This activity is great fun to try - 'Cracking the clues: making your own cracking clues to the Earth's past'.
Your pupils may have noticed that, when a pool dries up, it often leaves a muddy bed, which cracks into regular shapes (polygons) as the wet mud shrinks. Therefore, ancient mudcracks show us that the area where they are found must have been mud that dried out in the past. It must have been surface mud rather than mud laid down under deep water. So the cracks are key clues to the conditions in which the mud was laid down.
Polygonal cracking in natural materials is caused by shrinkage and the shrinkage is caused either by drying out or by contraction on cooling.
Please let us know how you get on with this latest Earthlearningidea. Which method did you use?