The south-west corner of Essex, mainly comprising Thurrock and Basildon, is heavily urbanised, but Basildon has the Langdon Hills and large areas of former plotland to the south of the new town, while Thurrock has the Mardyke Valley, a complex of ancient and new woodlands around Belhus, and the former chalk pits near Grays, now Chafford Gorges Nature Park. Along its southern fringe lie some of the best surviving marshlands of the Thames Estuary, the largest of which is the RSPB's Rainham Marshes reserve on the edge of Greater London.