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Rocks can be formed in three ways. Rocks formed by from lava on the earths surface or from magmas deep underground are called Igneous rocks. Secondly, Metamorphic rocks are formed when rocks have undergone profound changes due to pressure and heat. The last type of rock found on Earth are sedimentary rocks which form on the earths crust.
Fossils are much more likely to be found in sedimentary rocks. This is because of the way they are deposited as mud, sand or other material. Each stratum represents the deposition of sediment on the seabed, lake, river or land surface. This leads to stratification, which can be clearly distinguished as different layers, of which undoubtedly the KT boundary is the most famous and can be seen around the world.
Fossils may sometimes be found in other types of rocks as volcanic ash and dust may settle in water and preserve marine animals. Metamorphic rocks such as slate may contain fossils, although they tend to be distorted.
Sedimentary rocks are classified into there groups; detritial, organic and chemically formed.
Detrital or fragmentary sediments include sandstones, mudstones, shales, conglomerates and breccias. They are made from materials such as sand grains or mud particles that have come from weathered pre-formed rocks and then have been carried and deposited by the sea or rivers. Fossils found within these fine grained rocks are likely to be preserved in great detail. Marine shales and clays are rich in mollusc and arthropod fossils, whilst land formed sandstones and shales are likely to preserve vertebrates and plant fossils.
Organic sedimentary rocks are formed from organic plant or animal material. They tend to be pale in colour and porous. Coral limestones are formed from sediments from coral reefs, whilst crinoidal limestone is formed from seabed banks of crinoid stems. Lime stones are rich in fossils such as ammonites and gastropods.
Some sedimentary rocks are formed by a chemical process. These include Oolitic limestone which is formed from small oolith grains. They are composed of layers of calcite precipitated around a minute shell fragment or sand grain. These rocks are often found in shallow marine areas and can be rich in fossils of echinoderms, brachiopods, molluscs and corals.
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