Barrier reefs are characterized by reefs that are separated from the shoreline by deep lagoons. Barrier reefs tend to form from the sea landward, and are perhaps the rarest form of reefs. Barrier reefs are typically the result of glacial cycles that cause the sea level to substantially drop and then rise again. Barrier reefs can sometimes form from previous fringing reefs, but they do not always follow that pattern. Barrier reefs could form on any topographic rises, like hills in now submerged floodplains.