AbstractContinuous high‐frequency marine seismic profiling was used to define the extent of geologic units in the offshore areas of J‐Field, Aberdeen Proving Ground, Maryland, during March and April of 1988. J‐Field is an area that was used by the U.S. Army for disposal of chemical warfare agents, munitions, and organic solvents from the 1950s through the 1970s. A major concern at this site is the subsurface migration of hazardous wastes into offshore areas and eventually into the Chesapeake Bay. A 150‐foot (45.7 meter) deep paleochannel containing Pleistocene age fluvial and estuarine sediments was identified from boreholes constructed onshore. The paleochannel sediments consist of three lithic units. From bottom to top these units consist of gravel and sand, massive silty and sandy clay, and interbedded sand and clay. The seismic profiles were used to identify the extent of these units and map them in offshore areas. An accurate knowledge of the distribution of the geologic units in onshore and offshore areas is important to the investigation because the coarse‐grained paleochannel sediments may provide a preferential flow path for contaminated ground water and the fine‐grained sediments may impede the movement of contaminants into deep