Oldhamia radiata.

Thomas Oldham discovered fossils of radiating fans shaped impressions on Bray Head in 1840 when he was director of the Geological Survey. These fossils were named after him. For a while these were considered the oldest fossil in the world. Bray Head. Fossil hunting on Bray Head was very popular with the Victorians.


What is a fossil?

Fossils are the remains of once living animals or plants. People have been finding fossils in rocks for thousands of years, but until quite recently they didn’t understand what they were. Today we recognise that the fossils we find in rocks represent the ancestors of the animals and plants that are alive today.


Not many plants and animals are lucky enough be turned into fossils.

When an animal or plant dies its remains usually rot away to nothing. Sometimes though, when the conditions are just right and its remains can be buried quickly, it may be fossilised. There are several different ways fossils are formed. Here we go through the five steps of fossilisation to make a typical ‘mould and cast’ fossil.


Mould and Cast Fossils

  1. An animal dies, its skeleton settles on the sea floor and is buried by sediment.

An animal dies and its body sinks to the sea floor. The soft parts of the animal rot away, leaving only its skeleton. The skeleton is buried by sediment (like mud or sand) falling from the ocean above. The sea floor is an ideal place for fossilisation, which explains why many fossils are marine (from animals that lived in the sea). Land animals may die and be swept out to sea to be buried in the same way.

  1. The sediment surrounding the skeleton thickens and begins to turn to stone.

The skeleton continues to be buried as sediment is added to the surface of the sea floor. As the sea floor sinks, pressure increases in the lower layers of sediment and it turns it into hard rock.

  1. The skeleton dissolves and a mould is formed

Now buried at depth and surrounded by stone, the skeleton is dissolved by ground water. This leaves a cavity (or hole) preserving the shape of the original skeleton. This cavity is known as a natural mould.

  1. Minerals crystallise inside the mould and a cast is formed.

Water rich in minerals enters the mould, and fills the cavity. The minerals deposited in the mould form a cast of the mould. This cast has the same shape as the original skeleton, but none of its internal features.

  1. The fossil is exposed on the Earth’s surface.

Millions of years later, the rock surrounding the skeleton rises to the Earth’s surface (this happens during mountain building, earthquakes and other earth processes). The rock is worn away by wind and rain, and the fossil is now exposed, waiting to be found!

The children examine fossils we have found.


Today we used casting with clay and plaster to explain how mould and Cast fossils developed.

The children rolled out clay and chose natural materials from woodlands and beaches to press into it. We talked about positive and negative space.

They were each given a plastic circle. These formed sides around the clay to hold the liquid plaster.




It took half and hour for the plaster to set enough to take away the clay and reveal the relief images. There were great satisified comments as each opened their plaster work.

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