Two hundred and fifty two samples from 40 cores and 40 seafloor samples have been analysed using sedimentological and foraminiferid techniques. The results have been used to interpret the sedimentological history of Start Bay during the later phases of the Holocene marine transgression. They indicate the existence of a barrier–lagoon complex, with associated tidal marshes and rivers and an open sea environment c. 8000 years BP. With a continuing rise in sea level and accompanying barrier migration shoreward the Bay and coastline with enclosed freshwater lagoons were established. Geophysical evidence supports this hypothesis.