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How Fossils Formed
Alister Cruickshanks

The most common method of how fossils formed is once an animal or plant dies, it falls to the ground, and is covered by sediment. This is often sediments brought from water. In the diagram above, the ammonite died in a river, and sediments over time covered the ammonite (shown in the second diagram). Finally after hundreds of thousands, or millions of years, the land is eroded and the fossil can be seen.

Of the vast amount of prehistoric life that died, it is only a tiny amount that has survived the fossilisation process. The conditions when the majority of life died were just not right at that time, to preserve them. Most fossils are found in sedimentary rocks which were formed from the sediments of rivers, lakes and seas. The majority of the animal and plant fossils we find today, had originally died near these areas, got broken up and deposited on the beds of the rivers, lakes and seas. The sediments covered them and over time some of the layers grew so thick that many of them got crushed. The sediments compacted and over time and turned to rock. The rocks shifted, moved and became exposed to the elements. This process can take up to several hundred million years. Now as the rocks erode or are quarried for e.g. the fossils become exposed and can be collected.

The Best conditions for Fossilisation

1. The quick burial of animal remains in moist sediments. This prevents scavengers from eating and bacteria from decaying them.

2. The quick burial in volcanic ash. Many dinosaur bones in the American west have been found buried in volcanic ash.

3. The presence of hard body or plant parts .Teeth, bones , shell and wood for e.g.

4. Unchanging temperature conditions.

5. Ground water that is heavily mineralised.

6. Sediments that are very fine make a better burial than coarser gravels.

7. Calm conditions, so that remains are not broken up (by wave or currant action for e.g.).

Heavily mineralised water

Leaves falling during Autumn


AMBER - Is the fossil resin from trees and plants. Whilst the resin was still sticky it sometimes trapped insects, spiders and small animals such as frogs, preserving their external structures.

Amber from the Suffolk Coast

Fly in Amber

MUMMIFICATION - A few mummified remains of animals have been found in some caves where the conditions are dry and sterile. Usually only the bones are preserved this way, but occasionally, skin and other tissue can be preserved also. Mummification is not true fossilisation, just a pause in the disintegration process.

– Which we talked about earlier.

TAR – Insects and animals have been found embalmed in tar. Tar preservation can only remain stable for thousands of years, not millions.

PYRITIZATION – During fossilisation some fossils can be replaced by iron pyrite. Fossils preserved this way, how ever, become unstable when exposed to moist air. Pyrite fossils need to be kept in very dry conditions.

Pyritic ammonites from Charmouth

SILICIFICATION – Wood is often found silicified in freshwater and terrestrial sand and silt deposits. Weathering volcanic ash supplies the silica which gradually incorporates into partially decayed wood.

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