I came to our second Zoom talk knowing nothing about flint other than it was used as tools by Stone Age Man, as a child running on Hastings beach once the barbed wire had been cleared away and that we had flint in the South Porch of Chelmsford Cathedral, (Jackie) so I knew that at least there would be something for me to learn tonight and I certainly wasn’t mistaken! Ros gave us a very engaging lecture.
The familiar quartz material we know simply as “flint” came about as a result of particular circumstances which began 100 to 65 million years ago during the Upper Cretaceous period of the Earth’s history. Most of the low-lying parts of Europe were covered by the “Chalk Sea”. The climate was warm and there was no polar ice. The sea water was teeming with a collage of plants and animals, such as shellfish, scallops and sea urchins. Sponges were particularly numerous. Chalk (Calcium Carbonate) makes the sea water alkaline and causes the sponges to dissolve. The sponges and other marine plankton had absorbed silica (silicon dioxide, SiO2) which were then dissolved in the alkaline waters and drawn into the chalk sea bed. During this process, further chemical reactions occurred as a result of hydrogen sulphide bubbling up from deeper in the sea bed which combined with the oxygen to form acid. In this environment silica came out of solution and subsequently hardened into flint. Over time, the outside acquired a whitish cortex layer, 5 mm or more in thickness.
The processes described by Ros were illustrated by pictures and photos. We saw layers of flint in the white chalk cliffs along the south coast including Beachy Head, together with images from electron microscope studies showing the variety of internal structures. The shape of flint can vary between the neat, rounded pebbles found on the sea shore of my home town Hastings formed by sea erosion over millions of years - to the irregularly-shaped pieces that can be found in fields and gardens. Sometimes the flint piece contains strange lines that can be mistaken for fossils. These “Liesegang rings”, named after the German chemist R E Liesegang (1869-1947), are caused by iron and other minerals diffusing through the flint. Sometimes the flint contains quartz crystals or layers of the mineral chalcedony. Fossils are quite common, particularly bivalves or sea urchins but rarer finds, such as sharks’ teeth, can occur. Fossils of marine creatures such as sea urchins that have been involved in the formation of flint are sometimes “Caught in the Act”!
Flint as building material: Because we have no sandstone or other building materials in our area our ancestors used what they had and East Anglia is rich in buildings which include flint in their structure. However, the uneven shape makes it unsuitable for corners. Hence St Mary’s church, Broomfield, has a rounded tower; St Andrew’s, Willingale, has brick corners. Gradually they discovered that they could make patterns and “knapped” flint could be used decoratively, as with Dedham church and our Cathedral. Bridewell at Norwich cleverly used “nippled” flint.
Flint in aggregate: The gravel pits of Essex, being part of the ancient Thames river bed, are rich in flint and aggregate is extracted on a large scale. Examples include the Bulls Lodge quarry at Boreham and Highwood quarry at Dunmow. It is now Chelmsford’s major industry.