Sedimentary rocks are formed by the accumulation and consolidation of sediments . There are three main types sedimentary rocks . Clastic sedimentary rocks , such as breccia or sandstone , are formed from other rocks that have been broken down into fragments by weathering , which have been transported and deposted elsewhere . Organic sedimenatry rocks – for example , coal are derived from plant and animal remains . Chemical sedimentary rocks are formed by chemical processes . For example , rock salt is formed when salt dissolved in water is deposted as the water evaporates . Sedimentary rocks are laid down in layers , called beds or strata . Each new layer is laid down horizontally over older ones . There are usually some gaps inthe sequence , called unconformities . These represent periods in which no new sediments were being laid down , or when earlier sedimentary layers were raised above sea level and eroded away .