The Anglian Ice Sheet EarthCache
Hidden : 6/23/2016
Difficulty:
1.5 out of 5
Terrain:
1.5 out of 5

Size: Size: other (other)

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Geocache Description:


ANGLIAN ICE SHEET


THE ICE AGE - COOLING OF THE PLANET


The story of the Ice Age starts around two million years ago. At this time the climate was probably not too dissimilar to the present day but the temperature had been slowly dropping for tens of millions of years, ever since the balmy, tropical days of the dinosaurs. The oldest Ice Age deposit in our region was laid down in a shallow sea and is known as the Chillesford Sand (part of the Norwich Crag Formation). This sand occurs at the base of gravel pits in north-west Essex but it is extremely difficult to date due to the absence of fossils for much of its thickness. Following deposition of the Chillesford Sand there is a very fragmentary record of our climate and landscape over the next million years or so. This period, which makes up most of the early part of the Ice Age, is poorly understood but some evidence has been preserved, thanks largely to an early route of the River Thames.

THE EARLY THAMES


During the early Ice Age the Thames flowed to the north of London, through north Essex, Suffolk and Norfolk and out across what is now the southern North Sea to become a tributary of the Rhine; the evidence for this being a substantial thickness of what is called Kesgrave Sands and Gravels which, remarkably, represents the actual bed of the river. These old Thames gravels contain a variety of unusual pebbles from as far away as North Wales, proving that, at that time, the Thames, and its tributaries, must have been a huge river system draining the Welsh mountains and bringing their characteristic volcanic rocks into the Thames basin.

The gravels also contain large boulders of puddingstone and sarsens, which are very hard conglomerates and sandstones respectively. They are believed to be derived from pebble and sand seams in the Reading Beds, and which have subsequently become cemented by quartz. They have been put to use by man as ancient way markers at road junctions. The gravels have great commercial value and are worked in numerous gravel pits between Harlow, Chelmsford and Colchester, which was the route of the ancestral Thames at least 600,000 years ago.


During this time the River Medway flowed north across east Essex to join the Thames near Clacton, leaving behind a ribbon of distinctive gravel which can be found between Burnham-on-Crouch and Bradwell-on-Sea. There were also other northward-flowing tributaries of the early Thames. Evidence of these are the patches of gravel that are found on the tops of the hills in south Essex, such as the Langdon Hills, Warley and High Beach in Epping Forest.

THE ANGLIAN ICE SHEET


The regular pulses of climate change culminated in the Anglian glaciation, a severe cold stage about 450,000 years ago that allowed a great ice sheet to spread south into the region across the valley of the early Thames. A lobe of ice from this ice sheet blocked the Thames in the Vale of St. Albans causing a catastrophic change to the route of the river, diverting it south to its present position. Evidence for the existence of this ice sheet is a substantial thickness of boulder clay, or till, left behind by the ice as it ground southwards across the frozen landscape. The boulder clay, lying on top of the old Thames gravels, forms a distinct plateau over the north of Essex, now dissected by modern river valleys. Boulder clay can be found as far south as Hornchurch, which is known as the most southerly point in Britain that the ice penetrated during the whole of the Ice Age. The boulder clay contains rocks, called glacial erratics, that have been carried south by the ice, some of them from as far away as northern England and Scotland. The boulder clay also contains some fossils such as Jurassic ammonites and belemnites, brought here from the Midlands.

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.


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.

FACTS ABOUT THE FINCHINGFIELD STONE


Stones such as this boulder are known as glacial erratics and were carried to Essex by the Anglian Ice Sheet that covered almost the whole of Britain during the coldest period of the Ice Age, some 450,000 years ago. At this time Essex was situated at the southern edge of the ice sheet, which was up to 2 kilometres (over a mile) thick in places. Its extent is fairly well known because it has left behind evidence of its existence in the form of a rock called boulder clay, or till. A great thickness of Boulder clay exists across north Essex except in river valleys where it has been removed by erosion.

This boulder was probably discovered in a local field, having been washed from the boulder clay as the River Blackwater cut down through the boulder clay plateau.

As the ice moved it ground up and carried along pieces of the rocks over which it passed, just as glaciers and ice sheets do today, and when the ice melted an unsorted clayey residue called boulder clay, or till, was left behind. Boulder clay contains rocks transported long distances by the ice and known as glacial erratics. By matching rock types with known outcrops in other parts of Britain geologists are able to establish the direction of ice movement across the country from its origins in Scotland or Scandinavia.

Glacial erratic boulders of basalt and other igneous rocks are rare in Essex, especially those over about 50 centimetres in size. This example in Finchingfield, in a conspicuous position on the side of the road, is very unusual. The boulder may have been brought to Essex by the ice from Scotland. Analysis of the boulder’s composition may give a clue to its precise origin.

This history of this boulder is not known. It is assumed to have been discovered in a local field or pit and brought to the village for a practical reason, perhaps as a mounting block in the 19th century.

To log this cache please send your answers via either email or Message Centre, either of which can be accessed through my profile.


1)Describe the shape, texture and composition of the stone? is it round or flat, rough or smooth?


2)What part of the country is it thought this stone originated from?


3)What size is the stone?


4)Which two rivers are thought to have left this region in the area of Clacton on todays map and which river did they join up with? This can be seen with the deposits of Kesgrave Sands and Gravel across Essex. From where do some of the pebbles within these sands originate?


5)If you would like to post a picture of yourself or your team they would be most welcome with your log. This is not a requirement of claiming the find.



All information for this page has been sourced from a blog from 2007 and The Essex Field Club .

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