One of the most fascinating things about the desert is the prevalence of mud cracks. I remember hiking into the Pariah Canyon in Northern Arizona/Southern Utah and along the dry stream bed there were huge, dinner plate sized, mud cracks. They were so dry and hard that they clinked like glass and cracked loudly when we walked on them. When I was young, I used to take apart the mud cracks like puzzles and try to put them back together. Anytime a layer of clay silt is deposited by a rainstorm in a depression in the ground, when the clay dries it shrinks and forms the puzzle pattern mud cracks.
Like everything else in the world, mud cracks are studied by geologists. If the mud cracks are subsequently covered by another layer of silt and preserved and then through geological changes hardened into rock, they give us a record of the long ago drying event. They use mud cracks to determine the orientation of the original sedimentary beds that formed the cracks. The edges of the mud cracks curl upward and point out which direction was up in the original bed. Also, the cracks get thinner as they go down into the layers and point downward.
We had a recent thunderstorm in Mesa and the ditches on the sides of the canals filled with water. Of course, the water evaporates in a day (or even a few minutes or hours) but it leaves behind a thin sheet of sediment, dust from the canal road, that dries into the fabulous patterns of mud cracks. If you would like to see a number of different photos, go to the Earth Science World Image Bank.
Mud cracks are like many things I find while walking Arizona, they are fascinating, interesting and mostly ignored by everyone.
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