Rock of ages

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.

The British Geological Survey (BGS) was established (as the Ordnance Geological Survey) in 1835. The world’s first national geological survey, its original remit was to survey the country and produce a series of geological maps. Today, the BGS, which still produces the “official” map of the UK’s geology, is best described as a quasi-governmental organisation split between research, commercial projects and “public good”. Quite a lot of its work is now done outside Britain: at the time of writing, projects include studies of groundwater in the Philippines and volcanic activity in the Afar region of Ethiopia.

One week in early October, four members of the BGS set up camp in a self-catering cottage near the town of Tring in the Chiltern Hills, about halfway between London and Oxford. They were on a training exercise as part of a project to produce a new geological map of the chalk of southern England. On the day I arrived, the wooden table in the main room was covered with maps, books, a half-drunk bottle of red wine and a packet of chocolate digestives. Field leader Andrew Farrant, tall and thin, with steel-rimmed glasses, was drinking a cup of tea. He had a sort of leather holster attached to his trousers, from which swung a geological hammer with a surprisingly wicked-looking long, pointed end.

Farrant has been working on the chalk-mapping project on and off since 1996. “I would say that not enough attention is paid by the academic research community to understanding the geology of the UK,” he said. “If I was doing this [mapping project] in east Greenland, then I’d probably get funding for it – east Greenland is sexy. And people tend to think that because we have a geological map of the UK, it’s all been done, but actually you can still improve it.” The geology of the Chilterns, for example, was last mapped in 1912. Since then, the discipline has changed quite a bit. Geologists now know about plate tectonics and radiometric dating. There are laser-based distance measurements for elevation maps and digital terrain models and higher-definition Ordnance Survey maps, allowing hitherto unrecognised features to be recorded. All of this will affect the maps that are produced.

And when it comes to the chalk, these new maps matter in a way they didn’t in 1912, because since then, the population of the south-east has increased by roughly a third. In particular, this jump has put pressure on the region’s transport systems – often created by tunnelling though chalk to form such projects as HS2, the Gravesend tunnel and Crossrail – and the region’s water resources, much of them stored in the chalk aquifer. Imagine stumbling, blindfolded, through an unknown landscape, uneven terrain underfoot, and large, hard objects rearing out of nowhere. Without decent mapping, this is essentially the situation for a tunnelling engineer faced with an immense block of chalk. “Obstructions are a very big issue,” Mike Black, Transport for London’s principal geotechnical engineer, recalled in an interview in New Civil Engineering. “We spend a huge amount of time on desk studies trying to work out where everything is or where it might be.”

An unexpected flint band or hard rock stratum can shatter the shield of a £100m tunnel-boring machine. Hit a fracture or a seam of clay, and your tunnel – filled with men and machines – might flood with water. The Channel tunnel, for instance, doesn’t go in a straight line from A to B, but follows as much as possible a single layer in the chalk that is one of the most suitable for tunnelling. To plan the route, engineers looked at samples of chalk from boreholes and analysed the microfossils in order to find the best way through. “That saved Eurotunnel probably half a billion pounds,” Farrant told me.