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Sedimentary rocks are made up of little bits of material, called ‘clasts’, which have been cemented together. They form in places where material is moved around and deposited, like rivers, deserts, lakes and oceans. Quite often, sedimentary rocks will either include, or be entirely made up of, the remains of living organisms. Once the sand or shells have been deposited they are covered by layers and layers of more sediment. Gradually, over time, the pressure of all this material on top squeezes the clasts together and they become more stuck together, so they are no longer soft mud or sand, but a hard mudstone or sandstone. Sometimes the grains cement themselves together and sometimes the grains are stuck together by another material.
