Logs of driving tests and borings in the deposits under the tidal estuaries in the Chesapeake Bay region have been obtained at 14 bridge sites over the Potomac, Susquehanna, Rappahannock, York, and other rivers and over Chesapeake Bay itself. The estuaries of this region are submerged river valleys cut into the Cretaceous and Tertiary sediments of the Coastal Plain, but they now contain a fill consisting of sand and gravel overlain by sandy silt of alluvial and estuarine origin. The youngest deposits are semiliquid silts (river mud) probably deposited by marine currents. Studies of ground-water supplies in terraces less than 15 feet above sea level that border some of the rivers indicate that there may be two fills of different ages separated by an unconformity and that each represents a different period of low sea level during Pleistocene time. The gradients of some river profiles steepen near the master stream, an inheritance from the period of low sea level in Pleistocene time when base level was lowered. Swampy and braided stream reaches result from the adjustment of the channel cross section to the inherited steep slope.