Natural England - Shropshire (including Telford and Wrekin)

Shropshire (including Telford and Wrekin)

The geology of Shropshire is some of the most diverse and interesting in England.

Claverley Cutting

Road cutting through Triassic sandstone in Shropshire near to Claverley.

The rocks can be broadly divided into three representative areas: relatively younger deposits (Permian-Triassic and Quaternary) in the north and east; an intermittent central band of Carboniferous strata and an area of older rocks (Pre-Cambrian-Devonian) occurring in the south and west of the County. The differing geology of these three areas leads to distinct associations of landscape.

In the west and south-west of the County, the Shropshire Hills form an area of great diversity of landform, vegetation and landscape character comprising the Clee Hills, Wenlock Edge, Stretton Hills, Long Mynd, Stiperstones and the Wrekin. The oldest rocks in the County, and in England, are the Pre-Cambrian and Cambrian marine sediments and volcanics that form the ‘whaleback’ ridge of the Long Mynd and the other distinctive, steep-sided hills running north-east from the Stretton area up to The Wrekin. These outcrops are associated with a major fault line, the Church Stretton Fault, which runs through this area. To the east of the Long Mynd, the sharp, shattered ridge of the Stiperstones and the surrounding upland heathland provides exposures of Ordovician shallow water sediments. Similar rocks are also exposed in the Shelve area and yield rich fossil faunas of trilobites and brachiopods.

Running in a line parallel to the orientation of the Pre-Cambrian formed hills is the distinctive wooded escarpment over 25 km long, on Wenlock Edge, which is flanked on either side by broad, flat-bottomed dales (Apedale and Hopedale). These features are formed by limestones (Wenlock Edge) and intervening shales (the dales) of the Silurian age and which here yield abundant shallow marine fossils including many species of trilobites, brachiopods and corals. To the south-east of Wenlock Edge, the broad valley of Corvedale, formed in mudstones and sandstones of the lower part of the Devonian Old Red Sandstone, gradually rises up to an undulating plateau and the imposing bulk of the Clee Hills. These hills protrude from the Old Red Sandstone plateau and are formed of Carboniferous Limestone and Coal Measures, capped by a thick layer of basalt. The coal and the dolerite have long been exploited, the latter more recently for road construction. The sandstone plateau around the hills gives rise to red, silty, loam soils over silty clays.

Mudstones, marls and sandstones of the Carboniferous Coal Measure also outcrop in an intermittent, sweeping band of outcrop from the Wyre Forest, northwards to Newport and then across to west of Shrewsbury. Although much of the surface outcrop is of the barren Upper Coal Measures, the productive Lower and Middle Coal Measures have been exploited in the Coalbrookdale-Telford area and in the Shrewsbury area.

In the north of Shropshire, from a line roughly running between Shrewsbury and Newport, the solid geology is dominated by sandstones and mudstones of Triassic age, and a small outcrop of Lower Jurassic clay. Much of this area is, however, covered by a thick mantle of sediments deposited by the last great ice sheet and its meltwaters, some 15,000 years ago. The characteristic mosses and meres in the Whitchurch area represent hollows in the surface of the glacial sediments that have become infilled with silts and peats over the past 10,000 years.

Precambrian

Some of the oldest rocks in England occur in Shropshire, where, due to their general hardness and resistance to weathering, they commonly form high ground including the well known hills of the Wrekin, Caer Caradoc and Long Mynd. The outcrop of these rocks, in central Shropshire is associated with the major Church Stretton Fault which has through geological time strongly influenced both deposition and structure.

The Pre-Cambrian here comprise a range of rock types, which are mainly of sedimentary and volcanic origin, which have been deformed by pressure and heat over hundreds of millions of years. The Large upland mass of the Long Mynd comprises mudstones, sandstones and conglomerates that were originally deposited in a shallow sea. These rocks have been divided into two series. The eastern half of the area comprises the Stretton Series; mudstones and siltstones with massive sandstones and some beds of volcanic ash (tuff). These rocks, which dip steeply to the west, are well exposed on the eastern side of the Long Mynd in Ashes Hollow and the Cardingmill Valley. Overlying the Stretton Series, and occurring in the western half of the Long Mynd are the purple sandstones, mudstones and conglomerates of the Wentnor Series. These same rocks outcrop in a narrow band extending to the north-east to form Bayston Hill and an isolated block forms Haughmond Hill, north-east of Shrewsbury.

Running along the line of the Church Stretton Fault, a series of outcrops of Pre-Cambrian volcanic rocks occur. At the northern end, the upland bulk of the Wrekin and the adjacent area around Wrockwardine, are formed by tuffs, lavas and intruded igneous rocks such as dolerite and granophyre. To the south, similar rocks outcrop on the Hope Bowdler and form the well known and prominent hills of Caer Caradoc and the Lawley. Outcrops of Pre-Cambrian volcanic rocks (tuffs, lavas and dolerite) also occur on the eastern side of the Long Mynd where they form Pontesford Hill and Earl’s Hill.

Cambrian

Associated with the Pre-Cambrian rocks of the line of the Church Stretton Fault are a number of outcrops of rocks of Cambrian age. The sequence of rocks comprises sandstones and shales with some thin limestones, which yield fossil trilobites. Although it is difficult, from the scattered nature of the outcrops to define the geography at the time of deposition it is apparent that the original sediments were deposited in a shallow shelf sea, the lowest part of the succession representing the incursion over a low-lying area formed of folded Pre-Cambrian rocks. The main areas of outcrop are south of the Wrekin and on the eastern side of Caer Caradoc.

Ordovician

Rocks of Ordovician age (495-443 million years ago) occur as a number of isolated outcrops in the central and north-western part of the County. The main area of outcrop is the Shelve-Stiperstones area. Here, a thick sequence of shales, mudstones and sandstones occur. Except for the lowest beds (the Stiperstones Quartzite) which indicate the proximity of a shoreline to the east, the rocks were probably deposited in quiet and relatively deep water, with occasional influxes of coarser sediment and outbursts of activity from volcanic or submarine craters. The Stiperstones Quartzite at the base of the Ordovician succession is a hard white to grey sandstone and forms the prominent long ridge and crags of the Stiperstones. The weathered sandstone creates a sandy, free-draining and acidic soil which supports the characteristic heathland vegetation of the area. The overlying shales and mudstones are quite fossiliferous and yield many species of trilobites and brachiopods that lived on the sea-floor and fossil graptolites which were small floating colonial animals which were widespread in the Cambrian-Silurian oceans. Ordovician shales also outcrop in the Pontesford area (the Pontesford Shales).

Along the line of the Church Stretton Fault there is a long (30km), but narrow (3km) outcrop of sandstones and shales of Upper Ordovician age. The basal sandstone of the series of rocks forms the prominent ridge of Hoar Edge, to the east of The Lawley. The rocks of this outcrop contain an abundant fossil fauna of brachiopods and trilobites. This contrasts with the rocks of the same age in the Shelve area to the west where shales with graptolites dominate, indicating that the Ordovician rocks of the Church Stretton area were deposited in shallower-water.

Numerous igneous intrusions occur within the Stiperstones and Shelve area. The largest of these is an intrusion of dolerite which forms the main mass of Corndon Hill. There are also many smaller intrusions of basalt-like (i.e. lavas) rocks in the area. These igneous rocks were probably intruded during a period of earth movements at the beginning of the Silurian.

Silurian

Rocks of Silurian age (443 - 417 million years ago) form some of the most characteristic scenery of Shropshire and were also the subject of pioneering geological studies. The Silurian of the Welsh Borders has traditionally been split into three units on the basis of the type of rocks and the fossils they contain. The lowest unit, the Llandovery Series, forms a number of narrow, discontinuous outcrops against the older rocks at Shelve, the Long Mynd as well as along the northern part of the main Silurian outcrop of Wenlock Edge. These rocks comprise sandstones, mudstones and thin limestones. These sediments were originally deposited within a shallow shelf sea with much deeper water lying to the west and north-west. The marine and semi-tropical environment in which they were laid down is demonstrated by the occurrence of fossil brachiopods, crinoids (sea 'lillies') and corals. The middle unit comprises limestones and shales of the Wenlock Series containing abundant fossil brachiopods, corals and trilobites again indicating deposition in a shallow shelf sea. The resistant Wenlock limestones form the wooded escarpment of Wenlock Edge; while the softer shales have eroded to form the adjacent valleys of Hopedale and Corvedale. The Wenlock Limestone is well known for the small fossil coral reefs (known sometimes as ‘ballstone’ reefs) that are preserved in the sequence of limestone rocks. The upper unit, the Ludlow Series, can be split into rocks that were deposited in the shallow shelf sea and those that were deposited in the much deeper ocean basin to the west. The effective division between these two areas lies in the Presteign-Ludlow area in north-west Herefordshire. Here, shelf limestones and mudstones, rich with the fossil remains of corals, brachiopods and trilobites dominate, while just over the border with Wales, deep water shales and mudstones of the same age occur.

Devonian

The succeeding mudstones and sandstones of the Devonian (417-354 million years ago) of the Old Red Sandstone occupy the south-east part of Shropshire, outcropping from Ludlow north-east to Bridgnorth. The lower part of this thick sequence of rocks has, over the years, been split into a number of divisions. The basal red and green shales, sandstones and mudstones (the Downton Series) form the solid geology around Ludlow and floor Corvedale. These sediments were deposited on a wide coastal plain over which large rivers meandered and conditions varied between estuarine and freshwater. Fossils from these easily eroded sediments include large water-going scorpion-like animals known as eurypterids, armoured fish and estuarine brachiopods.

The middle part of the Lower Old Red Sandstone (the Ditton Series) comprises beds of red and greenish-grey silts and locally calcareous mudstone, which give rise to the rich red soils of south-east Shropshire. The alternating beds of siltstone and sandstone contain nodular limestones formerly known as ‘cornstones'. These beds form the lower slopes of Brown Clee Hill and Titterstone Clee Hill and the sandstones towards the top of the sequence give rise to the marked scarp above Corvedale. These nodular limestones were laid down on coastal deltas under a hot climate and contain the remains of freshwater armoured fish. The sandstones often yield the remains of some of the earliest known land plants, which must have grown close to water across the delta surface. A thick sequence of coarse-grained green and brown sandstones completes the Lower Old Red Sandstone sequence.

Carboniferous

Carboniferous (354 to 290 million years ago) rocks outcrop in a discontinuous belt across the County, from the Wyre Forest northwards to Newport and then across to west of Shrewsbury. These outcrops largely comprise Upper Carboniferous Coal Measures and give rise to a series of small coalfields, the most noteworthy of which are the Coalbrookdale Coalfield, the Shrewsbury Coalfield and the Shropshire part of the Wyre Forest Coalfield. Small outcrops of the older Carboniferous Limestone occur at Lilleshall and Little Wenlock in the Coalbrookdale Coalfield and Titterstone Clee Hill, but these are of very limited extent when compared with the bulk of the Upper Carboniferous succession.

The Coal Measures were deposited in a coastal environment where large river deltas were building out into shallow marine waters. Continuing deposition over the millennia led to the further building out of the deltas and the formation of an extensive low-lying, swampy land. The periodic flooding and building of the deltas along the coastline resulted in the deposition of a series of layers of coals (representing the compressed remains of the luxuriant swamp vegetation) interspersed with layers of shale, clay, sandstone and mudstone. The Lower and Middle Coal Measures contain the once economically important coal seams, together with clays and ironstone. The overlying Upper Coal Measures are mainly barren of workable coal seams and are largely composed of red marls, mudstones, siltstones and sandstones. At Coalbrookdale and in the Wyre Forest, the productive Lower and Middle Coal Measures are present at surface and at the former locality played an important role in the early development of the iron industry.

Triassic

The Triassic (248-205 million years old) Period in Shropshire is represented by red mudstones and sandstones which form the solid geology to the area north of a line between Shrewsbury and Newport, forming the southern extension of the large Triassic outcrop of the Cheshire Basin.

These rocks were deposited under arid, desert conditions. The Lower Triassic sandstones of the Sherwood Sandstone Group form attractive hills in the east of the county (e.g. around Bridgnorth) and emerge as a series of ridges from beneath the thick glacial deposits of North Shropshire (e.g. Nescliffe Hill, Hawkstone Park). The Sherwood Sandstone Group consists largely of red, yellow, and brown sandstones that often show colour mottling. Pebbles are scattered through much of the sequence and include the well known Chester Pebble Bed. The smoothness and roundness of the pebbles within the Chester Pebble Bed indicates that they were transported by a large and powerful braided-river system, probably on the margin of an arid, desert mountain range.

The mudstones of the overlying Triassic Mercia Mudstone Group probably represent wind-blown dust that settled in shallow salt-lakes and sun-baked mudflats on the extensive desert plain. The arid conditions under which these rocks were deposited are indicated by the occurrence of numerous layers of salt, which formed through the evaporation of mineral-rich water under the intense desert sun.

Jurassic

A small area of rocks of Jurassic age (205-142 million years old), although obscured by glacial drift, occurs south-east of Whitchurch in the Prees area. This comprises the clays, limestones and sands of the Lias which were deposited as layers of mud and sand in warm, tropical shallow seas which covered much of central England at this time. The Lias (if exposed) yields many fossils of marine creatures such as ammonites, bullet-like belemnites (the internal shell of extinct squid-like animals) and bivalves.

Quaternary

Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets have been instrumental in forming the landscape we see today.

Various deposits of Quaternary age occur throughout Shropshire, with the most extensive deposit being till (or boulder clay), which is formed in and beneath glaciers and ice-sheets. During the last glacial advance some 20,000 years ago, ice invaded from the Irish Sea area and deposited till, sands and gravels over much of Cheshire and North Shropshire. Till with large amounts of gravel and boulders, derived from the Lake District and southern Scotland, formed at the edge of and under this ice sheet and now thickly blankets the northern part of the County. Deposits of glacial till, sands and gravels, in the Whitchurch area have given rise to an undulating landscape which includes a number of ponds and lakes, known locally as mosses and meres, such as those at Ellesmere, Weald Moor and Whixall Moor. These areas of open water and peat have developed in natural depressions in the glacial drift left by the ice sheets which covered the area. These natural sites form, together with similar features in Cheshire a network of wetland habitats of national importance. Glacial deposits also occur widely but more intermittently outside of North Shropshire, and notably in the major river valleys.

The melting of the last great ice sheet at the end of the Devensian, some 13,000 years ago has left its mark on the Shropshire landscape. Huge amounts of water, with sediment, were released and carved new courses across the tundra-landscape. A number of steep-sided glacial meltwater channels, such as Cwmdale, can be seen on the eastern flanks of the Long Mynd to the north and west of All Stretton. Perhaps the most spectacular new watercourse to evolve from the chaotic process of de-glaciation was that now known as Ironbridge Gorge. This formed when water overflowed from a large glacial lake in the Buildwas-Coalbrookdale area, cutting across the original drainage pattern.

Geological Highlights:

  • The rocks of Shropshire, particularly those in the Ludlow-Wenlock area have played an important role in the development of modern geology. In 1831, the British geologist Sir Roderick Murchison undertook a series of investigations of the rocks underlying the Old Red Sandstone that outcrop along the Welsh borders. Through extensive fieldwork and description of the rock types and fossils, Murchison was able to group these previously undifferentiated rocks into a series of units very different from those of the other rocks of England. The results of his research was the establishment of the Silurian (named after the old British Silures tribe that inhabited the area) as a new division of geological time. His work was described in a massive two volume work called The Silurian System (1839) and the Silurian Period is now today firmly established as a distinctive period of time around the World.

  • Faults and fractures which cut the folded Ordovician rocks in the Stiperstones area carry mineral veins that are rich in galena (lead ore). Mining for lead has taken place here since Roman times and up to the early part of the 20th Century, and much of the Stiperstones area is pock-marked by abandoned workings.

  • The Ironbridge Gorge and Ironbridge itself is widely acknowledged as the birthplace of the Industrial Revolution in the 18th Century. The area lies within the Coalbrookdale Coalfield, and the productive Coal Measures along with, limestone and iron ore were all readily available for the mass production of iron. Much of this geological material was exposed in the area through the downcutting of the gorge caused by meltwater at the end of the last ice age some 15,000 years ago.

Local sites

The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible.