Probably, different hardness on the ground. I don´t know. It seems that some little birds visit those structures. are there any footprints out of the structures?

all the best

Quique

reminds me of the shapes left by feeding flamingos, when they spin around as they sieve sediments. See Fig. 1 in Rodríguez-Pérez, H. y Green, A.J. 2012. Strong seasonal effects of waterbirds on a benthic community in a shallow lake. Freshwater Science 31: 1273–1288.

It might be flamingos, they feed in the bay. The structures are between 30 - 50 cm across.

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