Between the bands of limestone and chalk are wide Tertiary clay vales with large flood plains, which sustain many small settlements and dairy farms. The most notable of the valleys are those of the Stour and Frome. The Stour flows into the north of the county at Gillingham, into a wide basin, the Blackmore Vale, where it collects from many small tributaries. The river then flows south, through a gap in the chalk and across the healthand of south east Dorset. The Frome collects water from the aquifer of the Dorset Downs, forming in the hills in west Dorset, and flowing through Dorchester into a wide estuary, Poole Harbour, where it reaches the weak sands in the south east of the county.
South-east Dorset, around Poole and Bournemouth, and the New Forest, lie on very unresistant Tertiary beds: Eocene clays (mainly London Clay and Gault Clay), sands and gravels. These thin soils support a heathland habitat which supports all seven native British reptile species. Dorset Heath, a species of the genus Erica, grows in this area, and in 2002 was chosen as a symbol of the county by Plantlife's "county flower" competition.
The River Frome estuary runs through this weak rock, and its many tributaries have carved out a very wide estuary. At the mouth of the estuary sand spits have been deposited turning the estuary into Poole Harbour, the second largest natural harbour in the world (after Sydney Harbour, though since artificial expansion Sydney's claim is disputed). The harbour is very shallow in places and contains a number of islands, notably Brownsea Island, famous for its Red Squirrel sanctuary and as the birthplace of the Scouting movement.
The harbour, and the chalk and limestone hills of the Purbecks to the south, lie atop Britain's largest onshore oil field. The field, operated by BP from Wytch Farm, produces a high-quality oil and boasts the world's oldest continuously pumping well (Kimmeridge, since the early 1960s) and longest horizontal drill (5 miles, ending underneath Bournemouth pier). The clay pottery produced by Poole pottery from the local clays is famous for its quality.
Geology of England
Geology of the United Kingdom
List of Dorset beaches
List of Wikipedia images of Dorset