East Anglian Treasures: In the latest of his archive essays, Ian Collins considers a material which has helped define and shape our region for thousands of years - flint.

Exploring the fields, lanes and beaches of north Norfolk from 1922, sculpture student Henry Moore began to collect the flints that abounded.

Their flowing forms would inspire a vast gallery of reclining figures.

Moore's sister, Mary, became headmistress of the flint-and-brick school at Wighton, near Wells, where it was hoped that pure air would aid their father, ailing from ruined miner's lungs. But he died within weeks of arrival and was buried in the village churchyard.

On holidays from the Royal College of Art, Moore chipped away at his monumental modernist carvings in the schoolyard, returning for a seminal holiday in Happisburgh with fellow pioneering artists Barbara Hepworth, Ben Nicholson and Ivon Hitchens in 1931.

At the end of a seam stretching from Dorset, several miles wide and metres deep, this workable stone is a silicate probably made from the bones of sea creatures laid down in layers of chalk in the Cretaceous period up to 100 million years ago. It has shaped the architecture of East Anglia, and of Norfolk in particular.

In Neolithic times, around 3000BC, little mines ran the length of this broad band, a network long since closing

and being reclaimed by the landscape.

But if you wander today in very modern pine forests north-west of Thetford you may come upon a clearing with overgrown mounds and hollows like a bomb-cratered extension of the nearby Stanford Battle Area.

Initially named Grim's Graves by the Anglo-Saxons after a pagan god, Grime's Graves is in fact an ancient industrial zone – a network of more than 350 flint mines, one of which is open to the public.

Visitors spared claustrophobia (and aged 10 and over, and wearing 'sensible flat shoes') can descend 10 metres down a ladder into one excavated shaft.

Narrow tunnels then radiate outwards and lead to low chambers where our Stone Age ancestors mined the flint using antler pickaxes and shovels made from cattle shoulderblades.

Over this 90-acre site – with spent mines filled by spoil from newer ones – the stone was amassed and fashioned into spear and arrowheads, chisels, knife and axe-blades.

A hard stone easily split, the bright black flint would have been rubbed to razor sharpness with wet sand - glittering even before it drawn blood. Amazingly, the last gasp of this primeval weapons industry came in the neighbouring town of Brandon in the flint-gathering era of the young Henry Moore.

Last gasp indeed. For in badly-ventilated workshops where the stone was quartered, flaked and knapped into small flints at a rate of up to 300 an hour, fine dust caused many an early death from 'knapper's rot', or silicosis.

Brandon workers had produced 356,000 flints a month for flintlock firearms in the Napoleonic wars – being as vital to the victory at Waterloo as Norfolk's Nelson was to Trafalgar. The industry declined with the advent of percussion weapons until, by the close, the last masters of a Stone Age skill were serving antique gun collectors.

As Brandon demonstrates more than anywhere else, flint was a dazzling construction material, as well as a flamesparking means of destruction.

Mined black, it could also be gathered as rounded, grey pebbles (or larger cobbles) on beaches. Romans used it in Burgh Castle and Anglo-Saxons in round-towered churches; from Norman times it was made into castles,

monasteries, manor houses, cottages, barns and city or garden walls.

From the late 13th century a regional innovation of church towers with an octagonal belfry on a circular ground stage gave East Anglia a unique heritage of amended or additional flinty beacons.

And from the 14th century the decoration of knapped flint became a vernacular art form – with styles and skills ascending to the exquisite patterning of flint and dressed stone known as flushwork.

Starting with St Ethelbert's Gate to Norwich Cathedral Close, in 1316, this was to be expressed in all manner of flourishes: crosses, crowns, ciphers, dates, initials. An entire alphabet would adorn Stratford St Mary church in Suffolk.

Chequerboard patterns topped off church towers, porches and prominent walls. For the fabulous King's Lynn Trinity Guildhall - now part of the Town hall complex - it came to cover an entire façade in a vivid display of medieval wealth and taste.

Grime's Graves, now in the care of English Heritage, lie seven miles north-west of Thetford off the A134. Shaft open March to October. Admission charges.