How mud cracks form
The Triassic rocks of East Devon formed in a desert, but at times, this landscape also had branching rivers and lakes. When the water evaporated away, the recently-deposited mud was baked in the sun and mud cracks formed. The next flood brought more sediment which filled the cracks. Later the sediment was cemented, in this case with gypsum, making the surface much harder than the sediment in which the cracks originally formed. As a result, the mud cracks themselves are preserved as ridges.
We are actually looking at this specimen from the underside. Imagine pouring concrete into crazy paving and then lifting it up – this is what you would see.
The lower photo shows underwater mud cracks in Corcovado National Park, Costa Rica. The mud cracks are hard enough to be flooded without turning back into soft mud.
Find out more on the rocks and pseudofossils page.