Fig. 1 Hypothetical cross-section of a salt marsh showing its development over time. As sea level rises, sediments, roots and rhizomes build up at the same rate to form peat. As the marsh rises, it expands out over mud flats toward the water, and also landward. The illustration shows a marsh surface, with its vegetation, at 2,000 years before the present (yrs bp). Past levels of mean high water (mhw) are also shown for 3,000 and 2,000 years ago and for the present.