Wiltshire’s rolling downs never actually rise above 305m (1000ft) but the feeling of space, aided by the immensity of the sky, is exhilarating. Where the sheep still graze, a rich lime-loving flora thrives including such Wiltshire specialities as the Burnt-tip Orchid, Tuberous Thistle and Early Gentian. Nationally, Wiltshire now accounts for over 50% of all the remaining chalk grassland in the country. At its major extent Salisbury Plain forms the largest area of unimproved chalk grassland west of the Ural Mountains in Eastern Europe, and straddles the centre of the county. The Marlborough Downs lie to the north and a more diverse area of chalk downland occurs in the south of the county. This includes Porton Down and extends westwards through Cranborne Chase into Dorset.
Geologically, chalk forms the upper part of the Cretaceous series of rocks with alternations of greensand and clays in the lower sections. Bands of flint, a form of quartz, are common in many areas.
In contrast to the chalk grasslands, the wooded clay vales, with their neutral meadows, lie mainly to the north and the west. Geologically, the clay vale is formed from the Oxford and Kimmeridge Clays which are separated by the sandy limestone of the Corallian beds on some of the higher ground.
Overlying tertiary gravels, the neutral hay-meadows represent one of Britain’s rarest habitats and provide a last refuge for that rare and beautiful plant, the Snake’s Head Fritillary. The heavy clay also favours the oak woodlands of Savernake Forest and the now fragmented Braydon Forest, once a Royal Hunting Forest. Where these gravels have been extracted commercially, both in Gloucestershire and Wiltshire, a veritable ‘lake district’ of wildlife-rich gravel pits have been created, The Cotswold Water Park. Wiltshire also hosts part of the limestone rich Cotswolds to the north west of the county. This Oolitic limestone forms the middle section of the Jurassic series of rocks that outcrop from Dorset to Yorkshire and divides England almost in a straight line into two contrasting halves.