Mud cracks, or desiccation cracks, form in bodies of shallow water such as lakes, especially playa lakes in deserts, in flood plains, and in many ephemeral puddles. They are the result of shrinkage of mud, which is a fine-grained sediment consisting of a mixture of silt and clay. Clays are very fine-grained platy minerals that attract and hold water molecules. The addition and loss of water causes the clay to expand and shrink. Mud cracks are often preserved in sedimentary rocks and indicate the presence of shallow bodies of water that dried up. These examples from Montana are about one billion year old.
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