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THE WREKIN AND THE ERCALL

 

The Wrekin is home to amongst the oldest rocks in the county of Shropshire (677 million years old). The Wrekin itself, despite having a cone-like appearance, is not actually a volcano, but is made up of thick layers of lava erupted from volcanoes. Geologists think that Shropshire must have been very much like Japan around this time: a volcanic island, sitting in an ocean, near a larger continent.

The Ercall is a hidden gem of Earth history. Standing at the bottom of the quarries you can see an obvious change in the rock, from a bright pink mass to pale grey layers. This change is internationally famous amongst geologists as it marks the change between the Precambrian, where there was very little life, and the Cambrian, where life suddenly exploded in lots of different varieties.

This part of Shropshire during the late Precambrian was at the margins of an ocean where two tectonic plates were coming together.

The plate to the west (as it it today, towards Wales) was moving into the plate to the east (in the West Midlands). Their zone of contact was in the vicinity of the present-day Stretton Valley, along a gigantic crack through the Earth's crust that we now know as the Church Stretton Fault Zone. The westerly plate sank beneath the easterly one and as it did so the sediments became piled up, forming the thick sequence we see today within the Longmynd. This is known as an Accretionary Prism.

As the westerly plate sank deeper into the mantle so the heat caused partial melting and volcanoes developed above, within the easterly plate. Their lavas and intrusions formed the rugged crags of Caer Caradoc and its associated hills, the subject of another itinerary. However, the volcanoes released a lot of sticky lava of type known as Rhyolite. This can be seen in the western part of the Ercall quarry, with its distinctive flow banding:
 

photograph by Ian Stimpson, Hypocentre

 

 

Deeper within the Earth's crust the melting subducting plate developed granophyric magma which pushed up into shallower levels, creating the main bulk of the Ercall quarry. The resulting rise of the crust caused uplift of the land surface which was then eroded. In early Cambrian times it became flooded with shallow water and covered in sand. The actual contact (called an unconformity) can be seen here in two photographs - it is in the centre of the more genera; view; in the more detailed shot note the coarse pebbles of the Cambrian sediments in the top half of the photograph; the lower half is the weathered top of the granophyre intrusion:
 

photograph by Ian Stimpson, Hypocentre

 

 

 

 

 

 

 

 

Fancy a trip to the beach? No need to leave the county, just go to the Ercall!  Within the Cambrian sediments you can find rock-hard ripples in the pale grey sandstone, just the same as those you see on the beach today after the tide has gone out, except here the tide went out 544 million years ago.

These features have made the Wrekin and the Ercall particularly special and are now protected as an SSSI (click here to see the reasons for notification).

 

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