Geology of Shropshire

Shropshire's Geology is very diverse and most geological periods of time, and most rock types, can be found within the county. There is also a large amount of mineral wealth in the county, including lead, barytes, limestone, coal and iron, which helped the area develop the industrial revolution west of Clee Hill and, later, in the Ironbridge Gorge area. Quarrying is still active, with limestone for cement manufacture and concrete aggregate, sandstone, greywacke and dolerite for road aggregate, and sand and gravel for aggregate and drainage filters. Groundwater is an equally important economic resource.

Upon looking at a geological map of the county, the most obvious feature is the Church Stretton Fault. This runs from far to the South West (Wales, arguably Ireland), entering the county of Shropshire in the south west, near the small town of Clun. It extends north eastwards through the county, dividing the county in two, before coming to an end in the Cheshire Basin, beyond the town of Newport. The fault itself passes almost through Church Stretton, immediately east of the Long Mynd, and also passes close to The Wrekin. Another, extensive fault exists in Shropshire, the Pontesford-Linley Fault, situated near the village of Pontesbury, south west of the county town of Shrewsbury.

The oldest rocks in Shropshire are of Precambrian age and are to be found at Rushton, a mile west of The Wrekin, as schists and gneisses. East of Shrewsbury, on Haughmond Hill, the sedimentary rocks are of somewhat younger Precambrian age, and are being actively quarried for use on roads. The hill itself provides an amazing view across large parts of mid Shropshire.

The Wrekin is a prominent hill near the town of Telford. The sedimentary rock types are varied around the area, but lava and volcanic ash (tuff) from various volcanic eruptions form this famous landmark. However, The Wrekin itself is not a volcano, and never was. The primary igneous rock on the Wrekin is rhyolite; this has a pinkish colour and is usually banded as it is a slow cooling viscous extrusive rock. A particularly good outcrop of rhyolite exists as you drive between Wellington and Shrewsbury on the new A5 by-pass. Intrusions of igneous rock have been quarried in the past at nearby Ercall Quarry. Here, the main type of igneous rock that can be found is Granophyre. At Ercall Quarry itself, you can see the contact (boundary) between Precambrian rocks and the younger (Cambrian) sedimentary rocks. In the vicinity these contain fossils from the period associated with "the explosion of life".

Not far from The Wrekin is the famous Ironbridge Gorge, named after the bridge that stands over the River Severn near Madeley. The geological events that took place here hundreds of millions, and then just tens of thousands, of years ago were what made the Industrial Revolution possible: deposition through geological time resulted in close juxtaposition of iron ore, coal and limestone (the basic ingredients for iron smelting), with a river large enough to facilitate transport of the products. The Gorge itself was carved out by meltwater flooding out from beneath a huge ice sheet that had moved here from the north and west, towards the end of the last ice age, some 17,000 years ago.

Much of North Shropshire is a plain which is a basin of Permian and Triassic New Red Sandstone, overlain by Jurassic deposits in a small area near Wem. This basin continues north into Cheshire. Faulting has occurred within the sandstones, because of basin extension during and after the infilling of the basin. Escarpments form small prominent hills within the plain. The basin is bounded on the east by the Hodnet Fault, which runs roughly from Shrewsbury to Market Drayton. East of this fault the sandstone is thinner. In the north west of the county near Oswestry are outcrops of Carboniferous Limestone and the Coal Measures.

The South Shropshire largely fall within the Shropshire Hills AONB.[1] These hills were formed on a continental shelf, but buckled up into hills at the time of a continental collision: the Variscan Orogeny. The most famous of these hills is probably the Long Mynd, which is Precambrian in age and forms the west side of the Stretton Valley. East of Church Stretton is Wenlock Edge, a Silurian limestone escarpment. In between lies a complete succession through the late Precambrian, Cambrian, Ordovician and into the Silurian. Wenlock Edge is the equivalent of the equally famous North American Niagara Escarpment, which comprises Middle Silurian (Clinton Group) to Upper Silurian strata. These two escarpments were most likely in the same locale prior to the separation of the North American and European plates at the end of the Mesozoic Era, 60 million years ago. This is evident by the presence of the Mid-Atlantic Ridge. South West of Church Stretton, and at the westernmost border of England with Wales, is the very rural area of Clun Forest. The rocks here are mainly Silurian in age, and whilst they are, on the whole, weak rocks that are easily eroded, the topography is very varied with an impressive landscape, and the hills were put to good use in Iron Age times with the use of fortresses on the hills across Shropshire, including Clee and Anchor.

Igneous intrusions outcropping at the surface in South Shropshire are few and small, but much larger bodies are believed to exist at shallow depths, evidenced by geophysical anomalies and radon-bearing groundwater. Several sills of dolerite intrude the Clee Hills – Brown Clee, Titterstone Clee and Clee Hill itself. These are some of the highest points in Shropshire, and serve as outcrops for Old Red Sandstone, and also various Coal Measures and limestone. Indeed, Brown Clee is considered to be one of the best exposures of Old Red Sandstone that exists. South Shropshire was on the border of the Old Red Sandstone continent, explaining why it appears so prominently in the Clee area.

The Quaternary geology of Shropshire is equally fascinating, dominated in the Devensian by major glaciers flowing east from Wales and south from the Irish Sea basin, meeting around Shrewsbury and moving south as far as the Stretton Valley. The Wye glacier moved up from the south into the southern part of the county which, together with the meltwater deposits, have combined to create a diverse and complex series of deposits.

The Geological Survey Memoirs provide the most comprehensive details of the local geology but unfortunately not all the county is covered and all are now dated - most were published nearly a century ago. The last traditional memoir was that for Sheet 166 (Church Stretton) published in 1968; the Telford Memoir incorporates the studies undertaken for the new town development, on which it concentrates (published 1995) and the most recent (2001) is for Sheet 165 (Welshpool), which includes a little of the extreme west of the county. Otherwise the most comprehensive description is contained within Peter Toghill's book.[2]

Details of local and regional studies are published by the Shropshire Geological Society.[3]

Notes
  1. Shropshire Hills AONB
  2. Toghill, P. (2006). Geology of Shropshire. 2nd Edition, The Crowood Press, Marlborough, 256 pp.
  3. Proceedings of the Shropshire Geological Society
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See also
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