Between 75 and 90 million years ago, this part of the country was under a shallow tropical sea. It was during this time that the distinctive chalk ridge of the South Downs was formed, by the build up of marine deposits. It represents the southern remnant of a huge dome of Chalk stretching across to Surrey and North Kent. Weathering and erosion during the last ice age sculpted the landscape into its valleys, distinct hilltops and ridges.
The chalk acts like a giant sponge, and stores water. This huge underground reservoir (or ‘aquifer’), feeds water from springs into streams and rivers. It also provides most of the residents in and around the Downs with their drinking water.
Soils on the Chalk are thin, well-drained and poor in nutrients. They support slow plant growth without the use of fertilisers. Pockets of more fertile soil can be found on the dip slope and in the valley bottoms.
By contrast, on the greensand areas to the North of the chalk, the soil is damp and rich in clay. It is here that the ancient woodlands and heathlands thrive, cultivated and influenced by centuries of careful management.
The distinctive Greensand ridge shares the same ‘scarp’ and ‘dip’ slope landform as the chalk hills. It was also formed from deposited sands and clays when this part of Britain was under the sea. The chalk layer over it however, has been worn away by weathering and time to leave the complex geology of Lower and Upper Greensand, and Gault Clay.
There are also areas of a chalky sandstone ‘Malmstone’. This is an historic, and locally distinctive, building material used in areas of East Hampshire and West Sussex.