Sediment banks have formed at several tidal passes through the arc of shelf islands which separate Florida Bay from the reef-tract lagoon. The Matecumbe Keys tidal bank has formed by vertical and lateral accretion during the Holocene rise in sea level, within the last 6,000 years. Tidal banks comprise coarser coralgal sand and mud on the seaward side and muddier molluscan-foraminiferal deposits on the shelfward side. This trend in composition corresponds to the gradient in environmental factors from more open marine, higher energy conditions seaward of the island barrier to more restricted, sheltered conditions on the shelfward side. The composition and variability of tidal bank sediments are intermediate to those of sediment banks in the reef-tract lagoon and those in the restricted bay. Comparison of tidal banks with other types of sediment banks in the South Florida area provides criteria for recognizing analogous banks in ancient carbonate bank sequences.