SHROPSHIRE ROCKS!

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Jurassic

The Jurassic 208-144 million years ago

In Shropshire, we don’t have many rocks dating from the Jurassic Period. Those that we do are found in the North of the county around Prees. What these rocks tell us is that like most of the rest of the country Shropshire was submerged beneath a shallow sea. These early Jurassic rocks are stuffed full of sea creatures called Ammonites and Belemnites, which lived in a similar way to a modern day Nautilus. They would shoot jets of water behind them to shoot through the water at high speed, evading predators.

The Jurassic has become famous for its land and sea monsters. Giant Plesiosaurs patrolled the oceans, feeding on fish and crustaceans, while enormous sharks were prey to even larger hunters. The waters of the Jurassic seas were no place for a swim.

On land, true dinosaurs had evolved, like the predatory Allosaurus and the gentle plant eating Iguanadons. These ancient beasts roamed lands of conifers, rivers and swamps. Although we don’t find any of their remains in Shropshire, it is possible that dinosaurs did walk the plains of our county. During the Jurassic, the British Isles were a series of small islands surrounded by shallow sea lagoons.

Jurassic rocks outcrop all around the country. The Isle of Wight is famous for its dinosaur bone beds and Lyme Regis is an ammonite hunters paradise. Closer to home, the limestone of the Cotswolds with its distinctive creamy colour tells a different story. Tropical lagoons surrounded by huge sandbanks create an unusual feature – oolitic limestone. Oolites are little round balls of limestone that build up around fragments of shell or sand. The motion of the waves rolls them backwards and forwards, slowly growing in size as more limestone is added.

The super-continent of Pangaea had broken up and the sea flooded in. The bombardment of the earth by large meteorites continued into the Jurassic as did an unusual type of volcanism – flood basalt. These immensely destructive volcanic eruptions have never been witnessed by Man and that is something we must be grateful for. Flood basalt lava flows are best described as a continuous outpouring of lava that goes on for thousands of years.

Basalt is very runny lava, not thick and sticky, so it flows very fast and doesn’t usually have a large cloud of ash and gas. Flood basalts are super eruptions, driven from deep within the earth and are probably associated with mantle plumes or hot spots. Layers of lava, many miles thick can be built up over the course of a flood basalt eruption. The opening of the Atlantic Ocean probably started a flood basalt eruption at the beginning of the Jurassic period and this was close enough to Shropshire for the effects to be felt.

Because flood basalts are unusually powerful, their plumes of hot gas might reach up into the high atmosphere. Basalt contains a lot of sulphur, which is a climate-altering element. Sulphuric acid droplets in the upper atmosphere reflect sunlight and can cause climate cooling.

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