The Great Divide


Hello and welcome to episode 1 of the Natural History podcast. The great Divide. England and Scotland.

It is not the obvious divide. Tea or Iron-Bru.  

No there is a bigger divided, one so vast it split the UK in two and very few of us even know anything about it. And no, it’s not Brexit either. 

What is it then? Well lets dive in to North West Scotland. To a place called. Knockan Crag. If there were no sign posts or even a visitor centre, 99% of people would pass by it. After all, it is a crag. Big walls of rock don’t offer much in the way of entertainment. But to a few educated eyes this crag represents ground breaking ( and quite literally) evidence . Much debate has arisen from this crag as Knockan Crag is the perfect example, of the Moine Thrust. Other crags are available.

So as a modern day visitor, what would you see? At first the obligatory car park and toilets, which will be post glacial deposit. Then we would over suit and boot, ice axe and campons, for the small  walk up to the crag. There, we see two different coloured rocks. A white band of limestone, sitting below a black band of metamorphic rock. Limestone is what the White Cliff of Dover are made of, basically dead sea creatures. Metamorphic rock is a stone that has been super heated and squashed and as a result has been turned in to a different type of rock. Hence the name metamorphic. 

So we have two these two different coloured rocks, white limestone and black metamorphic.  So what justifies this crag having its own car park? Well It’s because the rocks switched place. The older black metamorphic rock is sitting on top of the younger white limestone.

And this anomaly is being played out over a 100 miles along north west of Scotland. 

Something, you may have figured isn’t right. Something is going on, its breaking the rules and we can’t blame the dwarves. That would be a very middle-ages answers and we are trying to use science. Gone are the days of easy answers and exciting tales.

Now logic dictates that the longer something has been laying around, the further at the bottom of the pile it is going to be. Take laundry. The clothes you wore last week are at the bottom of the pile, and the clothes you wore yesterday, are at the top. This also happens with rocks, the newest rocks are closer to the surface and the older rock, below. This principle is call uniformitarianism. 

However that is not the case and in the 19th century the newly created subject of Geology was trying to figure this mystery out. Two of those detectives where 

 touring the north west of Scotland in 1850. They where called Sir Roderick Murchinson and James Nicol. They where examining the Moine Thrust, or our upside down rocks if you prefer. along its 100 mile expanse. 

We don’t know if the pair visited Knockan Crag, probably not, as there was no car park back then. But Murchinson did comment upon it and drew the conclusion that, as the black metamorphic rocks sat on top of the white lime stone. The limestone must be older, because of uniformitarianism. Which back then was a powerful argument, after all, how does a hundred mile band of rock switch places? For Murchinson, this was case closed, feet up and fire on. 

However his traveling companion James Nicol, wasn’t so sure.  He noticed that the rocks where distressed. Cracked, brittle, discoloured. Instead, Nicol, came up with a alternative, rather, out-there theory. 

He believed that the Dark metamorphic rock, was actually older that the white limestone, and at some point they switch places. He reckoned, the metamorphic rock cracked and the one half was pushed up and over the back of the white lime stone. Any rock on top of the now piggybacking older metamorphic rock would have been eroded away by the weather, resulting in what we see. 

So to clarify with cake. It would be a bit having a sponge cake. Cutting it in half and then placing the one half on top of the other. Finishing it off by eating the top layer.

How? Murchinson said from his fire side armchair.  Well Nicol, didn’t really have an explanation for how this happened, He just had a theory on what happened. Unfortunately, for Nicol 

Muchinson was an aristocrat with lots of power (and probably coal). So Nicol’s theory was basically binned. 

It was only after the death of Murchnson in 1872, did Nicol’s out-there idea began to be discussed more openly. Yet it took another 34 years, for Nicol to be vindicated. 

In 1906 the A-team of geology, were sent to Scotland to sort out the matter for once and for all. And they did, proving Nicol’s theory was correct, and this, earn’t him a huge amount of respect. Which was a shame, as he died 27 years earlier. But he was right! he explained ‘what’ was going on. It would not be until the 1960’s however, that we would have a how it happened. 

Fossil difference

“How, I don’t know how you managed that” my father declared. We where standing in the middle of a woodland in Shropshire. I had lead my parents up a hill,

 in to the woods, down some well maintained foot paths and then took a 90 degree turn off in to the wilderness of Shropshire, England.

I had no idea where I was heading, but down an incline hoping to stumble upon a stream or some exposed rock and I fund what I was looking for. A very shallow narrow stream, perfect. So I knelt down by the water and started pulling apart the soft stone.

The rocks where black shale, basically fossilised mud. The area we where in was once a shallow sea bed during the Silurian period, which was about 440 million years a go. That’s right, you could once had a beach holiday in Ludlow, it would been a darn sight warmer too as the country was 30 degree south of the equator. About where South Africa is now. 

But back to the story. I started pull the rocks apart, and with-in a few minutes. Victory! I had discovered a Trilobite. Leading my dad a from of be-musement, to surprise and back to the opening statement. A Trilobite is basically an extinct prawn and they where very successful. Evolving in to all sort of shapes and sizes. 

This Trilobite is nothing special, it was just the tail end and quite common. In fact so common, it has been found in Wales, Europe and even as far as Morcco and Libya. Yet if we travel 6 hours north, over the boarder in to Scotland, nobody has ever found that type of trilobite in Scotland ever, and dare I say never will. For the Scottish Trilobites of Cambrian to Silurian( Which is about 550-440 million YAG) are very different from those in England and Wales. You won’t find a Scottish trilobite else where in Europe, these specimens only occur there and … Canada. Yes, the  Scottish and Canadian Trilobites are of the same family. It seems even 440 million years a go, the Scottish wanted nothing to do with the English. 

Maybe there was some natural barrier in the way? Or Europe was a shallow sea and the Scottish trilobites liked deep water. Many ideas where thrown out there, but to the early pioneers of this subject, they just had no idea. It is just a crazy idea. How does an inch long Trilobite manage to cross the Atlantic? and then having reached Scotland, just give up on visiting Europe? Yes, there was a lack of Iron-Bru, which may have played a role, but I doubt it.

It wasn’t until the 1960’s did we really start to put the pieces of the puzzle together. For the Moine Thrust and the tale Trilobites do have a common answer. Plate tectonics.

The idea that the worlds continents move around, arbite very slowly.

I have already mentioned you could have had a beach holiday in Ludlow 440 million years a go and that is the result of plate tectonics. Continents moving around. England, Wales and southern Ireland where all on one one thin micro content called Avalonia. Scotland and Northern Ireland where connected to the much larger American plate. Newfoundland to be exact. England and Scotland used to be thousands of miles apart. You could have stood on the northern English coast, built a sand castle and swam in the now vanished ocean of the Iapetus.

 With Scotland not even on the horizon. And this is what explains our trilobite mystery. We can find the same trilobite in Canada and Scotland, but not in England, because there was an ocean in the way. and the same goes for the Trilobites from England, Wales and southern Ireland.

Rivalry between England and Scotland is ancient and it dates back 410 MYA when the two countries first came to blows. The two scrum halves started pushing against one another. 

The force was so great, it squeezed the land, and as a result the Lake District and Snowdonia mountains ranges where crated. I say Mountain ranges, as back then, they where as tall as the himalayans are now. In Scotland we see the creation of the Highlands. This event is known as the  Caledonian Orogeny.  An Orogeny is not a night out in Glasgow, but a technical name for the creation or building of mountains.

The collision of the two continents also explains the moine thrust. As the two came to blows, the ground started to become compressed and ripple. The force was great enough that further north in Scotland, the ground began to crack and slowly force the black metamorphic rock over the top of the White lime stone, creating our Victorian puzzle. 

And if you look now, the political and geological boundaries of England and Scotland are the same. It is remarkable that 410 MYA the boarder was created. 

So when you next cross over the border between England and Scotland, remember,  you are crossing the political boarder, an ancient ocean and to a different continent.

References/ further reading

The Geology of Britain, an introduction – Peter Toghill. Published- 2000

The Hidden Landscape – Richard Fortey. Published -1993

Published by Naturalhistorypodcast

I'm passonate about natural history, an interested that as sparked at university whilst studying it for a few modules. Now I want to introduce as many people as possible to this exciting world.

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