Relatively non-dolomitic limestone nodules which float in lithified dolomitic sediment are forming on a supratidal mud flat in the lower Florida Keys. Storm tides periodically deposit layers of lime mud above normal high tide level which dry and crack to form typical mud-crack polygons. These polygons erode into flattened nodules that subsequently become buried in relatively more porous and permeable sediment. Magnesium-enriched brines concentrated through evaporation are more readily transmitted through the more permeable sediment than are the less permeable nodules. Selective dolomitization of the morepermeable sediment produces features bearing a strong similarity to sedimentary structures commonly found in ancient dolomitic rocks.