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The Geology of the Weald

 

Geologically, the Weald of Southern England was formed by plate tectonics during what geologists call the Oligocene and Miocene periods between 34 and 5.3 million years ago. The underlying rocks, the clay, sand and chalk, are much older sediments that built up over millions of year at the bottom of what was called the Tethys Sea. This huge stretch of water separated the European and Asian landmass from Africa. They were deposited during the Mesozoic and Cenozoic periods, between 248 and 34 million years ago. As the African landmass moved progressively northwards towards the stable Eurasian landmass, these sediments were squeezed and slowly forced down into what the geologists call the ‘Tethyan geosyncline’.  

 

About 135 million years ago this area of Southern England was the northern edge of a huge freshwater lake. Rivers draining the surrounding European landmass carried silt and rock fragments which accumulated as horizontal beds of clay and sand. In places the clay is over 300 metres thick. The whole area sank about 110 million years ago and the sea broke through into this lake about five million years later in the middle of the Cretaceous period. As a result, the bodies of a host of marine creatures, mostly mollusca, brachipods and ammonites, built up on the sea bed. As their bodies were eaten their bones, teeth, claws and shells began to accumulate above the sand and clay. Here they were washed around in the tidal currents and had much of their surface features worn away. As the rivers drained the volcanic areas of Scandinavia and Northern Britain the warm, shallow water was rich in dissolved minerals, including green glauconite, an iron-rich silicate, As a result the porous remains became phosphatised. They absorbed the phosphate from the seawater and built up a hard coating, similar to encrusted plaque on your teeth. Over time they were buried under up to 100 metres of sand and fossilised.

 

With a warmer climate between 115 and 95 mya, the large shallow sea provided habitat for what the geologists describe as microscopic “coccolith biomicrites formed from the skeletal elements of minute planktonic green algae, associated with varying proportions of larger microscopic fragments of bivalves, foraminifera and ostracods”. When they died their skeletons accumulated in great depths and were compressed to produce extensive chalk deposits of Southern England, some of the youngest rocks in the United Kingdom.

 

Between 75 and 70 mya a great uplift of these Tethyan sediments started, producing earthquakes, volcanoes and what we now know as the Atlas Mountains, the Alps, Apennines, Pyrenees and other mountains around the Mediterranean. Between 34 to 5.3 million years ago the outer ripples of this mountain-building period created an anticline in this area. This was a huge, round-topped dome of rock, estimated to have been about 970 m. above sea level. Folding and faulting cracked the rock strata and, exposed to the elements, they dried out in the sun and wind and river erosion gradually lowered the dome.

 

During the last ice age between * and c.12,000 years ago, much of Northern Europe was covered in ice, hundreds of metres thick. Whilst it did not reach this far south, freezing and thawing further stressed the rocks of the Weald. When temperatures started to rise about 20,000 years ago vast quantities of glacial meltwater started to further erode the chalk and exposed sand and clays of the central area. The ice finally retreated about 11,000 years ago, leaving the landscape of the Weald much as it is today. Many hundreds of metres of the softer upper chalk had been dissolved and washed away. The only remnants are the gentle dip and steep scarp slopes of the North and South Downs. As the top of the dome was removed much of the sands and clays were also eroded. The highest point in Hampshire is Pilot Hill, which reaches the height of 286 m (938 ft).

 

http://en.wikipedia.org/wiki/Hampshire