Types Of Fossils – Part 1

Welcome back! Hope you all had a good Christmas! So, to begin the year, we will be looking at different types of fossils, in terms of how they have been fossilised. And we begin with the one most people are familiar with – Casts.

 When most people think of fossils, they often think of creatures that have turned to stone. This is not quite true, but the closest it comes to that are casts. So, what is a cast? Well you can actually try it yourself……

You will need:

  • Modelling clay or wet sand (modelling clay works better).
  • Plaster of Paris mix.
  • A shell or a toy animal (make sure it’s a toy – don’t try to actually fossilise your pet, they won’t appreciate it!).
  • Food colouring (optional).
  • Small paintbrush.
  • Cooking oil.

Start off by moulding the clay in your hands to make it nice and soft. If you are using sand, make sure it’s damp, so it holds its shape. Place the ball of clay/pile of sand on a tray and flatten it a bit.

Step 1 of making a cast fossil.

Take your shell and toy animal and press them firmly into the clay. Make sure you get them in deep, but make sure you don’t go through to the tray. Take the shell and toy out carefully and leave the impression in the clay.

Step 2
Make sure they are pushed down…..
Make sure you don’t ruin the impression when you add the oil.

Mix up your Plaster of Paris mix according to the instructions. I tend to go a tad on the thick side. You can also add a bit of food colouring to the mixture as well.  Carefully paint the inside of the impression with a very thin layer of cooking oil, to help get it out afterwards. Spoon the mix into the impressions in the clay and leave them to set.

Once they have set, carefully take them out of the clay – and there you are! A cast fossil!

 So what you have done there is replicate what happens when a cast fossil is made. The creature gets buried in sediment, and then gets destroyed, or rots away. A new sediment, often harder than what the creature was originally buried in fills up the space where the creature was and makes a perfect cast of that creature. There are many examples of cast fossils, you can see some of them below.

You actually made another type of fossil as well, one we’ll be discussing next week – a mould fossil!

To find out more about fossils, visit some of the websites below:



or visit the Genesis Museum of Creation Research – www.genesismuseumofcreation.com

Ammonite Cast Fossil. The sediment used to infill the impression was actually full of fossils, so it’s a fossil inside a fossil!
Another Cast fossil, this time a Nautilus. It has been filled with several minerals and agates.
A cast fossil of a trilobite.
A cast fossil of a Echinoid – a fossil sea urchin.