/or minerals cemented together. The fragments may be as small as a microscopic piece or huge boulders of pre-existing rocks compacted together to form an even bigger rock. Sand turns into sandstone, mud into mudstone, and pebbles and cobbles into conglomerate
Biological sedimentary rocks are composed of fragments of an organism’s remains. They could be shells or pieces of corals to form a type of limestone. They could also be plants combined with other elements to form coal.
Some sedimentary rocks may form through saturated precipitates that are cemented together such as in the case of oolitic limestones. Other sedimentary rocks may form when materials from a volcanic eruption are compacted and cemented together like tuff and volcanic breccia.