SHROPSHIRE ROCKS!

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Cretaceous

The Cretaceous 144-65 million years ago

We don’t find any rocks of this age in Shropshire, but the geological events of the time were responsible for developing the modern upland landscape.

Most Cretaceous rocks outcrop around the south and east of the country, although there are also important outcrops in Northern Ireland and Scotland (Arran). Chalk is the rock that is most associated with the Cretaceous, the white cliffs of Kent and Sussex being made entirely of chalk and flint.

Chalk is also a kind of limestone. It’s made up of the shells of a microscopic sea creature that floated throughout the warm cretaceous seas. Millions and millions of these tiny sea creatures lived and died to build up the hundreds of metres of chalk that make up those famous white cliffs.

Cretaceous earth was warm and wet, there were no ice caps and sea covered most of the surface of the planet. On land, the golden age of the dinosaurs had begun. The famous faces that grace our television screens so often belong to the Cretaceous period. Tyrannosaurus Rex, Velocirapter and Triceratops don’t belong in Jurassic Park, but Cretaceous Park!

By the end of the Cretaceous, all the building blocks of life as we recognise it were in place. The first mammals had evolved, along with the first flowering plants. The continents although mostly covered in water were starting to look familiar in shape – life was good.

India began its (relatively) high-speed collision with Asia at the end of the Cretaceous. This caused many flood basalt eruptions that covered much of the Indian region with 200,000 square miles of lava and, more importantly, greenhouse gases. This may well have been the precursor to the death of the dinosaurs; setting up global warming, changing the vegetation, leading to starvation and the downward spiral towards extinction.

The extinction of the dinosaurs at the end of the Cretaceous may have happened as the result of volcanic eruptions, but it ended with an even bigger bang.

A huge meteorite 6 miles or 10 km in diameter came burning through the sky and crashed down into the seas just north of the Mexican Yucatan Peninsula. Most of the meteorite would have vaporised on impact, as would much of the surrounding rock and water. This sent boiling hot steam and ash high into the atmosphere and shock waves out from the point of impact for hundreds if not thousands of miles. The region would have been instantly decimated anything living in the immediate area of the impact crater would have been killed, plants, animals - everything.

However, the meteorite still had a few surprises left up its sleeve. Debris from the impact thrown into the atmosphere came down all over the planet, starting wildfires where it landed, most of the planet burned. The clouds of ash and gas blocked the sunlight for years, altering the climate and preventing plants from photosynthesising. The food chain was broken. The shock waves went deep into the earth, probably triggering earthquakes and volcanic eruptions thousands of miles away from the impact site.

The final and deadly twist to the death of the dinosaurs was due to where the meteorite landed. The Mexican sea bed where the impact happened was rich in a mineral called gypsum. This contains lots of sulphur, which is a major contributor to serious long-term climate change. When the meteorite struck, it vaporised the surrounding rock, sending lots of sulphur into the upper atmosphere where it stayed for years, adding to the gases erupted in India. Sulphur acts to reflect sunlight and cool the planet, so the meteorite had a much more serious and long lasting effect on the global climate and the fate of the dinosaurs than might otherwise have happened. Linked with the flood basalt eruptions, the stage was set for a mass extinction.

We know so much about the far-reaching effects of the meteorite because it left a calling card. All objects from space are much more radioactive than the surface of the earth, in particular meteorites have lots of an element called Iridium. All around the world in rocks of this age, geologists have traced a layer rich in radioactive Iridium, a sure sign that the effects of this impact were felt all around the globe.

Luckily for us, one of the survivors of the devastation was a small rodent like creature, our distant ancestor. From this point on, mammals would become the dominant creatures, evolving and changing but humanoid life was still millions of years in the future.

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