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Silurian

The Silurian 439-409 million years

439 million years ago, Shropshire was taking its Caribbean holiday. After the global ice age at the end of the Ordovician sea levels rose once more and Shropshire is famous as being one of the best places to study this change. Geologists call rises in sea level marine transgressions, while falling sea levels are marine regressions. You can track the path of this transgression across the county and into Wales using different types of fossils as an indication of how deep the water was at any given point.

Brachiopod fossil, Wenlock EdgeOnce the marine transgression was complete, the stage was set for one of Shropshire’s finest moments in geological history – the creation of what would become Wenlock Edge. Patches of coral reef sprang up in this warm shallow sea, creating a framework for other animals to live in. Just as coral reefs today are teeming with lots of different kinds of animals, so were these Silurian reefs. Corals created the skeleton of the reefs with crinoids (sea lilies), brachiopods (shell fish), sponges and trilobites all living on, in or around the structure.

The rocks of Wenlock Edge that show these fossilised reefs were considered so perfect that they became the standard that all other reefs of this age were compared against. This slice of geological time is actually called the Wenlockian, and this isn’t the only thing that the Silurian in Shropshire is famous for.

Ludlow also has a namesake in geological time. Ludlow was the place where one of the most famous figures in the history of geology came to study Silurian rocks. Sir Roderick Murchison studied the fossils he found around Ludlow and discovered their wonderful secrets. This part of Shropshire was located in a shallow tropical sea, near the edge of a large continent, out to the west lay the deep ocean.

Trilobite pygidium, Wenlock EdgeThis shallow sea was prone to violent storms, which regularly churned the sand and created bands of shells that were washed up by the waves. The beds around Whitcliffe were laid down in slightly gentler conditions, allowing worms and all manner of burrowing creatures to destroy all the layering in the sands and mud. The frequent earthquakes did not help either. As they were so close to the active Church Stretton fault, the sediments were being continuously shaken, causing them to slide and slump into wonderfully contorted layers.

Around Clun, these movements along the fault dictated much of the sediments that would later make up the rocks here. We find mud and siltstone around Clun, representative of deeper water conditions. Occasionally fragments of Wenlock limestone or Whitcliffe limestone are found in these rocks which suggests that sometimes the earthquakes were so severe that they sent cascades of these sediments down into the deeper water.

Much of the activity along the Church Stretton fault was due to the closure of the ancient Iapetus Ocean. The gap between Scotland and southern England had closed, and by the end of the Silurian the ocean would have completely vanished. As the two landmasses collided the rocks were bent and folded, fractures in the earths crust released the pressure that was building up. Have you ever looked at a map of the British Isles that shows all the hills and valleys? Next time you see one look closely at it and you might just see a pattern there. From the Scottish Lochs to the Stretton hills you can see lines of hills and valleys that run from the north east to the south west. This is the lasting legacy of the closure of the Iapetus Ocean nearly 400 million years ago.

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