Mud Cracks
Images (c) 2003 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Everyone sees fresh mudcracks like these. Much rarer are fossil mudcracks like those below.
Mudcracks form where a layer of uniform sediment becomes desiccated. The thickness of the layer affects the size of the desiccation cells—thus the contemporary mudcracks above, in a thick clay layer, have large cells while the sandy mud of the riverbed dried out in thin layers and small cells. See how in some places, the cracks filled with sediment and stand out in a honeycomb pattern after the mud layer fell off.
It takes a very active environment, where sedimentation is fast, to preserve delicate features such as mudcracks. But here they are in a boulder of Pleistocene mudstone near La Grange, California. Sediment from the ancestral Tuolumne River, draining the Sierra Nevada, piled up in wide terraces and swiftly buried the muddy surfaces of the floodplain. (See more pictures of the locality.)
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