Uttlesford’s pebbles
It can be hard to see geology in Essex - we don’t have many cliffs or conspicuous landscape features - but it’s easy to find pebbles in gardens and fields, and see them in the walls of old buildings. Each pebble tells a story, and literally millions of years of history can be revealed by the shape, texture and composition of a pebble.
Uttlesford has a greater diversity of pebble types than anywhere else in Essex because most of our countryside is blanketed by a great thickness of boulder clay that was left behind by the melting of an ice sheet during the Ice Age (boulder clay is a good name as it’s usually a clay and it usually contains boulders, but it doesn’t always, and because geologists like to be more precise, they now call it ‘till’). This clay once covered the whole of the district but modern rivers have removed it in the valleys so it can now only be found on the higher ground. In the gardens and on the footpaths here you will often encounter an unusual pebble – perhaps a piece of limestone full of fossils from Lincolnshire, or a lump of gritstone from Derbyshire, or a dark-coloured dolerite from Northumberland, or maybe a colourful granite from Scotland? But the most common pebbles by far will be of chalk and flint – the bedrock of much of this area.
The Chalk underlies the whole of Essex but it only appears at the surface in Uttlesford and in Thurrock. The gentle hills of north west Essex and Cambridgeshire are effectively a continuation of the Chilterns, and the Chalk in Thurrock is really part of the North Downs of Kent. Chalk pebbles therefore occur in vast numbers in Uttlesford but look closely and you will see that they are usually quite hard, unlike our softer, local chalk. These chalk pebbles came from Cambridgeshire and countless trillions of them were carried south to be deposited in the boulder clay, or by colossal torrents of meltwater issuing from the edge of the ice sheet.
The story of flint pebbles is far more complex. The lifespan of a pebble largely depends on the hardness of the original rock and whether it splits easily. Flint is a type of quartz and therefore is extremely tough. All flint originated as flint nodules in the chalk, and was formed at a shallow depth in the white, limy mud on the floor of a tropical sea some 80 million years ago. This mud eventually hardened into chalk and once the sea had retreated a vast amount of chalk strata across southern England was uplifted and stripped off by erosion, releasing a vast number of flint nodules. These nodules were broken up, and when seas returned, around 55 million years ago, they were subjected to relentless battering. Angular fragments of flint are gradually transformed into the familiar rounded pebble shape and, given enough time, eventually reduced to the size of a sand grain. These ancient beaches have long since disappeared, the flint pebbles have been redeposited as layers of gravel, uplifted and exhumed again and been transported once more - a cycle that may have been repeated several times, and most recently moved again by the ice sheet during the Ice Age. If their roundness wasn’t enough of a clue, their beach origin is also revealed by the presence of small, crescent-shaped scars called chatter marks, which are percussion fractures produced when pebbles are violently thrown together. Our well-rounded flint pebbles have therefore had a long history, but other pebbles we encounter in Uttlesford can tell a very different story.
The Ice Age started around two million years ago when the world’s climate, which had been slowly cooling for tens of millions of years, flipped into an unstable state. We currently live in a warm spell (an interglacial) in this complex cycle of climate change. The Ice Age has so far witnessed a dozen or more of these interglacials, of 10,000 - 20,000 years duration, interspersed with cold periods, or glaciations, each lasting about 80,000 years. During the warmest interglacials there were hippos wallowing in Essex rivers, and during the coldest glaciations the ice sheets spread out from Scandinavia and Scotland to cover much of the UK. It was during the coldest of these periods, some 450,000 years ago and known as the Anglian Glaciation, that the ice sheet, hundreds of metres thick, covered the whole of Uttlesford and reached as far south as north London. This ice sheet diverted the Thames from its much more northerly course across East Anglia to roughly the course it follows today. It also deposited the boulder clay which, in places, is nearly 50 metres thick.
Boulder clay therefore contains numerous ‘foreign’ or ‘exotic’ pebbles which come from further north and identifying the type of rock, and where it is likely to have come from, enables us to build up a picture of the movement of ice sheets across the country. One such local pebble in Saffron Walden Museum, of a rock known as a rhomb-porphyry, proves that the Scandinavian Ice Sheet joined with the British Ice Sheet over what is now the North Sea and pushed a lobe of Scandinavian ice down what is now our east coast to reach as far as Essex. We know this because rhomb-porphyry is a very rare rock (a type of volcanic lava) and in the northern hemisphere is only found near Oslo in Norway. Other pebbles found are distinctive volcanic rocks from North Wales which must have found their way here via the former course of the Thames and subsequently picked up again by the ice. Others are from France (Brittany) and Cornwall and were brought here via several ancient river systems until again being finally reworked by the ice.
Boulder clay can occasional contain very large boulders, known as ‘glacial erratics’. One of the largest is the Leper Stone on the Cambridge Road north of Newport, which is a tough sandstone known as a sarsen stone. They are often found on farmland and moved by the farmer to avoid damage to the plough. There are several examples outside Saffron Walden Museum. The largest collection, however, is in Saffron Walden at the junction of Margaret Way and Gibson Gardens, where there is a remarkable grass mound containing at least 25 erratic boulders of at least 9 different rock types. They were gathered in the nineteenth century by George Stacey Gibson and placed at this spot which was originally the site of a summer house in his extensive gardens.
For further information see:
https://saffronwaldenhistoricalsociety.files.wordpress.com/2014/01/the-gibson-boulders.pdf
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