Bexley, lying as it does on the outer fringe of London, has many relatively large areas of open space. The ridge of higher ground in South London crosses the Borough from its high point of Shooters Hill, on the boundary with the Royal Borough of Greenwich, to end above the River Thames at Belvedere, where the land drops down to the old port of Erith. This high land, whose geology is the sand and pebbles of the Blackheath beds, and which results in heathland, provided the line on the old Roman road (Watling Street) ran between Crayford and Welling. The land falls away to the north of the high ground, across the Erith Marshes to the River Thames, which here makes a loop to the north at Crossness. There is further ridge of less higher ground from the west terminating at Sidcup.