©BSNHS 2014

At first sight, the area around Bishop’s Stortford does not seem to have much to offer in terms of geological interest. There are no mountains, no cliff faces or rock exposures and no active quarries or gravel pits that are open to the public. However, the district has an interesting geological history and many aspects of this can be discovered simply by walking around the area.

In terms of geological age, most of the local geology is relatively recent, the oldest visible rock being Upper Chalk that was deposited during the later stages of the Cretaceous period some 100 to 65 million years ago. Although not visible in the immediate vicinity of the town, chalk may be found outcropping in the banks of streams and ditches just to the northwest. This Upper Chalk is white and contains courses of flints, which were collected in former years for use as a building material and flints may be seen throughout the area incorporated in garden walls and buildings.

The Cretaceous period ended violently at an event known as the KT boundary, probably caused by a massive meteorite impact in the Gulf of Mexico which caused such devastation that the era of the dinosaurs came to an abrupt end. The upper layers of the chalk were stripped off by erosion and the flints from these layers were pulverised into sands and pebbles over long periods of time.

We have now entered the Palaeocene period and this is represented locally in the deposits of sand and gravel which are worked commercially as valuable building materials. Also during this period, around 55 million years ago, it is apparent that the climate became extremely hot, rather like the centre of Australia today and as sea levels changed some of the pebble beds became cemented by silica that had been dissolved in the sea water. This resulted in an extremely tough conglomerate known as Hertfordshire puddingstone, an in-situ exposure of which has been found in a quarry near St. Albans. The puddingstone layers were only a few metres thick and were broken up by subsequent earth movements, leaving boulders of the more strongly-cemented material which were then brought to their present locations by major movements of river water. Hertfordshire puddingstone is so hard that on being broken, the break passes through the pebbles rather than around them as would happen with most concretes. Puddingstones often contain pebbles that have been stained by iron compounds present in the water when they were formed, giving a rich orange and brown colouration and this feature was utilised in the manufacture of jewellery and ornamental pieces for which the stones were cut and polished. Puddingstones were used in ancient times to make querns for grinding cereals but were often broken up in earlier years due to superstitious beliefs regarding their origin. However, many remain and may be seen in various prominent locations in local towns and villages. The splendid specimen in Bishop’s Stortford by the canal basin on Adderley Road came from Chaldean Farm, Much Hadham and was placed in its present location to replace one found when the canal basin was originally excavated but subsequently lost. Pieces of puddingstone may occasionally be found in the beds of local streams or in areas of excavation.

The next geological period represented locally is the Eocene from around 55 to 34 million years ago, during which much of southern England was covered by a tropical sea in which layers of mud accumulated, forming the deposit known as London Clay. This clay is greyish in colour but weathers to chocolate brown on exposure to the air as a result of oxidation of pyrite contained within the clay and it is exposed where rivers have cut through the overlying material and also when foundations are dug during construction projects. Formerly used extensively in local brick and tile manufacture, London Clay has now largely been replaced by clays from the earlier Jurassic period which occur in Bedfordshire and elsewhere, since these clays experience much less shrinkage during the firing process and hence make more consistently sized products.

Much of the local area is covered in deposits that were laid down during the Quaternary period, also known as the

Pleistocene, around 2.5 million years ago until the present day. During this period the Earth experienced repeated swings in temperature, from freezing ice ages to warmer interglacial periods. As the glaciers advanced and retreated, vast amounts of material from more northern areas were transported and dumped over the area, leaving a deposit known as Boulder Clay, also known as glacial till. Moreover, large river systems, including the River Thames, were forced by the advancing ice sheet to more southerly positions. These rivers were responsible for the transport of huge amounts of sand and gravel which now exist in the area. The Boulder Clay is very interesting in that it contains many fragments of rocks that do not otherwise exist in the area, including sandstones from the Carboniferous period in the midlands and north, igneous rocks such as basalts from

the Whin Sill in northern England and boulders known as septarian nodules from Jurassic areas of central England. The mechanisms responsible for the formation of septarian nodules (which also occur in the London Clay) are not fully understood, but appear to involve the growth of lumps of mudstone at the bottom of the sea, followed by a contraction process that results in internal cracks developing within the lumps. These cracks were then filled with water containing dissolved calcium salts resulting in the formation of calcite crystals within the nodule. The Jurassic septarian nodules that may be found in the area often exhibit masses of scratches and score marks on their outer surfaces resulting from transport within the glaciers from their original locations. This combination of clay and calcite within the same boulder was not lost on earlier generations, who realised that these were the raw materials for cement and that by grinding and roasting the nodules, a material known as Roman Cement could be produced. The name septarian nodule is derived from the Latin word SÆPTUM, which means partition or enclosure and a superb example is on display in the Stable Block at Audley End House. The Boulder Clay also contains some fossils derived from Jurassic deposits further north, the most notable of which is a bivalve mollusc called Gryphaea. These curved shells were known as Devil’s Toenails by our superstitious forebears and may occasionally be found on the surface of recently ploughed fields in Boulder Clay areas.