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 What lies beneath the South Downs and the White Cliffs of Dover?

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normanhurst
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PostSubject: What lies beneath the South Downs and the White Cliffs of Dover?    Mon 05 Nov 2012, 00:41

One again I’m not sure where to leave this as it’s probably more a geology than a history topic… maybe history of the world.


Are the same sea creatures still with us that formed chalk and the white cliffs of Dover?
Why is there no sedimentary rock formed above the chalk… it’s covered with just a few inches of soil barely enough to support grass pasture… over the millennia what has happen to that covering, has it been eroded, if that’s the case, there are no bare places with no grass cover. What has stopped the annual growth and dieback from forming a thick loamy soil?


I’m very familiar with the chalk downs of Kent, and the masses of flint strew across the fields with pretty villages and churches constructed from it, I spent a lengthily and unproductive amount of time in my childhood bashing flints together hoping to achieve the magnificence of a polished hand axe similar to one a teacher once let me hold. In the cliffs of Dover you can see various lines ‘stratum’ of flints, but nothing else. I’ve been to grimes graves and been down a flint mine… quite fascinating, and the curator there kindly gave me a demonstration of flint knapping… enough for me to get the basics, and produce some bits I’m quite pleased with.


However, walk along the beaches at the foot of said cliffs and your walking on shingle… where does this shingle come from, what’s it made from and what rock lies beneath the chalk cliffs. For many years I had a small lump of stone a friend brought me from when he was working on the channel tunnel…
I sailed on sand and ballast dredgers, and don’t ever recall seeing a piece of stone the likes of what he gave me despite dredging in the Solent and English channel.


Where I live now I love to look for fossils on top of Portland or Kimmeridge Bay, and to see where dinosaurs left their footprints. And just yards away from the water’s edge at Kimmeridge, atop the cliff is one of the oldest oil wells still going strong.
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PostSubject: Re: What lies beneath the South Downs and the White Cliffs of Dover?    Mon 05 Nov 2012, 07:30

Good Lord, Norman, you do come up with some questions....

normanhurst wrote:
Are the same sea creatures still with us that formed chalk and the white cliffs of Dover?.

Basically yes, the same types of creatures that formed chalk still live in some areas of the world where they are still sinking, when dead, to the bottom as calcite-rich sludge which may in the far future become compressed enough to form chalk. But the exact species' that formed southern England's chalk, some 70 million years ago, have of course evolved over the subsequent millions of years to be replaced by other similar species but doing the same thing.

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Why is there no sedimentary rock formed above the chalk… it’s covered with just a few inches of soil barely enough to support grass pasture… over the millennia what has happen to that covering, has it been eroded, if that’s the case, there are no bare places with no grass cover. What has stopped the annual growth and dieback from forming a thick loamy soil?


Yes the covering has been eroded away. In essence the areas that we now know of as the South and North Downs became low lying islands about 50 million years ago, so there wasn't anything much being deposited or they were actually being eroded. Occasionally on the surface of the Downs you see large lumps of hard sandstone, but these blocks are actually older than the chalk and were moved into position by glacal action and come from the eroded core of the Weald between the N and S Downs or in the case of the Wiltshire Downs from further north and west. There are also a few, a very few, pockets of deposits that were actually formed on top of the chalk and survived being eroded away but these are rather unique. You can however still see nearly all of the sedimentary rocks that were deposited after the chalk was formed in the Hampshire Basin and the London Basin where the land remained low and deposition continued either side of the Wealden/Chalk island. Perhaps the best places to see all these beds stacked up in their correct order (or rather lined up since the land has subsequently been tilted) are Alum Bay and Whitecliff Bay at either end of the Isle of Wight. There are other good exposures of rock (actually these rocks are mostly clays and fairly unconsolidated sands) along the Kent, Sussex and Hampshire coasts too, and of course they've been seen in all the bore holes drilled throughout the South East. Between all these locations one can get to see the entire line up of the successive rocks that were deposited after the Chalk (late Cretaceous) through the Palaeocene and Oligocene periods (with a geological break - the very top of the Cretaceous, after the Chalk, is missing in the UK). And there is another gap in the succession after the Oligocene as the land rose and there was a period of erosion rather than deposition. We're still essentially at the point today although there are some pockets of more recent stuff deposited by post ice age flooding from melting ice-sheets and sea level rises. The chalk downs are basically bare or with only very thin soils largely because the chalk is nutrient poor (chalk is fairly pure calcium oxide with nothing much else), and very permeable, so that all rain water just soaks away taking with it whatever nutrients there might be. Deep loam cannot develop except where there are valley floors lined with clay washed out from the ice age glaciers. The glaciers themselves were all further to the north, the most of the chalk downs were never glaciated, but they were affected by permafrost and glacial fooding.

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I’m very familiar with the chalk downs of Kent, and the masses of flint strew across the fields with pretty villages and churches constructed from it.... However, walk along the beaches at the foot of said cliffs and your walking on shingle… where does this shingle come from, what’s it made from


Nearly all the shingle on the beaches and all the flints in the fields are the same stuff... flint eroded out of the chalk, and then rolled into smooth pebbles in the case of beach shingle. Of couse a lot of the shingle along the Sussex coast originally comes from a lot further away ... the sea in which the chalk was formed was huge and covered a very wide area (the same chalk extends under Holland, Belgium and Northern France) and that is only a fraction of it's original extent. And with sea currents the shingle moves location. Currently the movement along the south coast is west to east, hence the build-up of the huge mass of shingle at Dungeness, and also evidenced by shingle piling-up on the wesward side of all the groynes build out along the Hampshire, Sussex and Kent channel coasts to try and stabilise this eastward drift. Flint is almost pure silica and if I remember correctly originally derives from the silica deposited in the bodies of ancient sea sponges that were buried along with all the other creatures in the sludge that eventually went to form the chalk. The silica became concentrated along certain bedding planes and joints in the chalk hence the appearance of prominent lines of flint in chalk cliffs, such as at Beachy Head. Shingle is nearly entirely composed of flint pebbles because silica is very hard and withstands rolling around by the sea, unlike clayey or sandy rock, or indeed chalk itself which mostly comprise the local rocks. But if you look closer at shingle there is other stuff there too ... hard igneous rocks, like granite, basalt, schist etc, that would have originally come from Cornwall, Ireland, Scotland and Scandinavia, or maybe even further away.

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... and what rock lies beneath the chalk cliffs. For many years I had a small lump of stone a friend brought me from when he was working on the channel tunnel…


Under the chalk are the succession of older Cretaceous rocks, sandstones and clays mostly, which still outcrop on the surface in quarries in Sussex, Kent, Surrey, and which again can be seen in coastal cliffs, such as near Hastings and Folkestone or along the southern coast of the Isle of Wight etc. Below them, and going further back down the succession are the Jurassic rocks some of which you are already familiar with from the coast at Portland and Kimmeridge, and of course nearly the entire Lower Jurassic sequence can still be seen exposed along the heritage 'Jurassic Coast' either side of Lyme Regis. Under this lot I'm not so sure exactly, it all rather depends on whether and when the bit of the Earth that would become Southern England was under the sea or was land etc...

I'll see if I can find a geological cross-section through southern England and the Channel.



Interested to hear about your fossiling trips.... God I miss all that, I used to spend at least a day or so every month for many years either patrolling the beaches at Bracklesham, or in Pinhay Bay, west of Lyme Regis, or scroffeling around in Surrey clay quarries.


Last edited by Meles meles on Mon 05 Nov 2012, 11:56; edited 5 times in total (Reason for editing : Spellings and typos mostly)
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PostSubject: Re: What lies beneath the South Downs and the White Cliffs of Dover?    Mon 05 Nov 2012, 09:19

Norm, did you see these the other day? Not answers to your questions, MM has done that very fully, but an intriguing and thought provoking slant.
http://www.bbc.co.uk/news/science-environment-20154031

and
http://www.bbc.co.uk/news/science-environment-20154030
and
http://www.bbc.co.uk/news/science-environment-13335683

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PostSubject: Re: What lies beneath the South Downs and the White Cliffs of Dover?    Mon 05 Nov 2012, 09:25

Blimey MM... how I wish I'd bumped into you while fossiling... yes I've picked up some good ones over the years... but the ones from Kimmeridge, well they're a good three feet in diameter.

I'd be very interested to see a cross section through this area...

Very informative... it will take a time to ingest. thanks.
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PostSubject: Re: What lies beneath the South Downs and the White Cliffs of Dover?    Mon 05 Nov 2012, 09:34

Cross section, South to North from out in the channel, up through just east of the Isle of Wight, through the Portsmouth area, through the South Downs, the Central Weald, and North Downs to just west of London...the chalk is the pale green just below the beige, labelled 'Upper Cretaceous' in the section (I had to put it as a link 'cos when I posted the image you can't see the captions):

http://en.wikipedia.org/wiki/File:WealdBasinCrossSection.png

And there's these too from http://www.discoveringfossils.co.uk/hastings_fossils.htm
Note again how the rocks of the Weald form an upwards bulging fold or dome (an anticline), the top of which has been eroded away to leave the exposed chalk of the downs on either side of an older core.... and I believe there are a couple of locations in the area where this core has been so eroded that you can see bits of the very topmost Jurassic rocks (top of the Portland, I think) exposed at the surface in the bottom of a couple of valleys.


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PostSubject: Re: What lies beneath the South Downs and the White Cliffs of Dover?    Mon 05 Nov 2012, 13:20

normanhurst wrote:
Blimey MM... how I wish I'd bumped into you while fossiling...

But maybe we have bumped into each other .... I've been fossiling for ages, probably started seriously when I was about 14* and have met many, many fossil hunters over the years, from interested kiddies and their parents intrigued by pretty stones, through keen like-minded amateurs, to geology field students and crusty old professors. Everyone I've met without exception has just been happy to chat, to learn, to impart what they know, to proudly show off what we've just found, or commiserate on a lousey days hunting. So you never know, we might well have chatted at some time.

* Just as an aside, when I was 16 or 17 I used to go off with the geology teacher on occasional geological explorations around Sussex. We'd bunk off school for the afternoon in his battered old Mini (I think he used to get me excused PE which was an added bonus)... and just the two of us would go fossiling. Can you imagine doing that nowadays? A pupil alone with a teacher, exploring old quarries or dodgey cliffs, no care for health & safety, no liability insurance! We once nearly got cut off by the tide below Peacehaven cliffs and had to wade back to the seafront steps, and another time we got caught sneaking into a quarry, which we believed had ceased working, but unfortunately hadn't. Oh happy days!
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PostSubject: Re: What lies beneath the South Downs and the White Cliffs of Dover?    Mon 05 Nov 2012, 13:50

Meles meles wrote:
Can you imagine doing that nowadays? A pupil alone with a teacher, exploring old quarries or dodgey cliffs, no care for health & safety, no liability insurance!

Now a days they'd think there must be more nefarious reasons than health or insurance for a teacher to show interest and spend spare time encouraging students MM.
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PostSubject: Re: What lies beneath the South Downs and the White Cliffs of Dover?    Wed 07 Nov 2012, 01:43

Saw this, and wondered if they knew what it was all about.

http://uk.news.yahoo.com/white-cliffs-appeal-successful-000444452.html
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