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GLACIAL SAND AND GRAVEL
The landscape at this time is almost impossible for us to visualise. As the region was situated at the southern-most limit of the Anglian ice sheet, colossal volumes of melt water would have been continually released and the evidence for this is also preserved beneath our feet. In parts of East Anglia, boreholes have revealed deep, steep-sided valleys cut into the chalk bedrock and now completely filled with sand and gravel and hidden by a covering of boulder clay. Known as buried tunnel valleys or buried channels these remarkable natural features were formed beneath the ice sheet and were the main drainage routes for melt water. One of the best examples of a buried channel is the Cam-Stort Buried Channel that is present from Great Chesterford south as far as Bishops Stortford. You can't see it but in Newport it passes beneath your feet and is some 100 metres (300 feet) deep, almost half of this depth being below present sea level. Click here to see the maximum extent of the ice sheet in Essex.

Glacial gravel is present in many other places in Essex. It is recognisable as an unsorted residue of many rock types, mostly flint, laid down under these exceptional conditions.

THE HOXNIAN INTERGLACIAL - THE ARRIVAL OF HUMANS
The Anglian glaciation was followed, about 400,000 years ago, by a warm stage traditionally called the Hoxnian interglacial. Neanderthals made their way north from Europe during this period taking advantage of the retreat of the ice. The Thames now flowed approximately along its present course but in the Southend area it turned north to Clacton along the old valley of the Medway laying down what is known as the Thames-Medway Gravels. These gravels are well exposed in the cliffs at Cudmore Grove Country Park in East Mersea. At Clacton, before the construction of the sea defences, these gravels yielded worked flints which are the earliest evidence of humans in Essex.

Perhaps the most important Hoxnian site is Marks Tey Brick Pit, near Colchester, where lake sediments have revealed distinctive layers containing fossil pollen, revealing to geologists a remarkably detailed record of the vegetation living throughout this interglacial stage.

THE MODERN THAMES
The modern (post-diversion) Thames has numerous bench-like terraces on either side of the valley, the oldest being at the highest elevation. They were formed during subsequent cold and warm stages that are collectively known by geologists as the Wolstonian. Terraces are formed by the river slowly cutting down through successive flood plains and creating new flood plains at lower levels; each terrace containing spectacular fossils thereby telling us what animals were living in the region during each of these periods.

The working of brick clay at Ilford in the mid 19th century yielded the bones of hundreds of mammoths and woolly rhinoceroses in the former pits, and the largest complete mammoth skull ever found in Britain (the 'Ilford mammoth') was discovered here in 1864. To find out more about the Ilford mammoth please click here. Downstream, there are numerous other sites such as at Aveley, where a clay pit yielded the famous Aveley elephants in 1964.

THE IPSWICHIAN INTERGLACIAL
The greatest and the warmest interglacial stage during the whole of the Ice Age was the Ipswichian, about 120,000 years ago. This period must have been warmer than the present day with monkeys, elephants and lions in southern England, the bones of which were found beneath Trafalgar Square in the 1950s as a result of building excavations. Downstream, the foreshore at East Mersea is currently one of the best sites of Ipswichian age in Britain. Here there are highly fossiliferous sediments called 'hippo gravels', so called because the hippopotamus was remarkably abundant in our region at this time, but curiously absent during almost all of the other interglacial stages. Fossils indicate that humans and also animals such as the horse were absent from Britain during the Ipswichian. Each interglacial stage has a distinctive fauna, presumably because, in each case, some animals were not quick enough to migrate north as the climate improved and were halted by the reappearance of the English Channel as sea level rose.

THE MOST RECENT GLACIATION
Following the warmth of the Ipswichian came the intense cold of the Devensian stage when an ice sheet again spread south but this time reaching no further than north Norfolk. Permafrost conditions gripped this region, providing home for only reindeer, arctic wolf and similar species.

Fossils of these animals have been found in a gravel pit at Great Totham and further west in the Lea valley.

Perhaps the most spectacular evidence of the Devensian cold stage is the network of ice wedge polygons that are occasionally revealed by crop marks in fields during hot, dry summers. Ice wedges are formed when the frozen ground shrinks and cracks during times of extreme cold. Each summer the cracks filled with water which then froze the following winter widening the cracks; a process that continued for thousands of years. At the end of the glacial stage the crack filled with debris preserving them as ice wedge casts, which are now revealed as crop marks.

AND FINALLY.......
No review of Essex geology would be complete without a mention of the county's most remarkable geological event. On the morning of 22 April 1884 the most destructive earthquake to have affected Britain in historical times shook Essex. It is known as the Colchester earthquake because the greatest damage was caused to buildings in the Colchester, Wivenhoe and surrounding area. Remarkably no one was killed but there was considerable damage to over 1,200 buildings, including fallen chimneystacks, church towers and roofs stripped of tiles. This event is the more surprising because it occurred in a county otherwise devoid of significant seismic activity. To find out more about the Colchester earthquake please click here.

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Copyright Essex RIGS Group 2001