Over thousands of years, the land sank in the south (a process still continuing) to submerge many valleys creating today's characteristic rias, such as Southampton Water and Poole Harbour, as well as submerging the Solent. The estuary of the Solent River was gradually flooded, and eventually the Isle of Wight became separated from the mainland as the chalk ridge between The Needles on the island and Old Harry Rocks on the mainland was eroded. This is thought to have happened about 7,500 years ago.