Friday, April 26, 2013

The Downs and The Weald


The Downs of Kent and Sussex are a quintessential example of English landscape. The rolling Cretaceous chalk hills in the north and south come to an abrupt end at a pair of 200 metre high escarpments facing each other, gazing out over the older Jurassic rocks exposed below. Between them lies the dome of the Weald, filled with fields and meeting the sea in the recently emerged lands of the Romney Marshes. On a clear day (I admit that it can be a rare event) the other side is visible as a dim bluish line on the far side of the flat bottomed valley. This gash in the geology of South East England is not a fault bounded graben as the landscape might suggest, but England's own piece of the Alps.

This formation is an anticline, a type of fold with the oldest rocks in the centre. The area was recently uplifted as a distant consequence of compressive stress from the faraway collision of Africa and Europe that built the Alps and Pyrenees. The Weald is bordered by two ridges, a higher one in the chalk and a lesser one in the greensand formation below. The scarps have a shallower dip slope behind them. A geological cross section shows the different formations and how they relate to the structure at http://tinyurl.com/c7mh65a . The chalk escarpment goes from Surrey in the North (where it peaks at 269m) right the way down to the sea, by Folkestone and Eastbourne.

Famous places of geological interest include the white cliffs of Dover ( see our past story athttp://tinyurl.com/d9m83z6), Beachy Head (our past post is at http://tinyurl.com/cmdvty3) and the Seven Sisters. The landscapes offer stunning views of green fields, sometimes filled with wildflowers and a range of varied habitats determined by the underlying geology.

The Weald is drained by several rivers, cutting though the ridges. The Stour passes through Canterbury, one of England's oldest towns, and the religious centre of Christian England. Here St Augustine, who first converted us to Christianity, established its cathedral, which is made of sandstone from the greensand formation. The Medway flows past many monuments to our ancient naval glory such as Chatham Docks, where Blackbeard was hanged.

A potted geological history of the area would run something like this: An ancient landmass, called the London Platform was repeatedly uplifted and eroded in the Jurassic, depositing sand and clay cycles in a marshy plain, in which dinosaurs such as Iguanodon roamed. These are called the Wealden series and end with a muddy plain. A branch of the Tethys ocean then invaded in a marine transgression, depositing the two greensand formations (upper and lower), with the Gault clay sandwiched in between marking a period of deeper water.

As sea levels rose further, no land sediment could reach the area, and the chalk limestone was deposited (see our past post on flint and chalk formation athttp://tinyurl.com/d578uuz). At the end of the Cretaceous, uplift and erosion created a major unconformity (see past post athttp://tinyurl.com/ce7ceos), before some marine Tertiary rocks were laid on top. Erosion then created the current landscape. The highest parts of the Weald dome were removed revealing the cross section you can discover for yourself while driving around the area on a summer's day, stopping in an ancient pub out in the sticks for some liquid refreshment.

The final Alpine uplift and folding took place between 20 and 2 million years ago. The most intense burst was very recent, since 5 million year old marine sands were deposited in some places above the chalk. The ice ages then shaped the current landscape, when it was an open tundra. Permafrost related processes operated in a series of cycles as the ice waxed and waned, creating a series of v-shaped dry valleys with steep sides, also known as wind gaps and coombes. The best known one is Devil's Dyke, north of Brighton.

In glacial periods, impermeable permafrost caused runoff, which carved the valleys, and in interglacial thaws the chalk absorbed the rain and melt water and the valleys dried out. The sponge like chalk remains south east England's main aquifer. Where the chalk meets the clay a line of springs follows the whole length of the ridges. The recent burst of uplift also created a series of river terraces, which preserve records of England's earliest inhabitants and the Pleistocene paleoclimate. Several of the best preserved prehistoric flint mines occur in the Sussex downs.

The downs contributed to the development of Geology as a discipline. Being close to London, it was easy to reach and explore. The revealed strata were pieced together and seen to be similar to thos outcropping further west (In England and Wales, the further west you go, the older the exposed rocks). The anticline shows up on Strata Smith's first geological map. The first land dinosaur, iguanodon, was found in the Wealden series. They also were the dwelling place of Charles Darwin, who meditated and wrote 'on the origin of species' in his garden.


Image credit of the Wye downs: A.Welbourn.

http://www.kgg.org.uk/kentgeo.html

http://www.kentdowns.org.uk/uploads/documents/Mplan7.pdf

http://www.southdowns.gov.uk/looking-after/landscape/geology-of-the-south-downs

http://www.southdowns.gov.uk/learning/themes-to-study/landscape/geology/geology-through-time

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