Soft water-saturated estuarine deposits less than 10,000 years old underlie the southern part of San Francisco bay and the present and former marshlands that border the bay. Known locally as bay mud or as young bay mud, these deposits, and the estuarine environment that produces them, are of major importance in making decision on land use and development in the San Francisco Bay area. Knowledge of the distribution, thickness, and physical properties of young bay mud is critical to the feasibility, design, and maintenance of structures built on it. Fore this reason, numerous attempts have been made in the past to map or describe these characteristics (Mitchell, 1963; Goldman, 1969; McDonald and Nichols, 1974). The accompanying map of bay-mud thickness significantly revises part of an earlier compilation by Kahle and Goldman (1969) and includes new data from approximately 2400 boreholes, most of which have been drilled during the past 15 years. It also incorporates information on historic margins of San Francisco Bay and its tidal marshes (Nichols and Wright, 1971). Although this map was compelled mostly from data gathered during foundation investigations and construction projects, it is mostly from data gathered during foundation investigations and construction projects, it is not a substitute for such studies. Rather, the map provides regional information for land-use planning, seismic zonation, and design of foundation investigations.