A well preserved section of the south wall at Burgh Castle, showing Roman brick as well as flint
Local materials such as Cotswold stone or Welsh slate help man-made structures to blend in with the landscape. The shore hereabouts is made of sand, shingle, pebbles and cobbles; between Cley and Kelling the shingle ridge is the chief defence against the sea.
Flint is a variety of quartz, specifically that sort of quartz called chert, which is made principally of silica, silicon dioxide. It develops as nodules inside strata of chalk. In geology, a ‘nodule’ is a small, somewhat rounded lump of a mineral, or an aggregate of minerals, contrasting with the host stratum. Flint consists mostly of silica, probably deriving from the spicules of sponges. Chalk is a form of limestone derived from the shells of foraminifera and the calcareous remains of coccolithophores: both of these are single-celled animals existing, like the sponges, in huge numbers in the sea.
One theory has it that irregular holes and chambers burrowed in the chalk by crustaceans and molluscs fill with a gelatinous material that then becomes silicified with sponge spicules. As further layers of chalk are laid down, these are subjected to tremendous pressure, causing chemical as well as physical change in a process called diagenesis: the transformation from sediment to rock.
Geological upheaval can bring a chalk stratum to the surface, allowing erosion by water – whether by rainfall or the sea – and subsequent exposure of the flints.
The Stone Age is so called mainly because of the use of flint to make tools, weapons and other implements. It lasted for some 3.4 million years until about 4000-2000 BC, when metalworking was invented.
Two properties of flint make it specially suitable for the production of edged tools, such as blades: it consists mostly of siliceous materials which, like glass, can be made extremely sharp, and it is comparatively dense. The structure is microcrystalline, meaning that it comprises crystals so small that they may be seen only with a microscope. In the hands of a skilled knapper, blades can be produced that are easily capable of felling trees or skinning an animal, and of course nearly every municipal museum has its collection of flint arrowheads, spear-heads, axes, scrapers, etc. Many of these artefacts are the product of an advanced craftsmanship which peaked in the late Neolithic, about 5,000 years ago.
Flint varies in quality and suitability for knapping. The best flints were not always found on the surface: sometimes mines were dug in the chalk, a notable example being found at Spiennes in Belgium. In Britain perhaps the best known flint mine is at Grime’s Graves in Breckland. At St Roche’s Hill, near Chichester, collapsed galleries have been found that contained human skeletons. This has led to speculation that the dead miners were slaves, their bodies not considered worth the labour of recovering, especially if the flint seams there had been all but worked out.
The flint cobbles used to face our walls come in every shade of grey and brown, the colours being determined by the composition of the gelatinous material of which the original nodules were formed, with softer and differently coloured portions being worn down sooner. Wave action also repeatedly throws each cobble against its fellows, marking the surface or chipping bits off.
The cobbles are generally rounded but no two are alike, deriving their shape from the cavities excavated aeons ago by long-dead creatures. No doubt most of those we see in walls were at one time much larger and have been knocked about, ground down and polished by the sea, the detritus surviving as shingle and sand. It is hard to conceive of the immense period during which a single cobble becomes smooth, just as it is hard to conceive of the natural forces involved. The cobbles in our walls have in large part been crafted by the Moon, whose gravitational pull drives the world’s tides.
They have been further crafted by the men whose judgement decides which cobbles to place where, and if you look closely at a well made flint wall you can see both taste and discretion at work. And if you look more closely still, at any single cobble, whether in a wall or on the beach, you will find that it is unique and, in its colouring and lines, as subtle and beautiful as anything else to be found in the natural world.
