A rising sea level has drowned the Avon River estuary which drains the Windsor Lowlands (Unit 511). The river cuts through resistant Horton sandstones between high banks. Wherever the less-resistant Windsor rocks are encountered, wide valleys have been excavated. The extensive karst topography that developed on the gypsum (part of the Windsor Group) is prominent. Uplands of the Atlantic Interior (Region 400) define the Avon River's drainage basin.