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Mud cracks
I took this photo in the wash that runs down the center of our canyon. Not all mud cracks when it dries, and some muds crack more than others. Why?

Answer:

Dirt and mud are composed of different things in different areas. The cracks that form when it dries are usually due to the amount of clay. Clay minerals expand when they get wet, and shrink as they dry. The shrinking clay can also cause layers of clay to curl. The upper surface dries first, shrinking and pulling the layer into a curl. The more clay there is in the mud, the more it will crack when it dries. Mud cracks are important in dry areas, because they allow rain to enter the soil faster, instead of just running off.
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