How fossils are formed

The formation of fossils is actually quite a rare process. There are very specific conditions that need to be met before a living thing can be turned into stone. Normally, when something dies, it rots away and leaves no trace behind - however, if it is buried rapidly there is a chance that it could be preserved. 

About 99% of the fossil record is composed of marine animals. This is because in a marine environment, the animal can be buried quickly by sediments such as sand or mud, and oxygen will be limited; this prevents scavengers from taking too much, and slows down decomposition. The Isle of Wight is a fantastic anomaly to this with our large amount of dinosaur fossils. This is because the geological environment in the Cretaceous times here used to be alluvial (silty) floodplains with river systems, lakes and brackish lagoons, and so dinosaur remains would be washed into rivers or buried quickly by flooding. 

So, the key steps for creating fossils are:

1. The animal dies

2. The soft tissue gets eaten and decays

3. The remains are rapidly buried by sediment

4. More layers of sediment get laid on top - creating sedimentary rocks

5. Mineral-rich water will seep into leftover bones and turn them into stone (crystals can also be left behind in empty spaces - this is why the bones we find have a honeycomb-like structure as the internal bone structure has been preserved)

Step five can sometimes look a little different, especially in marine animals with shells. Instead of the water turning the remains into stone, it can dissolve them, leaving behind an imprint (a mould). This imprint will then be filled in by other minerals which creates a cast. Footprints are also formed in this way - for example, a dinosaur leaves a footprint behind (the mould), which gets filled in by sediments, creating the cast. A lot of the footprints at Hanover Point at Compton were formed in this way - that’s why they look 3D instead of being a footprint like we would leave behind. 

So how do we find these fossils if they are buried underneath layers of sediments? Rocks are exposed to the surface by uplift from geological forces. As the rock is exposed, it will be subject to weathering events, eroding the layers until eventually the fossils can be exposed. You can see the eroding effects of the sea all along the Isle of Wight coastline. 

Want some tips of fossil hunting on the Isle of Wight? Read Here