Read the interview with Geologist and Naturalist, Mel Wrigley -The story of Chalk
Mel Wrigley works for Eurotunnel and the White Cliffs Countryside Partnership, who aim to conserve and celebrate so much of the landscape heritage of the Dover and Folkestone coastal area. Having worked in the field of conservation for over 30 years, Mel has worked on numerous projects that focus on the Geology of the area. At a recent Dover Arts Development event, Mel offered a fascinating insight into the ancient chemical and geological processes that had led to the formation of the white cliffs that so much of the CHALKUP21 trail walks upon:
Dover is world famous for its iconic geological feature – The White Cliffs – of gleaming Chalk rock, the very symbol of England, and, likewise, the poem ‘Dover Beach’, with its opening line…“The sea is calm tonight, the tide is full and the moon lies fair upon the Strait…” by Matthew Arnold is famous in English literature.
Where does the story of the ‘White Cliffs – the story of the Chalk – begin?
Imagine…. you’re in a time machine …at this place of ‘Dover’…and we go way back in geological time, set your time machine clock to about 100 million years ago….and we go back to the Cretaceous Period !
You’d find yourself swimming around in a lovely, warm, tropical turquoise sea, something like the Bahamas of today. At a balmy 29 degrees C …you’d be enjoying the warmth and seeing cuttlefish- like animals called Belemnites, and curly, ammonites swimming about you in that sea … BUT watch out! These creatures would be being hunted by giant, marine reptiles like Ichthyosaur and Plesiosaurs and they’d eat us too!
Cretaceous means ‘the Chalk forming’ time.. And …it this era which ranged from 65 million to 142 million years ago when the temperature of the Earth’s atmosphere was much higher than it is today.
At this time, the Cretaceous atmosphere was altered by the effect of massive volcanos erupting, which spewed out vast amounts of gas. In turn, these gases caused a huge increase in Carbon Dioxide and the global temperature increased causing global warming! This caused the Polar Ice Caps to completely melt, and sea level rose around the world, to about 100 metres higher than it is today, (about the height of the cliffs today) which consequently caused much less dry land to be available! What land there was, was ruled by the Dinosaurs!
In that warm tropical Cretaceous ocean, the carbon dioxide dissolved from the atmosphere into the sea and minute algae called coccoliths absorbed it and made their shells from the chalk. When they died they fell down to the ocean floor, forming deep, porridge-like sediments, which got thicker and thicker; and got squashed by the sheer weight of the sediment above; it got compressed, and compacted into solid rock, and later in time this rock got uplifted by earth movements to become the chalk rock that we know today.
Astonishingly, CHALK is formed from what was once living organisms, similar to how COAL is formed by the remains of land plants; CHALK is formed from the remains of Marine Algae.
The FLINT is formed within the chalk rock. Flint is only formed in Chalk.
Flint is a glass-like substance composed of microcrystals of silica (a sort of glass like substance) derived from the remains of microscopic animal plankton called radiolarians and from the spicules of sponges. It’s used as strengthening in sponge bodies.
So FLINT too is a product of living animal matter! Isn’t that astonishing?
How did the English Channel form? Formation of the Channel….
Until quite recently it was thought by geologists that the English Channel was quite young, formed about 8,600 years ago. That was until recent research uncovered more evidence which suggests that the channel is much older than first thought.
From the Cretaceous, we Come forward in time, in our time machine, to only half a million years ago. If we looked from the top of the White Cliffs of Dover towards France, we wouldn’t be looking at the ‘sea’ but at a continuous, rolling hillside stretching over to join what is France today. This structure was the so-called Land Bridge and the geology on either side of the channel is the same formation.
Britain and mainland Europe were physically connected 500,000 years ago by this Land Bridge. Our Stone Age ancestors would have walked back and forth across that Land Bridge, as they hunted Woolly Mammoth and other large prehistoric (and now extinct) animals.
In a period of cold, of Ice Age, the Ice Sheet (glaciers) were moving down from the north. The front edge of the ice sheet (glacier) was roughly in a line from Bristol to London and stretched out and up into the North Sea.
The Land bridge acted as a dam and held back a vast glacial lake (thought to be twice the size of Wales). The Thames and Rhine rivers were draining into it. A river overtopped the dam, (here near Dover) a large waterfall fell over the dam and created a deep ‘plunge pool’. And this was surprisingly discovered when the geoengineers made boreholes to check the rock, for the route of the Channel Tunnel. They came across soft permeable sediment in the bedrock of chalk. The original route for the tunnel had to be changed to avoid the plunge pool sediment otherwise the tunnel would have leaked!
The prehistoric Channel river flowed down the centre of the channel through low lying marshy land. Of course, sea level was much lower than it is today because the water was tied up as ice in the poles and in the ice sheets.
Suddenly, about 450,000 years ago, probably caused by an earthquake, (because we’re on a fault line) the land bridge was broken and a catastrophic, cataclysmic event occurred as the land bridge was broken and the colossal pressure of the glacial lake water caused a rupture, and millions of cubic metres of water per second, came thundering through the collapsing chalk, and it scoured its way down the channel cutting deep gorges and grooves in the bedrock of the channel. It is thought to be the largest geological megaflood in Europe, and possibly the in the world, similar to one known of in Utah & Colorado in the Scablands, USA.
Suddenly, the physical land connection with mainland Europe was cut off by water! The English Channel –Dover Strait was born! Rocks were transported from that glacial lake, and debris from the broken chalk land bridge is found in deep grooves on the channel bed off the Isle of Wight.
Finally, I’ll leave you with this thought?……. Maybe the nomadic members of our Stone Age tribes literally got cut off from their family groups, when the land bridge busted? Who knows? Our connections are greater than we think. Genetically, we’re all much more related and connected than we would probably ever imagine.
Melanie Wrigley M.Sc; B.Sc (Hons)