Hampshire Basin

In former times the Frome and Solent rivers would have drained much of the basin from west to east, fed by tributaries flowing from the north and south.

Today the western part of the basin drains via the rivers Frome and Piddle into Poole Harbour, and via the Stour and Avon directly to the English Channel.

The main basin consists of an asymmetric syncline in the Cretaceous chalk, with a gentle dip southwards from Salisbury Plain terminating abruptly at a near vertical monocline in the south.

The chalk can be visualised as a thin layer draped over rigid blocks of older rocks at depth, which have moved vertically due to the Alpine Orogeny.

These blocks have moved vertically relative to each other during late Palaeozoic and Mesozoic times, resulting in considerable variations in the thickness of the various formations deposited over them before the chalk.

The oldest beds, the Lambeth Group ('Reading Beds') and Thames Group ('London Clay') outcrop in narrow bands towards the perimeter of the basin, from the coast at Studland, around the perimeter of the Dorset heathlands,[5] north and east past Romsey, swinging southeast past Eastleigh and eastwards towards Chichester, Worthing and Shoreham-by-Sea.

It is likely that the London and Hampshire basins were initially part of a single larger area of deposition covering the whole of southeast England during the Palaeocene.

Geological map of southeastern England and parts of France, showing the Hampshire Basin in its regional context.
North-south cross-section of the upper crust of southern England, showing the Paleogene London Basin to the north and Hampshire Basin to the south. Also visible is the inverted nature of the Weald , which was a basin during the Early Cretaceous and thus has a relatively thick Lower Cretaceous sequence. Vertical exaggeration 1:5.
Geological map of the Hampshire Basin