Clastic rock is a secondary or derivative rock composed of fragmented materials – for example, conglomerate, sandstone, shale, etc., which have been formed out of the remains of previously existing rocks. Besides the large class of sand-and-gravel rocks, clastic rocks also include many rocks of organic origin, such as certain limestones, composed of the debris of shells and corals; coals made of the remains of plants; and some ironstones consisting in whole or in part of organic debris. Fragmental volcanic rocks, such as tuff and agglomerate, come also into the same division.