How do cracks appear in mud flats during the period of drought?

Cracks in mud flats
When a mud flat dries up due to incessant evaporation of water the mud contracts, setting up a uniform tension throughout the area and as a crack opens under tension, the material to either side is compressed perpendicular to the length of the crack. Gradually, squeezing builds up. The easiest way to relieve this pressure is for a new crack to open at right angles to the first one. Such rifts occur simultaneously all over the perched flat and the result is a complex pattern of polygons like the one shown in the accompanying photo. In a few cases, some of these cracks may be half a meter wide and double that on depth.
If the layer of mud is thin enough, the more rapid shrinking of the upper surface will cause the edges of the polygons to curve upward and mud curls will appear. If on a slope, these tubes of dried mud will simply roll away.

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