Staggering geology of southern England via the Coast Path
England's geology becomes increasingly older as you travel westward along the south coast. Meanwhile, the colours of the cliffs shift from splendid white to leaden grey, warm gold and deep red. Sedimentary strata give way to deformed, plutonic and mantle rocks in Devon and Cornwall, all the way to Land's End at England's southwestern tip. All along this coastline, the Coast Path winds past rugged cliffs where the geology of England is exposed in sweeping, breathtaking views.
By Kathelijne Bonne. Photos by K.B. unless otherwise stated. Support GondwanaTalks with a one-off or recurring donation.
Nowadays, everyone flies, but in the past people from 'the continent' took the ferry to England. For those on board, the first sight of this land was the wall of white cliffs of Dover, made up of chalk. They glisten in the sun (when it shines) as they face south. Even the Roman name for the British Isles, Albion, is a reference to the first colour you see as you approach England from the sea.
White Cretaceous Coast: White cliffs of Kent and Sussex
This horizontally layered white chalk, from which chalk for blackboards is also made, is so widespread in Western Europe that the period it was deposited in is named after it: the Cretaceous after creta in Latin meaning chalk. This is the last period of the Mesozoic era, best known for dinosaurs although much other fascinating life existed then too.
Tiny creatures in particular left an indelible footprint. The chalk cliffs and other calcareous deposits consist almost entirely of the skeletons of plankton that was adrift in the seas submerging continental shelfs surrounding the island archipelago that was Europe in Cretaceous times. Phytoplankton extracts carbon dioxide from the air to make billions of tiny skeletons, and therefore plays the biggest role of all in the carbon cycle. Its impact is larger than that of forests.
In western Europe, the Cretaceous basin stretched across England, France and Belgium. In France for example, chalk deposits of the Alabaster Coast (famously painted by Monet and other impressionists) stretch all the way to the Champaign region, where marl, a mixture of chalk and clay, provides the ideal soil for grapes.
We head a long way west, far beyond Beachy Head near Eastborne, the highest chalk cliff in England. The best way to explore the cliffs is by walking the Coast Path that hugs the entire coast but you can choose to do bits of it. Heading west, we go back in time, to older strata of the Jurassic Coast.
Golden cliffs of the Jurassic: West Bay Cliffs in Dorset
The cliffs at the locality of West Bay have a striking golden-yellow colour and are truly magnificent when basking in the evening sun. The Coast Path ran above the cliffs, but has been rerouted inland as the slopes have become unstable.
The yellow sandstone was deposited during the Toarcian (184 to 174 million years ago), a stage of the Early Jurassic. Why are the strata yellow? In the shallow Jurassic sea, large amounts of clay and organic material were deposited. In places where there was little oxygen, bacteria converted sulphates from seawater into sulfide, which reacted with iron in the sediment on the seabed to form the mineral pyrite. When the strata were later uplifted and exposed to air and rain, the pyrite oxidized into the remarkably ochre-coloured oxide limonite.
Lead-grey Jurassic Coast: ammonites and marine reptiles of Lyme Regis and the Jurassic Coast
On the border between the counties of Dorset and Devon, the earliest Jurassic is exposed in high, steep, lead-grey cliffs, where the rocks protrude in horizontal slabs.
We're in the town of Lyme Regis. Mary Anning (1799-1846) lived here and unearthed and studied Jurassic marine animals. Mary is buried at the local church, alongside several siblings who died young and her brother Joseph, with whom she discovered an ichthyosaur when she was only eleven. The ichthyosaur was a large marine reptile that resembled a giant fish. It had a huge eye, probably one of the largest eyes that ever existed (trilobites, a Paleozoic class of marine arthropods, reveal much about the evolution of the eye).
Ammonites, which are cephalopods related to squid and with a beautifully coiled shell, are extremely abundant on the Jurassic Coast. They first appeared in Palaeozoic times and only a few species survived the Great Dying at the end of the Permian, but those surviving species diversified greatly in the Mesozoic and flourished in the Jurassic and Cretaceous.
Ammonite Pavement
The Ammonite Pavement in Lyme Regis is an extraordinary stretch of coastline, accessible only at low tide. Countless ammonites lie exposed in the limestone layers of the 199-million-year-old Blue Lias Formation (Early Jurassic). They had survived another, smaller extinction event at the end of the Triassic period and had only just begun to recover.
This rocky pavement is enjoyable to walk on barefoot, as it is pleasantly flat and gently undulating. You can hop from one loose rock slab to the next. Rock slabs have broken away along the old seafloor bedding so you do walk on the petrified mud in which the ammonites settled. The Ammonite Pavement is sprinkled with large, rounded boulders that also contain ammonites.
Small tide pools brim with snail shells and seaweed, all little worlds of their own. Fortunately, this stretch of coastline lies well away from the main beach near Lyme Regis. Large piles of rotting seaweed are lying there and they stink terribly. The south coast of England is indeed undergoing a harmful algal bloom. After unusually heavy rainfall, increased erosion has washed sediment and nutrients into the sea, feeding the algae — but that is another story.
The red coast of the Triassic: Jacob's Ladder Beach in Sidmouth
Further still to the west and back in time, during the Triassic, Britain – and other parts of Europe – lay in the interior of the supercontinent Pangaea. This part of the world was a hot, barren landscape. Sand was redistributed by wind and rivers. Near lakes and lagoons, a few horsetails and conifers managed to survive and ended up as fossils.
Amphibians, reptiles and fish have been found in the thick red layers of the Helsby Sandstone Formation, which is now spectacularly exposed at Jacob's Ladder Beach in Sidmouth, Devon. The red colour is a result of oxidation of iron in the dry, warm climate.
When it rains, these steep red cliffs erode into the sea, which also takes on a bizarre reddish hue. The Coast Path that runs on top of the cliffs offers a delightful walk across pleasant vegetation towards High Peak, from where you can see a few stacks lying in the sea below.
Granite in Cornwall: moors, tors and Land's End
Further west, the Jurassic cliffs give way to the older Permian, and then there is a sudden major change. The geology of western Devon and Cornwall no longer consists of horizontally layered strata like the outcrops we have seen so far. These older rocks are the 'fruits' of the Hercynian Orogeny, a phase of mountain-building that affected the whole of Europe: continents collided to form the supercontinent Pangea. You may have noticed that the Devonian, the geological period exposed here in deformed, broken and folded strata, is named after the county of Devon where we are now.
Cornwall is dotted by large granites, which rose like 'bubbles' of hot magma beneath the mountains that have since eroded away. The granite was subsequently exposed by erosion. Acidic soils with heathland vegetation, 'moors', have developed on the granite and now form grazing grounds for semi-wild ponies. 'Tors' rise from the landscape; bizarre granite formations created during the ice ages. A massive chunk of granite juts out into the sea at Land's End, the westernmost part of southern England.
Earth's mantle, magma and the 'Moho' in Cornwall: Lizard Point and Kynance Cove
The Lizard Peninsula, the southernmost headland of Cornwall, is an 'ophiolite' and one of Britains most unusual pieces of geology. Rare rocks risen from great depths reveal a cross-section of an ancient ocean floor and pieces of the Earth's mantle that have been thrusted upwards due to the immense forces of the Hercynian orogeny. A line stretches across Lizard peninsula: the 'Moho', or Mohorovičić discontinuity which is the boundary between the Earth's mantle and crust, named after the Croatian geophysicist who lived from 1857 to 1936.
A stunningly beautiful little bay on the Lizard Peninsula is Kynance Cove. Rocks consist largely of serpentine, a green-red mottled rock formed from ancient ocean floor. The boulders on this particular beach consist of serpentine and gabbro, polished by the powerful Atlantic tide. I pick up a few reddish-black pebbles and throw them back into the waves. They clatter and crunch as the surf rolls over them.
It is unimaginable that these pebbles come from the miles-deep bowels of the earth and now roll restlessly on a modern-day coast, which is plagued by algal blooms and admired by walkers (you can only reach Kynance Cove on foot), but at the same time, it is undisturbed by our human dramas. That thought is unsettling but also soothing.
I'll take one small pebble with me, as a memento of a spectacular coastline with incredible geology, and of a country which is the birthplace of geology and the nature of which inspired many people I learned so much from, like Jane Goodall, Richard Fortey and David Attenborough.
Read on to discover who Mary Anning was, and how she defied a male-dominated society as a woman of science. Also discover the Tethys Sea, marine algae and how they inspire artists, the Permian mass extinction, trilobites and supercontinents.
Kathelijne, editor of GondwanaTalks: I am intrigued by how earth, life, air, ocean and societies interact on geological and human timescales.
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