Christmas is a time of hope. The days may be dark and dismal but the corner has finally been turned. The sun is beginning its journey back to the north, and from here on the days will get lighter and longer. The new year has started. In Christianity, it is the birth of a baby that signifies this change. The celebrations are of course older than that. In northern Europe, the winter season started mid November with the darkest days of the dying year still ahead. It was a frightening time to wander in the primeval, haunted forests, looking for food and firewood. There was a reason why spirits needed banishing. In some countries, this start of winter is still celebrated on 11 November, whilst in the US it gave rise to halloween. The darkness of halloween and the light and hope of Christmas punctuate the winter season. It amazes me that some who wish to ban Christmas are happy with halloween. Pratchett would have pointed out that you can’t have one without the other.
Seeing a robin (the European, dainty variety, not the much larger and more self-confident American one) in the snow as a depiction of peace is a uniquely British fantasy: this is a bird that will not tolerate other robins, and will fight its brethren to the death in order to keep possession of the garden and its food supplies. It is the bull of the bird world. It does not believe in Peace on Earth. Only we do. Similarly, seeing a snow-covered Christmas landscape as being romantic and homely is a recent change. In the past, snow and ice meant cold and hunger. We have shaped snow into a symbol of peace. But don’t underestimate snow. It doesn’t only do peace. It is also quite capable of destruction.
The prime example of snow power is Iceland. Some 10 per cent of the country is covered in glaciers. They impact the land far beyond the glaciers themselves. Every time an Icelandic volcano heats up, the ice gets it. It melts from the bottom, and huge meltwater lakes collect underneath the glacier, where no one can see them. Eventually the glacier can no longer contain the lake: the ice barrier begins to lift, the water finds an exit and begins to flow in ever increasing quantities. A jokulhlaup has started. These floods can be impressive, but also very destructive. The coastal ring road invariably needs rebuilding. A look at the map of the south coasts of Iceland shows the areas of devastation. These were once rich farm lands, but as the glaciers grew in the Middle Ages, the jokulhlaups came and destroyed them. One can imagine what Iceland would have looked like at the end of the ice age as the main glacier that covered 90 per cent or more of the land began to melt. The jokulhlaups must have torn the land apart.
Jokulhlaups are not confined to Iceland. Its combination of ice and fire is rare, but glaciers can melt from other causes. The climate change at the end of each of the ice ages is a prime example of glacial melt. The sequence of events is somewhat predictable. The ice melts, and the water collects at the lowest accessible place. The way out remains barred by the big glacier, and a lake forms behind the ice wall. If things go well, the lake will slowly trickle out. If things don’t go well, it will patiently sit there until its chance comes. One day the ice dam will give way, and the lake will suddenly empty. This happened in North America, more than once. One time, a huge in-land sea emptied towards the west. The Washington Scablands still carry the scars. Another time, a meltwater lake the size of the Great Lakes combined emptied eastward, and the huge amount of fresh water made the Atlantic Ocean notably less salty. That turned out to be a bad idea. The fresh water stopped the overturning of the sea water which normally happens in the North Atlantic. 30% of the winter heat in Europe comes from this overturning. Within just a few years, Europe was thrown back into the depth of the ice age, just at a time when the young humanity had spread north again, into the new land. England became fully depopulated in this Younger Dryas. This single flood, which may have lasted weeks or months, set the world back by a millennium.
But before you conclude that North America has had all the fun, such floods have happened elsewhere too. At the time they happened, the sea level was much lower than it is now, because so much water was locked up in the immense glaciers. At times the sea level was more than 100 meters below the current one. And so the floods happened on what is now the bottom of the sea, safely submerged. This hides the evidence, and in any case, who looks for massive floods under water? Only a mad scientist would do that. But now those mad scientists have found something different: a jokulhlaup to end all jokulhlaups. And it happened, of all places, in England.
The great British break-off
500 kilometer long, 100 meter deep, and between 30 and 200 kilometer wide, the English Channel separates the English South Coast from France. It used to be called the British Sea, as far back as the Roman era. (The word ‘Channel’ was introduced into general use by Shakespeare. Of course, the French have their own name: they now call it La Manche.) This deep barrier between France and England is an essential part of the English psyche. They wouldn’t be bothered by a land bridge to Denmark or the Netherlands. But France is a step too far. Peace on Earth is easier to maintain if there is some distance. But how did the Channel form and why is Calais still so close? Why is Britain an island?
It was long before Brexit. Britain had been part of the European Union for as long as Europe had existed. The first attempt to break away was when the Atlantic Ocean began to form, and a rift developed from Greenland southeastward. The rift split Scotland from Scandinavia, and traveled south, in the process creating the North Sea basin. But it failed. The north had become separated, but the south remained solidly fixed to the continent. The rift gave up: a new one developed further to the west and this one became the Atlantic Ocean. Things could have been so different. If the North Sea rift had succeeded, England, Scotland and Ireland would have been part of the North American continent. Now, they remained connected to the rest of Europe.
In the west, the break between Cornwall and Brittany had long existed, the result of a rift in the Permian which re-activated several times. The rift came with volcanic activity, forming the Exeter volcanic series (also known as the Exeter traps). The English Channel Fault (in fact a series of faults) extended further east over the aeons, up to the Isle of Wight but no further. The basin it had created slowly subsided. The Chalk Sea came in and covered a large area of Europe, depositing thick layers of chalk. Eventually, the land rose up again. But the Channel west of the Isle of Wight remained a basin. However, this left a fairly wide land area between the Channel and the North Sea which included the chalk hills, running along England and continuing into France. The chalk formed a 200-meter high backbone to the land bridge.
The map show roughly what the region was like a million years ago. The North Sea, the scar of the failed proto-Atlantic, was now a bay. The south of England bordered the proto-Channel, another sea arm. In between these two seas was the broad land bridge connecting Kent to northern France and Belgium with its chalk core.
Now the ice came. The ice ages began with a long period of slow cooling. Suddenly, a critical level was reached and what had been a stable, albeit chilly climate became wildly unstable. Temperatures plummeted, and the glaciers came. They lasted for 100,000 years: then the ice melted and life recovered in what became a balmy period. But the heat of summer lasted only 10,000 years, and winter returned with a vengeance. Again the ice ruled, seemingly forever, with plenty of halloween but Christmas was a ghost of the future. Finally, summer did return, again only for a brief interlude. Four times the cycle repeated. The ice ages were not uniformly cold: there were variations in climate, and the ice waxed and waned. But the ice fully disappeared only in the brief, much warmer interludes. We are now in such a warm interlude.
The 3-kilometer thick glaciers locked up tremendous amounts of water. That water was borrowed from the oceans, and in consequence sea levels fell. During the depth of the ice ages they were lower by as much as 120 meters. Now England really was an integral part of Europe: most of the North Sea was dry land, and even the English Channel was a sloping plain extending to beyond Ireland. As the ice melted, the sea rose again. But it could not break the barrier that separated the seas north and south. At least, not without help.
When ‘the ice age’ is mentioned, it normally refers to the most recent one, which lasted from 110,000 to 12,000 years ago. It is the one we know most about; it is called the Devensian in England (other countries use different names). There were three earlier major glaciations during the quartenary: the Wolstonian (200,000 to 130,000 years ago), the Anglian (425,000 to 480,000 years ago) and the Beestonian (620,000 to 675,000 years ago). Again, these names are used in England only: every country has their own name for them. Each ice age cleared the landscape of most of the signs of the earlier ones, and therefore we do not know nearly as much about the preceding ice ages.
We know that the most severe period was the Anglian. Its glaciers came furthest south, and therefore left signs in the landscape that still survive today. In the UK, the Chiltern hills, north of London, mark the furthest extent of the Anglian glaciers. The Thames was forced out of its bed at St Albans, and it found a new bed further south, paving the way for London. In the west, the ice reached the Scilly Islands.
A river runs through it
Almost 50 years ago, bathymetric mapping of the Channel revealed a long, straight depression running west off Normandy. Called the Hurd valley, it lies 50 to 100 meter below its surroundings. The name is old: it dates from the 18th century when the lowest point was called Hurd’s Deep. The deepest point is 180 meter below current sea level. The straight channel was quickly shown to have been shaped by erosion, and was identified as a river channel. But which river?
There are two current rivers which could have fed the Hurd Valley, and both flow from France. These are the Seine and the Somme. Neither are particularly large. Larger rivers, the Rhine and the Thames, flow into the North Sea instead. It was a bit of a mystery how these two small rivers could have eroded such a deep channel. The depth provide a further enigma. River valleys form on dry land. The Hurd valley is too deep for this: it would have been a sea arm. How could a river erode a deep channel under the sea?
As the bathymetry became more accurate, it was found that the Hurd Valley was at the end of a series of stream beds, carved into the bottom of the eastern Channel and starting at the Dover Straits. This became known as the Channel River. One of the streams lines up with the Seine, and seems to delineate the ice age extension of this river. The other stream beds came from the Dover Straits, but without a clear connection to any current river.
An interesting aside, the reason that the bathymetry improved so much is GPS. Of course GPS does not work below the sea: sea water does not transmit radio waves. So how could GPS possibly help? It turns out that the dominant source of error was the position of the boat. The depth is measured by sonar, using the length of time before the echo returns. That gives the distance from the sonar to the bottom. The sonar is mounted on a boat, which goes up and down with the tides and rolls with the waves. That gives an uncertainty of several meters. With GPS, the position of the sonar itself can be measured to centimeters, even correcting for the effect of waves. It opened up an underwater world.
The sonar revealed the underwater riverscape off the English south coast. Amazingly, the mapped surface was free of sediment: the bottom of the Channel showed bed rock. The strong tidal currents in the Channel appear to have swept out the sediments. The river bed, carved in bed rock, dwarfed anything known before. At 15 kilometer wide, this was by far the largest European river known. It didn’t run straight: there was a bit of a meander, with a branching appearance. Beyond the Isle of Wight it fed three parallel arms which entered into the proto-Seine and from there into the Hurd Valley, but much of the water appears to have gone on, but in an indistinct channel as if in a large body of water.
Within the main streambed, there were a number of higher areas, clearly islands. The shape of these were like tear drops, with the tip in the downstream direction. Such a shape forms when there is a high rate of erosion: the shape minimises the drag and thus stabilises the island against the water flow. The shape in such a river is rare. It does occur in rivers that deposit sediment but these islands are not made out of sediments: they are made of bed rock. The shape is found in stream beds on Mars.
The islands are surrounded by ~1–2 m deep, 200–500 m wide and 8–15 km long scours into the channel floor. These scours form flow lines which curve around the islands. To create the islands required that 30 meters of bed rock was eroded away, something no current European river is able to do. The shape of the islands, the scour marks, the size of the river and the depth of the erosion all point at an origin in a massive flood. The size of the river suggests a flow rate of at least 106 m3 per second. This is four times the rate of the Amazon, and brings it in the ‘mega flood’ domain (in fact the term ‘mega’flood refers to a flood rate of a million cubic meters per second or higher).
Several river beds coming from the English coast, extended during the ice age, terminate at the deep river bed in the floor of the Channel. But the termination points are ten meters above the floor of the river. This is typical for situations where the river suddenly deepened, leaving the tributaries no time to respond by deepening their channels. This is a second argument for a sudden formation of the Channel river. It was formed in a catastrophic flood.
The stream bed can be traced to near the Dover Straits, but there they change. They terminate in a number of deep depressions. In fact these depressions were known. During the building of the Channel tunnel, a deep channel called the Fosse Dangeard was found, but at the time it was not understood. The bathymetry now showed a series of elongated, bowl-shaped depressions, up to 10 km long, 5 km wide and 120 m deep. They are located near where the Channel is at it narrowest, sandwiched between the white cliffs of Dover and the equally white cliffs of Calais. Apart from the size, these were the kind of plunge pools that form underneath waterfalls. But to form 100 meter deep holes in bed rock would have required waterfalls 200 meter tall. In fact, the height of the white cliffs would do nicely.
Everything was now coming together. At one time, water was spilling over the land bridge which connected England and France. This was during the ice age when the water in the Channel was 100 meter below its current level. But the water in the North Sea, on the other side, was 100 meter above its current level. Something had gone wrong in the North Sea.
The dammed sea
The ice had come and a huge glacier was sitting over the north of England, stretching to Denmark. The big rivers had found good flow beds in the dry North Sea basin, flowing north and emptying into the deep ocean, off Norway. But now this became blocked and the rivers had no way out. An inland sea was filling up. Northern Europe is an area of low plains, and the sea would have been able to spread, but the ice had come far and blocked off all escape routes. The only way was Essex.
The modern flow rate of the Rhine is about 100 km3 per year. At one point during the ice ages, the available sea area for it to flow into was reduced to perhaps 10000 km2 (this may be a low estimate which excludes the flooded parts of the Netherlands and Germany). The sea water rose by 10 meters per year. It quickly found the lowest points on the land bridge, and began to overflow. A river now drained a sea. On the far end of the land bridge, a waterfall formed. Initially, it was tall but not overly voluminous, as it only needed to balance the flow rate of the Rhine. And the Rhine may not have been as large as it is now: the ice age was a dry period. Things remained in balance, although large areas got their feet wet and a river now covered the bottom of the Channel.
But now the ice age began to wane. Temperatures rose a bit, and the glaciers began to melt. But the melt water did not yet have anywhere to go. The route into the deep sea off Norway was still blocked by the ice. The water volume entering the North Sea basin increased dramatically with the melting ice. Thus, the flow rate across the land bridge increased further and further. And chalk is not the hardest material to begin with, and wet chalk is even weaker. The waterfall cut back into the chalk, and finally reached the North Sea basin. The breached barrier collapsed and a whole sea drained instantly. Over 1000 km3 of water hurled to the exit. The chalk didn’t stand a chance.
At a flow rate of 1 million cubic meter per second, the sea drained in 10 days. (The flow rates probably peaked even higher, but our estimate for the drained volume may be a few times too low so this compensates.) At first, like in any jokulhlaup, a sheet of water covered the land, in this case the English Channel. The flow started to cut deeper and deeper channels into the bed rock, which formed the Channel river which we have now discovered. The torrent found the Hurd’s Deep and thundered into it, deepening it into its current channel.
When it was all over, the landscape had changed forever. Gone was the land bridge. There now was a deep gap where once the chalk hills had connected smoothly. The edges of the gap were the white cliffs. But Britain was not yet an island. Sea levels were still very low, and the North Sea basin was drying out again now that the inland sea had drained. Soon, the rivers would resume flowing north across its marshy plains. Perhaps 10,000 years later, the sea had risen far enough to come back in. In flood after flood the land was conquered, and now there was nothing to stop it at the Dover Straits. This was when Britain finally stood alone, separated from the continent.
So when did all this happen? The answer is, we don’t quite know. It wasn’t the most recent ice age. A date 450,000 years ago has been suggested, in the Anglian when the ice had come furthest.
So Britain has had close to half a million year to develop its unique culture. The melting ice isolated and shaped the country; the scars of that event have been hiding underneath the English Channel ever since, only to be discovered in our age of GPS. We now know the incredible magnitude of the flood that separated England from France. In the seemingly eternal winter of the ice age, it was a halloween flood worthy of the occasion. But after halloween comes Christmas. The ghost of Christmas past turned an unimaginable disaster into the making of a country. Exiled on an island, the isolation gave rise to the most outward looking and culturally diverse nation the world has seen. No other nation could have turned a fighting bird on frozen snow into a symbol of peace on earth, set against the native Scrooge. The inscrutable British mind, with its cultural creativity, perpetual self-doubt and self-assured superiority, was a gift from this ice age calamity.
Iceland is the land of the jokulhlaups. But England had its own Jokulhlaup, a megaflood that re-shaped the country. Ice melt caused a calamity that not even Jurassic bed rock could withstand. On that Christmas card, labelled Peace on Earth, perhaps that robin is not the only deception. The snowy landscape itself hides a killer. Volcanoholics know well that peace does not come the Earth. Peace is humanity’s answer to a far-from-peaceful world around us. It is not about how the world affects us. It is about how we respond.
This post drew almost exclusively on the work by Jenny Collier: A megaflood in the English Channel: Astronomy & Geophysics, Volume 58, Issue 2, 1 April 2017, Pages 2.38–2.42, https://doi.org/10.1093/astrogeo/atx062
As I am writing this Santa Claus is making his way around the world delivering presents, covering a complex merging of traditions. There have been many calculations on how fast the reindeer sledge needs to travel in order to visit the entire world. The energetics of the journey are considered rather less frequently. The reindeer get their energy from carrots left out by the expectant children. Father Christmas himself gets mince pies to eat (and a certain amount of alcohol, leaving a whiff of suspicion of driving a sledge while under influence). At the speed the reindeer are traveling, air drag is the main drain on energy. The required speed is 600 miles per second. Taking into account the size of the reindeer and sledge, and assuming they fly aerodynamically (legs tugged up behind) and supersonically, the power required to counter the drag is 10 terajoules per second. In terms of carrots, that makes 60 million carrots per second or 6 trillion carrots over the entire flight.
Now mince pies have a 6 times higher energy density than carrots, explaining why Father Christmas is slightly bulky. So it would be much better to feed the mince pies to the reindeer. That would leave the carrots for Father Christmas, who would lose some of his surplus weight and as a side benefit gain a more aerodynamically favourable shape.
How does this compare to the Channel flood? The total energy released by the water falling down into the Channel was around 200 pentajoules, which could have kept the reindeer going for about 5 hours. Or alternatively and perhaps less in the spirit of Christmas, it could have powered the entire UK electricity grid for about 2 months. Assuming that the average person pays around 1 pound per day for electricity, this makes the Channel flood worth 4 billion pound. To end this post on a high note, now we finally know the monetary value of separating the UK from Europe.
And a final warning to Father Christmas and the reindeer: be careful with breathing while flying in the open air at altitude. There is a lot of ash and sulphur around from the recent explosion at Bezymianny.
Albert, 24 December 2017