Over part of south-central England the widespread shallow-water carbonate sedimentation represented by the White Limestone Formation (Great Oolite Group: Middle Jurassic), culminated locally with the formation of a sun-cracked, stromatolitic micrite indicating a supratidal environment. The emergent area can be mapped as a broad strip running at an angle to, and offshore from, the coast-line of the ancient Anglo-Belgian landmass. As a result of the emergence of this carbonate area a polarization of environments occurred. To the north-east a series of clay-rich sediments containing lignite and a brackish and fresh-water fauna were laid down: a lagoonal environment is indicated. To the south-west fully marine shelf carbonates continued to be deposited. We suggest that isolation of the northerly area of reduced salinity from the open sea was due directly to emergence of this carbonate area, which can thus be specifically interpreted as an island barrier or spit.