KENT GEOLOGISTS' GROUP
What are fossils?
Fossils are the remains or traces of animals or plants preserved by natural causes in the rocks of the Earth's crust. For example, if an animal dies, and is buried rapidly in a muddy sediment, its hard parts are likely to be preserved as a fossil. Fossils of marine animals are by far the most common because most sediments are laid down on the sea floor.
How are fossils preserved?
When an animal dies its soft parts usually decay rapidly leaving only the shell or skeleton. The soft parts are, therefore, only rarely found as fossils. Corals, snails (Gastropods), lamp shells (Brachiopods), and similar have a 'hard' shell that is often preserved unaltered. Sometimes the shell may be entirely replaced by a mineral such as Pyrite (Iron Sulphide, or 'fool's gold') or it may be dissolved from the rock by groundwater leaving only the impressions of the inside (internal mould) and the outside (external mould) of the shell. Plants and Graptolites are usually crushed so that all that remains is a carbonised impression.
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