Exploring a heritage of chalk and flint and a periglacial story of patterned ground.
Zoom in to the Google Map to find the points of interest.
Grimes Graves is an attractive area of chalky grassland pockmarked with over 430 prehistoric flint mine pits. It is one of the best places in Britain to see the links between geology and archaeology. The earliest pits were dug as vertical shafts in late Neolithic times, about 4,600 years ago, to reach rich seams of flint nodules. There are history displays in the visitor centre, and one of the mines is open to view.
A flint mining reconstruction by Alan Sorrell. [Image by Alan Sorrell, courtesy Julia Sorrell]
Grimes Graves is sited on Chalk bedrock of Turonian age, laid down about 90 million years ago in the Cretaceous period. Several flint horizons occur in the Chalk at this level, and the richest are known as the Brandon Series. With some effort, miners could access the flint here and in other places in the Brecks where the Brandon Series lies close to the surface (in historic times including Lingheath - see Trail site 2). The fact that the prehistoric miners knew where to dig suggests they may have had a practical understanding of the local geology. Each pit could yield up to 60 tons of flint.
Grimes Graves mining. [Image courtesy Beverly Curl]
When you visit the mine at Grimes Graves, take a look at the walls of the shaft as you descend the ladder. The uppermost layers of chalk are crumbly and look somewhat like cottage cheese; this is due to the effects of frost shattering during the Ice Age. The chalk lower was not reached by the frost and is tough and blocky. It contains several flint layers. The best of these was the productive ‘floorstone’, lying over 9 m (30 ft) down at the bottom of the shaft; it was reached by outward-radiating galleries. There are also some thin ‘marl’ bands made of clayey material; these are the result of fallouts of volcanic ash in the Cretaceous ocean.
A Mine Gallery.
Looking north-westwards across the shallow valley from Grimes Graves, you can see an area of periglacial patterned ground (sites A and B on map). N.B These are within the Danger Area so cannot be visited on foot. Plants of sandy and chalky soils are growing side by side as ‘vegetation stripes’. These are the result of Ice Age processes over 10,000 years ago, when repeated freezing and thawing churned the subsoil into a contrasting sandy and chalky pattern, forming polygons on level ground and stripes on sloping ground. In this way, alternating stripes of chalk downland and acid heathland can be seen growing close together – a speciality of the Brecks landscape.
‘Breckland stripes’ near Grimes Graves. Heather is growing on the sands. [Photo: CR Stevenson]
The site is on Open Access land but managed by Historic England. It is marked on Ordnance Survey Explorer map no.229 ‘Thetford Forest in The Brecks’ (c. TL 810 902), and signposted from the A1065 and A134. There is a Visitor Centre with museum displays and restroom facilities, which has seasonal opening hours (April-October). Car parking charges apply when on driving onto the site.