Poole Harbour - Geology

Geology

The harbour lies on a band of weak gravel and clay which is easily eroded by the rivers and sea. This band is bordered by two bands of chalk, the Purbeck Hills and Isle of Wight to the south, and the Dorset Downs and South Downs to the north. The clay extends west up the Frome valley to Dorchester, and would originally have extended east beyond Portsmouth in Hampshire. Before the last ice age the River Frome continued to flow east through what is now the Solent, joining the Stour, Beaulieu, Test, Itchen and Hamble, before flowing into the English Channel to the east of the present day Isle of Wight. A relatively resistant chalk ridge ran continuously from the Purbeck Hills to the Isle of Wight, which the rivers could not break through. When the glaciers of the north of the island of Great Britain melted, the south of England sank slightly, flooding the Solent valley and Southampton Water to form their characteristic rias (flooded estuaries). About 7,000 years ago, increased erosion from the sea and the increased flow caused by the change in climate broke through the chalk hills, cutting the Isle of Wight off from the Isle of Purbeck and flooding what is now the Solent and Christchurch Bay, leaving Poole Harbour as the estuary of the Frome.