SHROPSHIRE ROCKS!

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Great Bolas

The riverside cliff on the floodplain of the River Tern just south-west of Great Bolas exposes the abrupt contact between the Permian and the overlying Triassic.

The lower (Permian) beds are composed of sand that was formed when Shropshire was around 25° North of the equator and conditions were very similar to the present day Sahara desert. Imagine enormous sand dunes tens of metres high marching across a barren rock landscape being blasted by a never-ending wind.

The steep bedding of the dunes have been preserved in the rocks that you can see in this cliff.

It was a barren and desolate place, inhospitable to life and very few fossils are found in these red sandstones. If you pick up a piece or touch some rock you’ll feel how crumbly it is, sand grains come loose on your hands really easily. That’s because the 'glue' that holds the grains together is clay and rust. The clay comes from the breakdown of feldspars (commonly associated with quartz in sand, and often found in igneous rocks such as granite), and the rust is iron oxide which comes from the breakdown of iron-bearing minerals also found in igneous rocks (olivines, pyroxenes and amphiboles in particular). It’s this rusty coating that gives the rock its bright red colour. The igneous rocks suggest that the sediments were derived from mountains containing granite and similar rocks. Combined with the evidence of wind direction in the sand dunes, this indicates the source rocks were in what is now north-west France, in Brittany.

Overlying the sandstones are a variety of sedimentary rocks: conglomerates (once coarse river gravels) and sandstones, but lacking the steep cross-bedding of the Permian beneath. They’re not quite so bright red in colour and have got lots of pebbles in them of all different shapes and sizes. This dramatic change in rock type tells us that there was a sudden change in the environment, that North Shropshire was no longer a desert.

These thick pebble beds are a type of rock called conglomerate, a kind of natural concrete. Conglomerates can be made in different ways, slowly, with the pebbles being sorted, in water like rivers, by size and then sand and mud filling in the gaps later, or rapidly, where an enormous flash flood just picks up all the material lying in its path and then dumps it all randomly.

This sudden change happened at the end of the Permian, the time when the Earth nearly died and may be related to the serious climate changes that had occurred. Plant and animal species living on earth experienced a mass extinction event, something that was so catastrophic that it wiped 96% of all plants and animals off the face of the planet forever. In northern Shropshire we have an exposure of this point in time: a dramatic unconformity between the Permian and the overlying Triassic.

The contact is somewhat irregular (as can be judged in this photograph, which provides a close-up detail) and is believed to be the result of erosion by flash flooding in otherwise arid desert conditions. The apparently deep downcutting of the conglomerate into the underlying sandstone is partially an effect of perspective: we are actually looking at the edge of the eroded channel in which the gravels were deposited.

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