The Jurassic Coast and the Exeter traps
Our ammonite was dead: to begin with. It hadn’t always been.
It was a funny place. In typical English fashion, The Old Vicarage B&B was a mix of comfort and obsolescence. The old manor house had large rooms, and was beautifully decorated. It felt like stepping centuries back in time, but with a few upgrades – such as a light switch. England idolizes its past. This is the nation where one company advertises ‘Victorian Plumbing’. Who but the English would buy into that? But as a matter of fact, idolizing the past does not mean having to live in it – just making it look that way. In spite of its appearance, the plumbing in The Old Vicarage was fully modern. Live like Charles Dickens, but with the conveniences of today – that is the English way.
We have come here for another journey back in time. The southwestern coast of the UK is a gift from the past. Our world may be shaped by geology, but it gives sparingly. All land has a volcanic origin. The separation between the dense material from the mantle and oceanic crust (basalt) and the much less dense material of the continents happens in molten rock, where the densest material sinks and the lighter material rises. Once the continental baby is born, it can grow. The oceanic plates pull the continents along, and can underplate them. Collisions thicken the crust and throw up mountains. Volcanoes erupt and add their volume to the land. But it is not all plain sailing: all land is subject to erosion which brings down those mountains. Sometimes the land sinks below sea level, and now sediment can add its layers: once the land resurfaces, it has changed and the original surface is deeply buried. Desert sands can also bury the past. Geology is like Ebenezer Scrooge of Dickens’ Christmas Carol, giving little and taking back what it can. But sometimes there is an unexpected gift in the landscape; a curtain is pulled apart to reveal a ghost of Christmas past.
Our particular curtain is at the coast, where a steep cliff forms a dramatic full stop to the landscape. A pebbly beach has formed at the bottom. The cliff reveals what is hiding below the ground. It shows well-defined horizontal layers, clearly sedimentary: this land was deposited under water. The dark blueish rock is known as Blue Lias. It is quite a soft rock which is easily eroded. There are frequent landslips and rockfalls here which add their debris to the pebbles below. On this sunny day there are a fair number of people on the beach but it is not crowded. The coast lacks the grandeur of South Africa’s Otter trail or Australia’s Great Ocean Road (the one with the twelve apostles), but there is a quintessential beauty to the landscape with its century-old villages. But beauty is not what the people are after. They are looking for the ghost in the rocks: fossils.
The coast is known as the Jurassic Coast, and it is a Unesco World Heritage Site. Unlike Jurassic Park, the Jurassic Coast actually belongs to the era it is named after. It stretches for 150 kilometer along England’s southern coast, from Weymouth in the east to Exmouth in the west. But it is under attack. The longevity of the ancient cliffs is short-lived. Year after year the rocks fall and the waves carry the debris away. The Jurassic Coast is in retreat. It is that retreat which is bringing the fossils.
The Jurassic coast was first made famous by Mary Anning. She lived in the early 1800’s in the small coastal town of Lyme Regis; a carpenter’s daughter from a poor family who had limited education. But she had a unique ability to find and identify the local fossils. Her finds became legendary and brought her international fame as a collector. Mary Anning was possibly the most famous fossil hunter of her time. The Royal Geographical Society (never the most progressive of societies, perhaps understandable for an organisation so firmly rooted in the past) did not accept women as members, or even let them attend meetings, and she could not publish there. Others published her discoveries: some gave her credit, some did not, and some gave her money in payment. None recognized her as a fellow scientist. Mary Anning died young, at the age of 47, of breast cancer.
Charles Dickens’ Christmas Carol was published three years before Mary Anning’s death. It is a famous story of change and redemption. It also about child poverty and a lack of chance in life. Mary Anning knew that from experience. Her background and gender became an insurmountable obstacle to a full recognition of her ability. Perhaps it isn’t just geology that plays the role of the miser Ebenezer Scrooge: scientists themselves can also do the job just fine. And expressing regret cannot be enough. To quote Dickens: “No space of regret can make amends for one life’s opportunity misused.”
Twenty years later Charles Darwin wrote an article about Mary Anning, He mentioned that her memorial window in the church has an inscription stating ‘her usefulness in furthering the science of geology’, and pointed out that “Geology wasn’t a science when she began to discover: Mary Anning helped make it one”. But change has been slow, and even nowadays science is still not a world of equal opportunity. Just one example: many geologists still publish their papers behind paywalls. It denies too many the right to read the work they paid for.
Back to the reality of the past. The most famous discoveries of Mary Anning were her ichthyosaurus and plesiosaurus fossils. In 1811, as Darwin told, she saw some bones sticking out of a cliff. (She herself said it was her brother who first found it.) Darwin described it as “Hammer in hand, she traced the position of the whole creature, and then hired men to dig out for her the lias block in which it was embedded”. This was the first known ichthyosaurus. Cuvier wrote about a later specimen: “..the ruins of a former world. It had a lizard’s head, a crocodile’s teeth, a trunk and a tail like an ordinar quadruped, a chameleon’s ribs, a whale’s paddles, whilst its neck was of enormous length, like a serpent tacked on to the body”. Cuvier’s famous fossil, still on display in Paris, was found and extracted by Mary Anning.
Nowadays the Jurassic coast is best known as a place to find and collect ammonites. The tourists we saw wandering the beaches were looking at the rocks and stones, and were trying to break them up with their hammers. (The hammers can be rented on the beach). It is legal to collect fossils from the beach; collecting from the cliffs themselves is not allowed as they are property of the land owner (often the National Trust). In any case it is better not to venture too close to some of the cliffs: there is always a risk of a rockfall. An ichthyosaurus can still do serious damage to the keen collector! (And on a serious note, fatalities among the tourists are not unknown.) The fossils will end up on the beach eventually: the tourists just have to be patient. And if that once-in-a-lifetime-find is not to be found, they can always be purchased from the fossil shops that still line the main street of Lyme Regis, in the footsteps of the fossil shop of Mary Anning. A decent ammonite is quite affordable: it can cost less than the rental of the hammer! The local museum provides an intermediate way: it allows the tourist to see some of the old fossils and their history without having to claim finder-keeper ownership. It is small but well kept, and it nicely depicts the world of the past that Mary Anning was envisaging as she extracted her fossils.
The fossils of the Jurassic Coast are notably marine in nature. The paddles noted by Cuvier left little doubt that this dragon lived in water. No Megalosaurus here! (Tyrannosaurus was not Jurassic. If that had been found here, it would perhaps have been called Tyrannosaurus Regina, in memory of the carpenter’s daughter who gave the Jurassic its world renown.) The only outliers are the pterodactyls, creatures of the air caught in the water. The first one was, of course, found by Mary Anning, in 1828.
The most amazing part of the Jurassic Coast is something not many people get to see. Most people head for Charmouth and walk east along an accessible beach with finds enough for everyone. But even better sights are on the other side. We started in Lyme Regis and turned west, towards a rocky stretch which drowns at high tide. A guide is strongly recommended. Our guide first showed us some fossils incorporated in the stones of the local houses, before taking us past the harbour and a parking lot, on to those rocks. Knowing when this is safe is a major part of the responsibility of the guide: it can be dangerous to go at the wrong time. The other responsibility is to make sure that we left our hammer at home: collecting fossils here is strictly forbidden. After a while we reach a stretch with stepped, layered platforms. We step on and off the different layers as we follow the guide. The layers dipped a bit towards the cliff on a shallow angle. One of those layers is the guide’s goal. Standing at its edge, a world-famous view appears. Every part of the surface is covered with circles, some tiny, some half a meter or more across. They are ammonites, and this surface is their graveyard. It is called the ammonite pavement.
This too is a Unesco World Heritage Site. It may not exist for much longer: the storms break it up and every year the pavement is a little smaller. The guide pointed out the recent breaks. A large fragment was saved after such a storm. It is now kept in the National History Museum in London. It should have been on display in Mary Anning’s museum. If the Royal Geological Society wants its redemption, perhaps one start could be to arrange for this to happen! This particular curtain on the past may soon close. The past can be temporary.
The ammonite pavement is a 30-cm thick limestone slab which formed 200 million years ago. The surface consists for 40% of ammonites! The area was in the subtropics (30 degrees north of the equator), when this was a shallow, at times muddy sea. The mud collected on the sea floor, a bit like Scrooge’s gravy. The ammonites which were swimming in the shallow sea would on their death sink and become buried in the mud. Some were buried complete, others as shell fragments. Look hard and you can still see those fragments in the pavement, together with tell-tale trails made by burrowing worms. The ammonite pavement formed during a time when rather little mud flowed into the water, over a period perhaps as brief as a thousand years. This allowed the concentration of ammonites to build up so high. Later the mud flowed faster again, and the ammonite-pavement-to-be became buried. The layers that we saw above the pavement were far less rich in fossils.
Why did the shells survive in the mud? In a way, they didn’t. The living shells consisted of aragonite, a form of calcium carbonate which slowly dissolves in water. But the shallow sea at times had low oxygen levels, and this slowed down the dissolution. When this happened, there was time for the aragonite to slowly change to calcite, a more stable form of calcium carbonate. The anaerobic mud protected the shells and gave calcite its chance. Often, only the bottom half of the ammonite is present in the pavement: the part of the ammonite that was covered in the mud had time to fossilize, whilst the part sticking out above the mud dissolved into the water and left us no trace.
Sometimes iron sulphide finds its way into the chemistry, and the calcite becomes replaced by pyrite. This gives a golden glow to the fossil, and in fossils shops the pyrite shells may be given pride of place. But pyrite can quickly oxidise when out of water. If such fossils are kept under high humidity, the pyrite will oxidise to iron sulphate. This is an expansive mineral: it takes up four times the volume of the pyrite. The expansion causes the fossil to fall apart. Once the process has started, the decay is unstoppable. It is best to keep the fossil at a humidity below 40% – but it is better to leave the pyrite ammonites in the shop. It is called fool’s gold for a reason.
Follow the coast
Fossils are a true ghost of Christmas past. They seem to shout at us: “Come in, — come in! and know me better, man!” wanting to show us a glimpse of life in the past. But there are other, fainter ghosts. They are shadows, and not easy to see against the landscape. They are the landscape. Geology does not need fossils to show us the past. Dead as a doornail they may be, but the stones themselves are geology’s legacy.
Not all the Jurassic Coast is Jurassic. This can be seen best from out at sea. The horizontal layers are clearly visible in the cliff, but from a distance it becomes clear that they are not horizontal. There is a bit of a tilt down towards the east. Each layer dates from particular epoch; the one above will be younger. Because of the tilt, the cliff is younger towards the east. Over 150 km, the age difference adds up enough that the eastern-most areas of the Jurassic Coast are Cretaceous, not Jurassic. (‘Cretaceous Coast’ would have had a nice ring to it.) The central areas are, true to the name, Jurassic. The western-most area is Triassic. The change from Jurassic to the older Triassic is clearly visible in the rocks. Go too far west and the fossils become rare. The region around Lyme Regis and Charmouth is best for fossil hunting.
The age of the 150-km long coast ranges from 250 millions years in the west to 65 million years in the east. For England, this long epoch was a quiet spell in its geological history, when the country was in the centre of a supercontinent, Pangea. This may seem surprising, in view of the ammonites and ichthyosaurus and ammonite fossils! The super-continent had come together in stages. One of those stages had formed Laurussia, the northern continent. Gondwana joined soon after, and completed the coming together, the Christmas family reunion. England was at the heart of the union.
Before going into more detail about the reunion, let’s first look at this surprising sea. Where did it come from? This had to do with the coming break-up. For by the time the coming-together party started, the break-up of Pangea was already in progress. A new ocean was extending across the super continent. It had started where is now China, and from there grew westward, dissecting Pangea. This was the Tethys sea. (There were several phases in its growth, but that goes beyond the story.) During the Jurassic the Tethys extended across southern Europe, into adjacent Mexico (the Atlantic ocean did not yet exist). The Tethys brought water into the centre of the continent. As the new sea grew, Europe subsided. At the same time sea levels rose, perhaps because of the growing activity under the sea which pre-saged the break-up. The combination of subsidence and sea level rise caused wide ocean bays to form across Europe, and these extended into southern England.
From the late Triassic this sea covered much of southern England. London and the Cornish mountains remained above water, as did Wales. This was the shallow, subtropical, perhaps idyllic sea that was the home of the ammonites, in which ichthyosaurus hunted and above which pterodactyls soared – no doubt nesting on the nearby land in Wales and Cornwall. The climate was seasonal; rainfall and temperatures fluctuated through the year. The closest sea shore, complete with lagoons, was in south Wales. A small archipelago had formed around Bristol. This large bay was the environment in which the Blue Lias was deposited, the grey blue layered mudstone which forms much of the cliffs around Lyme Regis. These were the rocks from which Mary Anning dug out her fossils, and this was their history.
But history itself has history and geology builds on older geology. There are more ghosts to be found. The sea had brought life into a lifeless region. In the middle of the continent, this had been a hot and arid desert, the Sahara of its days. To explore the deeper desert past, go west, to the oldest end of the Jurassic coast where the sediment of the sea ends and the red sands of the desert take over. The ancient town of Exeter is here, along the river Exe. At one time this was a Roman army base; an odd place perhaps, so far from anywhere. The town continued after the Romans had left, and in the 11th century a cathedral and a castle were build. They are still here. The castle lies on a ridge overlooking the river. The building stone was taken from the local red sandstone, laid down in the days of the desert. But the ridge that the castle stands on is not sand. It is basalt, and it is a ghost from another past.
The basalt was erupted 280 million years ago. It crops up in several places around the centre of Exeter centre and is known as the Exeter volcanics, or more optimistically as the Exeter traps. The castle ridge is a remnant of the fissure vent. It is a small remnant of a significant effusive eruption; originally the basalt must have spread much further, although in no way wide enough to warrant the title ‘traps’. And it too found its way into the town buildings. Whereas the castle and inner and outer walls of the cathedral were build from the desert sandstone, the loose material between the inner and outer cathedral walls, and the stones of the city walls, are volcanic, mined from the various local outcroppings.
Not all magmatic heat reaches the surface. The thick crust below mountains can become hot enough to partially melt. The silicious liquid creeps upward but it isn’t able to reach the surface, and instead solidifies on the way. This is granite, and it too is found here. It is no longer deep below the mountains: the long years of erosion have brought it to the surface.
A line of such granite outcroppings runs along the spine of Cornwall, from Dartmoor in the east to the Scilly Isles in the west. Dartmoor is the bleak location of the Hound of the Baskervilles: it is just north of Exeter. It has the same age as the potassium-rich basalt of the Exeter traps: it is dated to 280 million years ago. (The granite further west is a bit older.)
And this brings us to another story, twice as old as the Jurassic Coast. Just like the carpenter’s child has an echo of Christmas, there is an another echo of Christmas here, that of the child refugee. For England had first arrived as a migrant. Quoting Dickens: “the spirit within him should walk abroad […] and travel far and wide”.
The story takes us back to a time before the mid-continental desert. The world consisted of three parts. In the east was Baltica, the ancient shield of northern Europe. In the west was Laurentia, the North American continent. In between was the Iapetus ocean. To the south was Gondwana, across the 4000-km wide Rheic ocean. (In mythology, Rhea was the sister of Iapetus.)
This was when the world was coming together. First the Iapetus ocean closed, 450 million years ago, forming the northern continent of Laurussia. The collision caused a mountain chain to form. Nowadays these are the mountains of Norway, Scotland and America’s Appalachians. Gondwana was approaching. But where was England?
It was in none of these places. England had originally been part of Gondwana, part of what is now South America. A microcontinent split off and began to drift northward at some 10 cm per year. The microcontinent is called Avalonia, and it contained England. (Another microcontinent, Carolinia, may have been attached to Avalonia at the eastern end, or it may have drifted independently nearby.) Avalonia arrived at Laurussia shortly after its formation. It extended Laurentia into the future USA, added England to Scotland, and created north Germany.
Avalonia is often depicted as a long, thin sliver of continent. In reality it may have been a bit less extreme in its dimensions. The docking can lead to significant shortening of the terrane, leaving it much longer than it is wide.
While Avalonia docked, the Rheic ocean began to close behind it. It was a complex process, much less straightforward than the closure of the Iapetus ocean before. Another microcontinent had split off from Gondwana, and this arrived just ahead of the main continent. It is known as Armorica, and it plays an important part in the story. While Avalonia had come from South America, Armorica was a fragment from Africa. Now Armorica docked to Avalonia, and extended Laurussia further. Armorica added Brittany and the Channel Islands, and the tip of Cornwall. Surprisingly, Iberia was also part of Armorica, attached to Brittany. Iberia would later rotate 180 degree to its current location.
The docking was complete by 400 million years ago. The complex process left a fragmented series of mountains across Europe, from the Czech republic to Cornwall. Together this is called the Variscan orogeny, the assembling of Europe.
But this was not the end. After the merger of Armorica and Avalonia, it appears that they briefly separated again. A spreading ridge along or close to the suture formed a new ocean. One suggestion is that it resulted from the attempted subduction of a spreading ridge of the closing Rheic ocean. This temporary basin is called the Rheno-Hercynian ocean. It remained a narrow gulf, extending from central Europe westward, a bit like the Gulf of California. It lasted 20 million year; its closure caused the fold belts of Cornwall and Southwest Iberia. Continental collisions can take time.
The full closure of the Rheic ocean lasted 100 million years. The last fold around Cornwall is dated to 290 million years ago.
But the final closure was still not the end. A few million years later, the granites from Dartmoor to the Scilly Isles formed (if you’d like to know, it is called the Cornubian batholith) and shortly after this the Exeter volcanics happened. Mountain ranges which form from continental collisions have little volcanic activity, because of the lack of subduction. They do form granite, because the pressure at the root of the thickened crust is sufficient to melt the lightest component of the crust, which then move upward but never get near the surface. The suggestion has been made that a thin part of continental crust of Gondwana had been subducted underneath Europe. This melted, and it produced the vast plutonic intrusions of which the granite remnants are still present. But the Exeter volcanics are basaltic, not continental. This suggests Exeter may have benefited from the old spreading ridge of the Rheno-Hercynian ocean, now stuck below the continent.
After the final merger, a quiet phase followed. These were the dry years when England was at the heart of a supercontinent, when rain was scarce and a massive Sahara spread across the country. Like a never-ending 2020, the social isolation lasted for more than 50 million years, with only sandstone to show for it. These were the glory years of Ebenezer Scrooge, leaving us little or nothing.
A Christmas present
Among the most memorable phrases of the Christmas Carol are the words of Scrooge’s nephew: “I have always thought of Christmas time, when it has come round—apart from the veneration due to its sacred name and origin, if anything belonging to it can be apart from that—as a good time; a kind, forgiving, charitable, pleasant time; the only time I know of, in the long calendar of the year, when men and women seem by one consent to open their shut-up hearts freely, and to think of people below them as if they really were fellow-passengers to the grave, and not another race of creatures bound on other journeys. And therefore, uncle, though it has never put a scrap of gold or silver in my pocket, I believe that it has done me good, and will do me good; and I say, God bless it!” Geology is like that. It creates nations but leaves them with only faults and fossils. But in fact, in this case there was a more economically valuable legacy. It may even have been the reason for that Roman legion in Exeter.
The valuable legacy is cassiterite. It is a tin ore, and tin became a valuable commodity in the bronze age. Bronze is an alloy of copper and tin. On the map of Europe of Strabo, Western Europe is rather poorly depicted. But off the coast of Iberia, two islands are indicated, and names as ‘Cassiterites’. This must have referred to Cornwall, albeit by someone who clearly had never been there. The tin mined here was widely traded. The famous Nebra sky disk, the oldest known map of the sky, was made with tin from Cornwall. The Cornish (and western Devon) mines also produced lead, copper and arsenic. More recently, wolfram was mined. There was wealth in the ground. The mines were active until fairly recently, but are now mainly a tourist attraction. They are, in fact, another Unesco World Heritage site.
The ores formed from material brought up during the final days of the coming together. It was not only in Cornwall. The ancient mines in the Czech republic, now known as the Erzgebirge (Ore mountains), also date from this period. The most famous location here is the Joachimsthal, a valley where the metal used for minting coins came from. The German word for valley is ‘thal’, and this became a byword for coins. It morphed into the old word ‘daalder’, and from there into the more familiar ‘dollar’.
The wealth of this Christmas present thus ended up in the people’s pockets. And it left us with yet another ghost of the past: an echo in our language that goes back to the halcyon days when the Rheno-Hercynian ocean closed. Exeter is the tip of an iceberg which brought us hidden riches.
Mapping of the ore veins in Cornwall has shown that they only occur above the crust of Armorica. The part of Cornwall and Devon derived from Avalonia lacks these deposits. But the intrusion which brought up the riches happened only after the two parts had come together. It should not have known about suture. It appears that the deep crust of Armorica must have been different from that of Avalonia: one contained these riches, the other did not. After all, they came from very different parts of Gondwana.
And this brings us to the end of the story. It has taken us from the Jurassic Coast to a much earlier time, when Europe was assembled. Our land has a hidden past. It has left us ghosts in the landscape, wanting to teach us, if we know how to listen. We are all children of an ancient land, living in the past, the present and the future. Our self-found ammonite may not be as impressive as those in the shops, but what a story it has to tell.
Albert, December 2020
More on the story of the English Channel
From all of Us to all of You
Merry Christmas and Happy Holidays!