This month’s blog from our Community Geologist, Dr Ian Kille, compares flint and chalk as building materials
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On a recent trip to Flamborough Head I found myself both at home and somewhat disconcerted. Flamborough Head is made of chalk, very white and with all the fossils you would expect to find from the Cretaceous Period. Chalk is what I was brought up on. The water that came out of our taps came from an artesian well and had been filtered through chalk. Below the London Clay that made our garden impossibly claggy to dig, there lay a massive saucer of chalk cupping the clays, sands and river terraces which underpinned the London Basin. Less than 10 miles away southwards the chalk emerges as the North Downs, forming the massive pediment of a worn away arch – the other half of which is the South Downs. The Downs, North and South, were my escape – Box Hill and Beachy Head, Hope Gap and the Silent Pool all beautiful places to be as well as offering up a rich array of fossils. Inoceramus liabatus, Spondylus spinosus, Echinocorys scutata, Micraster cortestudinarium and Sporadoscinia aclyonoides were all happily collected and labelled as a teenage buff.
Why then, the Flamborough fluster? Part of it was being in the north when I’m used to going south of London to see chalk. But that shouldn’t really be a surprise as it is well known to me that the chalk outcrops that forms the anticline of the Weald (framed by the North and South Downs) is mirrored by a syncline which emerges the other side of London. After all I lived in St Albans for many years and regularly visit a musical retreat perched above the Hughenden Valley in the Chilterns and know of their chalkiness. These chalklands are part of a great arc stretching from Salisbury through the Chilterns curving north to just east of the Wash and reaching the sea between Kelling and Hunstanton in Norfolk. The chalklands then continue north of the Wash from Skegness right the way up the coast to Flamborough where they once again rise to make cliff scenery nearly as flamboyant as that around Beachy Head.
There were two other things at Flamborough which shifted my pre-conceived notions of chalk and were probably the cause of my disconcert.
Not long after we arrived in our lodging in Bempton, my partner Rachael asked in surprise, if the house opposite was built of chalk, commenting that it didn’t seem to be a plan to build a house out of chalk as it is so soft. I muttered about Chalk Rock and Melbourn Rock, bands of harder chalk within the southern, generally soft chalk, which are used as building materials. I was however surprised to see how many of the older buildings were made of chalk – it seems that northern chalk is, in general, harder than southern chalk.
I was also surprised that the only other common traditional building material was brick. I remember from the chalklands in each of Surrey and Sussex, Hertfordshire and Hampshire, Suffolk and Norfolk, that many of the older buildings frequently featured flints.
Flints make for beautiful looking houses. They are, however, a challenge for the builders as flint is so hard, fiercely sharp when broken and comes in irregular lumps. The history of flint as a building material goes back at least to Roman times. Whilst it is not an easy building material to use, it is extremely durable and freely available (at least in the southern chalk). Having observed and considered the sandstones used to build Hadrian’s Wall, it was fascinating to return to St Albans in February of last year, moments before the pandemic kicked in. Walking from Waitrose (an essential in St Albans) south into Verulam Park towards the Abbey, the path is bordered on your left by a Roman Wall. It is this which featured as Mystery Rock number 20 for the Hadrian’s Wall Community Archaeology Project. It is constructed from a mixture of brick and flint.
The Romans recognized that to make stable walls out of flint it is more effective to mix it with layers of rectilinear material. As with Hadrian’s Wall the Romans once again show their ability to choose materials and build with them to produce remarkably durable structures. Continuing down the hill to the River Ver and then up to the Abbey also allowed for an exploration of the complex built history of the Abbey. This provided another lovely, but very different, example of stone reuse. Significant portions of the transept and the northern wall of the nave are made of reused Roman material. Other walls in both the transept and the southern nave feature newer knapped flint used in much larger faces supported by stone quoins. This reuse matches the way that Roman stone is reused in medieval churches in the Tyne valley except that the materials being reused are very different.
Back in Flamborough we headed out to the coast and found that the absence of flint in the buildings is reflected in an absence of flint in the cliffs. We started our exploration on the beach at South Landing, just south of Flamborough and headed towards Danes Dyke and Bridlington to the west. Here it also dawned on me there was another major difference. The wave-washed cobbles between the chalk boulders by the cliffs consisted of a curious mix of Carboniferous, Jurassic and even older rocks, none of which occur locally. Looking up at the cliffs the reason is clear, with a 10m plus band of boulder clay topping the cliffs. These wave-washed cobbles had hitched a lift on a glacier and were dumped within the boulder clay as the ice departed (some 12 thousand years ago), and now the ice-transported contents are eroding into the North Sea. This ice-sheet didn’t reach Sussex, so the cliff tops at Beachy Head just have a thin layer of chalky soil, and the beach cobbles are exclusively flint (with the odd bit of brick where a house has fallen in). Further exploration of the coast at Flamborough Head and Thornwick Bay, revealed that there were some flints to be found, but different in character from those further south. These flints were grey and not clearly distinguishable from the chalk. At Birling Gap in Sussex the layers of flint nodules band the cliff in dark black, contrasting with the brilliant white of the cliffs. Individual nodules when broken are a beautiful shiny translucent black and have a rind of a porous mixture of flint and chalk.
Back home, reflecting on the trip to Flamborough, I was reminded once again of my university tutor, Professor Harold Reading, and one of his sayings; a geologist is only as good as the number of rocks they have seen. I’m certainly seeing chalk differently thanks to our visit to Flamborough. Maybe it’s time now to plan trips to the Cap Blanc Nez and then to the Jasmund National Park in Germany and Møns Klint in Denmark, chalklands all. There are also chalk deposits in north America, Australia and Egypt. Apparently, the Champagne region of France is underlain by chalk too – chalk and cheese (and a little wine), now there’s a thought!