 These are the two best defined of the belt of large shallow closed depressions which account for the total topography along the southern edge of the Mendip Hills.The Brimble Pit depression has a large old lake sediment floor now pitted with sinkholes, while the Cross Swallet Basin feeds to a single active sinkhole around which are well preserved terraces of both rock and sediment.Both depressions have marginal cols feeding to overflow channels now permanently dry.