Geology

The Solent estuarine system is located at the southern margin of the Hampshire Basin, an elongated asymmetrical downwarp of mainly Tertiary deposition. The Lower Cretaceous Wealden Beds, which consist of freshwater marls and estuarine/shallow marine shales, are the oldest outcropping strata. These beds are overlain by the Lower Greensand and the shallow marine clays and sands of the Albian Gault and Upper Greensand.

The chalk of the Solent comprises three distinct units: the Lower, Middle and Upper Chalk. The Upper Chalk forms the vertical cliffs and stacks at the Needles but onshore crops are only found on the Isle of Wight and over the northern part of the area. This chalk contains extensive fossils and flint nodule bands.

The structure of the geology in the Solent shows the presence of three zones in the crust: the crystalline basement rocks; the Variscan Orogenic Basement, comprising the strongly folded and faulted Precambrian and Palaeozoic Rocks; and the relatively gently deformed Permian, Mesozoic and Teritary Sediments.

The present Solent System is the result of the middle section of the Holocene 'Solent River' system, a lowland riverine system which, during the Pleistocene, integrated all the consequent rivers of the basin and formed one of the major tributries of the English Channel. The evidence suggests that the Isle of Wight was separated from the mainland abround 7,000 to 7,500 years BP.

The River Frome was the source of the River Solent, with four other rivers the Rivers Avon, Hamble, Itchen and Test being tributaries of it. Seismic sounding has shown that, when the sea level was lower, the River Solent incised its bed to a depth of at least 46 metres (151 ft) below current Ordnance Datum.

The Southern Coastal Group have produced an animation of the formation of the Solent.