Geology Poole_Harbour

a quiet corner of harbour, looking south brownsea island

the harbour lies on band of weak gravel , clay eroded rivers , sea. band bordered 2 bands of chalk, purbeck hills , isle of wight south, , dorset downs , south downs north. clay extends west frome valley dorchester, , have extended east beyond portsmouth in hampshire. before last ice age river frome continued flow east through solent, joining stour, beaulieu, test, itchen , hamble, before flowing english channel east of present day isle of wight. relatively resistant chalk ridge ran continuously purbeck hills isle of wight, rivers not break through. when glaciers of north of island of great britain melted, south of england sank slightly, flooding solent valley , southampton water form characteristic rias (flooded estuaries). 7,000 years ago, increased erosion sea , increased flow caused change in climate broke through chalk hills, cutting isle of wight off isle of purbeck , flooding solent , christchurch bay, leaving poole harbour estuary of frome.