Precambrian (ending with the Ediacaran)

The Precambrian of Shropshire largely exposes rocks formed in the youngest phase, the Ediacaran 700 – 545 million years ago

Late Precambrian Earth

Let me set the scene for the beginning of the Shaping of Shropshire story. For millions of years an enormous continent had dominated the planet. Ice ages pulsed across the face of the earth sending sea levels up and down. Scotland and Northern England lay on the other side of the globe from the Southern parts of the country. Shropshire was about 60° south of the equator, about where the Falkland Islands are today lying on the edge of the fractured continent.

Plate subduction

This picture shows just what was happening to Shropshire at the beginning of the Shaping of Shropshire story. Oceanic crust was being forced underneath the edge of the old continent. Water and sediment carried down by the slab of ocean crust caused the mantle to melt and rise up, punching through the crust above and erupting as volcanoes. They would have been powerfully explosive volcanoes, full of gas and water, just like a shaken can of fizzy drink when you open it. These eruptions continued for millions of years, building up layer upon layer of lava and ash, thousands of metres thick.

Explosive volcano

At the same time as all this violence was creating Shropshire’s volcanic past, rivers were quietly building up yet more Shropshire rocks. These rivers wound their way across the barren continent. There were no plants or animals living on land at this time, so the bare soils were stripped with ease from the land surface. Mud and sand were carried down to the sea and deposited on huge mud flats and estuaries.

Mud sediments

This was happening some distance away from the volcanoes, but still the occasional ash flow would reach the mouth of the river. The shallow sea basin that formed behind the volcanoes was getting steadily filled with mud and sand from the river. Eventually it would contain about 7000 metres thick of mud, silt and sand.

Towards the final millennia of the Precambrian, the volcanoes had stopped their eruptions and the rivers were no longer scouring the land. The forces that had driven the ocean crust beneath the continent now lifted and folded these new rocks into mountains and valleys. Enormous squeezing caused the rocks to break and melted rock deep within the crust. These breaks, or faults released some of the stresses within the rocks, allowing sections of crust to slide past one another. The molten rock would shoot out along these faults, or rise slowly through the crust, melting its way through the surrounding rock layers.

The final stages of the Precambrian were dramatic to say the least. The Church Stretton fault system was born, moving huge slabs of crust up and down, side to side for millions of years to come. Volcanoes, earthquakes, disappearing seas and new mountains would give way to a quieter time. By the late Precambrian the global ice age had come to an end, as had the volcanic eruptions. The next chapter of Shaping of Shropshire has explosions of a totally different kind.

Shropshire Precambrian sites:
Caer Caradoc
Haughmond Hill
Lyth Hill
The Long Mynd
The Wrekin