Exposed Pleistocene marine limestones of the Florida Keys are often coated by laminated 1-to-6-cm-thick calcitic crusts. Heretofore these crusts have locally been identified as indurated marine algal stromatolites similar to the soft, marine, living algal stromatolitic mats of the Florida Keys, which border and occasionally even coat the encrusted bedrock; such juxtaposition is now considered merely coincidental.

C14 dating of five different crust samples reveals a time of formation (within the last 4395 ± 90 years) during which the land surface was above sea level. Field relationships and laboratory evidence also indicate subaerial origin. Three general types of crusts are: (1) microcrystalline rind, (2) dense laminated, and (3) porous laminated.

Similar laminated crusts found in subsurface cores suggest emergence followed by submergence of the Key Largo reef in late Pleistocene time.

Proper identification of such subaerially formed laminated crusts, to distinguish them from similar-appearing crusts formed in marine environments, is necessary for correct interpretation of paleoenvironments and former sea level fluctuations. Thin crusts may be the only evidence for recognizing some ancient unconformities.

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