A vertically stable, step-like thermohaline structure is observed throughout a continuous, 400 m conductivity-temperature-depth (CTD) profile taken near the Erebus Glacier Tongue. McMurdo Sound, Antarctica. The pattern is best developed between the sea surface and 250 m depth, the interval corresponding to that of the irregular underwater profile of the Glacier Tongue. The steps average 17 m in thickness and typically display discontinuities of 0.1¿C in temperature, 0.04% in salinity and 3.5¿ 10-4 g cm-3 in density. The observations are compared with theory and laboratory experiments of cell developement and lateral flow near ice melting into vertically stratified salt water. At this location, subsurface seawater is inferred to remain above the in situ freezing point year-round, and contains sufficient heat to account for much of the Glacier Tongue thinning by basal melting. An adequate volume of meltwater would result to produce the measured salinity steps. We discuss related observations and some implications of this process for ocean circulation and biological productivity in the Antarctic.