HOW ARE FOSSILS FORMED?

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Fossils have been discovered for many years, although it was not too far back in our history that it was generally believed that fossils could not exist in a world that God created.  Since then we have come to accept Darwins' theory of evolution and the existence of prehistoric creatures and plants that lived in a world where man did not yet exist.

Fossils are the preserved remains or trace of a dead organism. They can be formed in several ways. Dead animals and plants can be preserved in amber, peat bogs, tar pits, or in ice. Casts or impressions, such as foot prints, can be covered by layers of sediments which eventually become rock and so preserve the casts. Hard body parts such as bones, shells and leaves, can be covered by layers of sediments and over time the parts are gradually replaced by minerals.


Fossils can be so detailed that they can show fine features such as feathers, scales, spores and even movements in the surrounding sediment.  All this gives us a fascination for fossils, especially the really huge dinosaur ones!. 

Most organisms decay without leaving a fossil record but sometimes when conditions are favourable for preservation of plant and animal material fossils are created. Fossils can be found throughout the Earths' sedimentary layers including sandstone, mudrocks, siltstone, shale and, limestone. These layers are primarily laid down by moving water, layer upon layer, in a process known as hydrologic sorting. 

After the death of an organism sediment may be deposited rapidly on it, which will bury it quickly. Over time more sediment is deposited on the skeleton which is buried deeper.  

If conditions are right then some creatures will become fossils. A soft muddy sea bed, low oxygen levels and minimal light help to preserve organic remains.  In addition the body must not be disturbed by predators or strong currents.  As a result most of the fossils found are marine animals (95%), followed by plants (4.5%), land invertebrates including insects (0.25%), and vertebrates - mostly fish (0.0125%).

The weight of this sediment compacts the sediment grains together and pushes out any water, and the soft sediment slowly turns to rock in a process called Lithification. Minerals contained within the sediment slowly replace the minerals present within the skeleton, filling any voids left as the skeleton dissolves.  This leaves a re-mineralised copy of the original skeleton.

Rocks can remain undisturbed buried deep with the bedrock for many millions of years. However, this bedrock can be disturbed by tectonic forces when neighbouring plates collide.  If the bedrock is shifted upwards the rock will start to erode and reveal fossils buried within it.

Depending on the geological conditions fossils may be washed up gently on the sandy shore shore if they are preserved in soft clay, or they may have to be carefully extracted from hard rock using hammers. 


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