How Do Fossils Form? Make Moulds, Casts and a Fossil Cliff

Fossils are one of the best bridges between science and history: they’re clues from living things that existed long before us. Understanding how they form, and how we read them, is a great hands-on topic.

Part 1: moulds and casts

You need: modelling clay (oil-based, such as plasticine), salt dough (flour, salt and water), and things to press: a seashell, a leaf, a key or a coin.

Press a shell firmly into flat clay and lift it out. The dent is a mould (like a fossil imprint left in rock).
Press salt dough into the mould and let it set. When you peel it out, you have a cast, a copy of the shape.

Real fossils form in similar ways. A shell is buried in mud, the mud hardens into rock and the shell dissolves, leaving a mould. Minerals can then fill the mould to form a cast. In other cases, minerals replace the shell’s material bit by bit.

Part 2: build a fossil cliff

You need: a clear plastic cup and four dry materials in different colours (craft sand, sand, rice, lentils or couscous), plus small model fossils.

Pour a layer of one material into the cup. Press a model fossil against the glass.
Add the next colour on top, with a different model fossil, then repeat for four layers.
Swap cups with another team, and ask them to work out which layer was laid down first and which fossil is oldest.

The answer: the bottom layer is the oldest, since layers pile on top over time. This principle, that in undisturbed layers older layers lie below younger ones, is how geologists put fossils in order.

What fossils tell us

When a team finds a seashell in a cliff far from any sea, it’s evidence that the place was once under water. Fossils show what lived where, how life changed over time, and what climates were like. A cliff of fossils is a book read from the bottom up.

Why most living things don’t become fossils

Fossilisation is rare. It usually needs quick burial, hard parts like shells or bones, and conditions that protect the remains. Most animals and plants decay without leaving a trace, so the fossil record is incomplete, and scientists work with what they find.

Safety

If you collect real fossils, ask permission and follow local rules: in some places, collecting is restricted or illegal. Don’t hammer rocks without eye protection and adult supervision.

The activities above are How do fossils form and what can they tell us?, the first lesson in Restless Earth: Fossils, Soils, Volcanoes and Plates. The unit continues with What is soil made of? and Where do volcanoes and earthquakes happen? Sign up for three lessons free.