Swathes of England’s landscape were shaped by the immense block of chalk that has lain beneath it for 100 million years. For a long time, even geologists paid it little heed – but now its secrets and symbolism are being revealed

On the British Geological Survey’s map, chalk is represented by a swathe of pale, limey green that begins on the east coast of Yorkshire and curves in a sinuous green sweep down the east coast, breaking off where the Wash nibbles inland. In the south, the chalk centres on Salisbury Plain, radiating out in four great ridges: heading west, the Dorset Downs; heading east, the North Downs, the South Downs and the Chilterns.

Stand on Oxford Street in the middle of the West End of London and beneath you, beneath the concrete and the London clay and the sands and gravels, is an immense block of white chalk lying there in the darkness like some vast subterranean iceberg, in places 200 metres thick. The Chalk Escarpment, as this block is known, is the single largest geological feature in Britain. Where I grew up, in a suburb of Croydon at the edge of south London, this chalk rises up from underneath the clays and gravels to form the ridge of hills called the North Downs. These add drama to quiet streets of bungalows and interwar semis: every so often a gap between the houses shows land falling away, sky opening up, the towers and lights of the city visible far in the distance.