The presence of salt in isostatically uplifted coastal marshes well removed from tidal sources suggests that the salt is relict in origin. This was confirmed by the measured downward increase in salinity towards the deeper and older Tyrrell Sea sediments and by the presence of water isotopically heavier than the present-day meteoric or tidal waters. The low permeability of marsh sediments and the vertical distribution of salt indicate that diffusion is transmitting salt toward the surface, where it is subsequently removed by surface flow. Salt concentration decreases with distance inland, where the salt-loss processes have proceeded for a longer time. The chloride concentration of the deep pore water suggests that the total salinity of the postglacial Tyrrell Sea was 21–25 g L−1.