EXTREME BEDROCK CONTROL on BARRIER ISLAND MORPHOLOGY AND EVOLUTION

A high energy barrier island system on the west coast of Scotland comprises carbonate-rich sand beaches and low dunes (machair) backed by tidal basins or remnants of a back-barrier lagoon. The barrier islands are separated from each other by tidal inlets or stream outlets. This system rests on a planar bedrock surface(a strandflat) on which topographical irregularities strongly influence both plan- and profile morphology of the barrier island system. The result is a highly variable island chain with marked differences between adjacent islands. Two distinct barrier island styles exist. In depressions on the bedrock surface, a barrier shoreface is present whereas on elevated sections the beachface rests directly on bedrock. Topographic highs influence the planform morphology by providing fixed points for development of headland-embayment cells, tombolos and salients. The system is evoloving under rising relative sea level by barrier migration in which the sediment volume is preserved but is redistributed onshore and alongshore, creating spatially variable evolutionary patterns in response to underlying topographic variability; the past and future evolution of the system under relative sea level rise is strongly influenced by the bedrock surface morphology. The system will be extinguished under rising sea level when the landward limit of the low-lying ebdrock surface is reached. The bedrock surface exerts the dominat control on barrier architecture in all elements of the island chain (the shoreface, beach, dune, inlet/outlet and back-barrier environments.