SHROPSHIRE ROCKS!

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Devonian

The Devonian of Shropshire, previously known as the Old Red Sandstone, 409-363 million years ago

Imagine an enormous barren, burnt continent braided by rivers. This is what you would have seen 409 million years ago in Shropshire. Still south of the equator in the tropics, the land was drenched by rains, fuelling the rivers and their endless attempts to wear away the rocks.

Most of the rocks we find of Devonian age are red sandstones, mudstone and siltstone, typical river sediments. In fact, the continent was thought to have been so red that geologists have called it the Old Red Sandstone Continent. These river sediments built up to enormous thickness’, four kilometres thick in places. The rivers were busily wearing down the newly formed Scottish Caledonian mountain ranges created by the collision of north and south Britain. They flowed down the relatively low lying south of the country, depositing much of the debris they carried over Shropshire and the surrounding area.

The red colouration of the sandstone gives geologists an immediate clue that the rocks we find of this age were deposited on land. Most rocks contain iron. When this iron is exposed to air and water, it goes rusty, so when you see red rocks they have literally rusted over time. Rocks that have a lot of iron in them but haven’t had a long time exposed to the air and rain are usually an apple green colour, a big difference from rusty red.

While the rivers were continuing their ceaseless task of eroding the land, plants and animals began to look at this new environment as a potential home. The first plants made it on to land in the Devonian, simple plants started to colonise the continent turning it from red to green. These were not plants as you or I would think of today. There were no flowers or trees only mosses, ferns and fungi. Once plants had made it onto land animals were soon to follow.

Fishes appear in these river sediments, quite different to the sleek shapes we think of today. Heavily armoured with thick tough scales and heavy plated heads the fossils of these prehistoric fish can be found in the Devonian rocks of Shropshire. Their modern day relatives, the Coelacanths (pronounced see-lo-kanth) were thought to have been extinct, but examples are still occasionally found in the nets of deep-sea fishermen. These fish have thick fleshy fins, an adaptation that would make the transition from water to land much easier. During the Devonian, the first amphibians decided to stretch their newly found legs on dry land and stayed. These first land animals are the ancestors of all land dwelling creatures found today.

This huge continent, created by the collision of three smaller continents continued to rumble and shift throughout the Devonian. Almost all of Britain from the far north of Scotland to the South west of England was lifted out of the sea. Eventually the rivers would erode the land down to sea level over Shropshire and again the sea would cover the land.

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