Mystery Topography

The County retention pond adjoining our back yard may be flooded up to it's storm drains or completely dry, depending on rainfall activity. When it is dry, I've noticed the depressions (bottom photo) in the dry earth in an area often covered with water.
I've come up with several possible explanations for their appearance - none to my satisfaction.
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Brownbird - perhaps I'll climb over the fence and check them out - cover me! =:o)   - January 10th