 Tunbridge Wells is on the Kentish border with East Sussex, about south of London; the original centre of the settlement lies directly on the Kent/East Sussex border, as recalled by the county boundary flagstone that still lies outside the church of King Charles the Martyr.The town is at the northern edge of the High Weald, a ridge of hard sandstone that runs across southern England from Hampshire along the borders of Surrey, West Sussex, East Sussex and Kent—the town's geology is illustrated by the exposed sandstone outcrops at the Wellington Rocks and High Rocks (a Site of Special Scientific Interest due to its exposed gulls), and the quarries at nearby Langton Green from which sandstone was taken to build houses in Tunbridge Wells.The town is sited at the head of a valley that runs south-east to Groombridge; like the River Teise, which originates in Tunbridge Wells, the stream in the valley is one of the many tributaries of the River Medway, which runs through a much larger valley north of the High Weald.