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Carboniferous

The Carboniferous 363 to 290 Million years ago

The Carboniferous was a time of extreme transformation. The huge Old Red Sandstone continent had been eroded down and once again the sea covered the area that would become Shropshire. Britain was at the Equator; shallow seas covered much of the planet, laying down huge thicknesses of limestone. Limestone is made up of a mineral called calcium carbonate, which many sea dwelling creatures use to make up their shells and skeletons. When these animals die, their shell fragments and hard parts then form layers of calcium carbonate, which becomes limestone over time.

Limestone can also be made by chemical means, without relying on creatures extracting it from seawater. Have you ever tried to grow salt crystals by leaving a glass of very salty water on a warm windowsill? As the water evaporates, the salt crystallises on to the sides and bottom of the glass, leaving a thick crust of pure salt. Limestone can be made in a similar way, but on a much bigger scale. Warm shallow seas are continually losing water into the sky through evaporation. This means that that there is less water available to hold all the dissolved salts, including calcium carbonate, so just like the glass of salty water, crystals of calcium carbonate start to form. Thick layers of Carboniferous limestone cover much of the country. The North Pennines are famous for their limestone pavements, which means that during this early part of the Carboniferous, most of Britain was covered by warm, shallow seas.

But as we have seen so often in the history of our planet, these peaceful scenes would not last. Once more the forces of plate tectonics would bring continents together creating mountain chains and upheaval. Much of Britain was pushed up out of the sea again, the south of the country would experience folding and faulting once again, while Shropshire remained virtually untouched. Occasional pulling apart of the crust allowed molten rock to break through and create thin lava flows. There were mountains to the north of us, worn down by huge rivers transporting their cargo of mud and sand to the sea where they built up enormous deltas, much like the modern day Mississippi and Nile rivers do today. This new soft land would rapidly become colonised by plants, with thick fern forests springing up creating lush tropical swamps. The plants that grew on these deltas and around the lagoons were not plants that we would easily recognise today. They were giant relatives of club mosses, tree ferns and horsetails, all plants that survive today but in a much smaller form.

Throughout the Carboniferous, global sea levels changed dramatically. Part of this was due to the creation of a new supercontinent, but it may also have been due to an intense period of glaciation. Ice ages come and go itís not all ice all the time. Because Britain was at the equator, we donít see the direct effects of ice, just the changes in sea level that it brings. During an ice age, more of the planetís water gets locked up into the ice caps so sea levels drop. When the ice caps melt again, sea levels rise. Carboniferous Shropshire rocks show a number of these rises and falls. Low sea levels are represented by the delta sands and layers of coal formed from the remains of dead plant material. Higher sea levels mean that these deltas were submerged beneath the sea and thin layers of limestone were deposited on top of the sand and coal seams. This is a classic Carboniferous cycle of sandstone, coal and limestone, repeated over and over again until Shropshire was lifted completely out of the reach of the sea.

The Carboniferous earth would have seemed very strange to us, in fact it might even have been harmful to our health. Oxygen levels in the atmosphere were much higher then, maybe over 30%, compared to 21% now. This made the air much thicker and richer and enabled some quite strange animals to evolve. Flying creatures were perhaps the weirdest, giant dragonflies being among them, the biggest of them having a wingspan of 75cm, quite a bit larger than our modern day insects. Reptiles also evolved during the Carboniferous, leaving their footprints in the rocks that are now just outside Bridgnorth.

By the end of the Carboniferous, Shropshire had crossed the equator continuing on itís northwards journey and was right in the middle of the biggest continent that our planet has ever experienced.

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