On an aeromagnetic map of the Chesapeake Bay area, the northeastern part of the bay coincides well with a deep, “flat” magnetic low, and the upper part of the Delmarva Peninsula east of the bay coincides with detailed magnetic highs; the two areas are separated by a steep, straight gradient that matches the eastern shore of the bay. On the basis of magnetic and geologic evidence, we interpret the Chesapeake Bay magnetic low as a buried Baltimore Gneiss dome, bounded on the southeast by a normal or reverse fault marked by the steep, straight gradient; mafic and ultramafic plutonic rocks probably underlie the southeast side of the fault zone. The flatness of the Chesapeake Bay magnetic low, as opposed to the detail of the anomalies on either side, however, suggests that an abnormal thickness of nonmagnetic sedimentary rocks also coincides with the low. This could reflect a buried Triassic basin or, more probably, a thickened section of Coastal Plain sedimentary rocks deposited in the fault-bounded basin. The present course of the upper part of Chesapeake Bay is probably inherited from the pre-Pleistocene course of the Susquehanna River, but the river's course may have been determined by the fault zone. Thus, this zone and similar en echelon fault zones along strike may explain the sudden change in course of major rivers in the central Atlantic Seaboard region.