Geology and mineral resources in Shropshire

Shropshire has a diverse geology with a wide range of mineral resources and rocks representing most of the major divisions of geological time. Broadly speaking the geology of the county can be divided into three areas:

1. An area of relatively younger Permo-Triassic rocks often overlain by thick glacial deposits in the north and east.
2. A band of Carboniferous strata extending discontinuously across the county from north-west to south east.
3. An area of older rocks (Lower Palaeozoic and Pre-Cambrian) occurring in the south and west of the county.
The differing geology of these three areas leads to distinct types of landscape and mineral resources.

North and East Shropshire
The thick glacial sands, gravels and boulder clays of the Cheshire basin extend southwards into North Shropshire, where they produce a flat or gently rolling landscape, which is punctuated with hummocks and lakes formed by receding glaciers (e.g. the country around Ellesmere). Glacial deposits also occur intermittently outside of North Shropshire, especially in the major river valleys south of Shrewsbury. Glacial sand and gravel is one of Shropshire's most important mineral resources.

Permo Triassic rocks occur extensively in the east of the county around Bridgnorth and emerge from beneath the thick glacial deposits of North Shropshire to form a series of attractive sandstone ridges (e.g. Nescliffe Hill, Hawkstone Park). These rocks were deposited in desert conditions at a time when Britain was nearer the equator. They include the deposits of fossil sand dunes, desert rivers and dry lakes. Permo Triassic sandstones are an important historic source of building stone and are still worked today for high quality cut stone (dimension stone) at Grinshill, near Wem. Permo-Triassic conglomerates are also a source of sand and gravel.

Central zone
Upper Carboniferous rocks occur in a discontinuous belt from north-west to south-east across the county. They contain coal bearing strata giving rise to a number of small coalfields. The most well known of these are the Coalbrookdale Coalfield (Telford & Wrekin) and the adjacent Broseley Coalfield, both of which have a recent history of opencast coal and clay mining. Underground mining occurred in all of the county's coalfields during the 20th century. The coals represent the remains of trees growing in a tropical swamp. The fireclays which often underlie the coals formed from the soils upon which they grew.

Lower and Middle Carboniferous rocks occur locally beneath the Coal Measures sequence. Lower Carboniferous dolomitic limestone (limestone containing magnesium) occurs in the Oswestry area, where is worked for roadstone and agricultural magnesium lime at Llynclys Quarry. Dolerite, an igneous rock found locally in Carboniferous rocks is also an important source of roadstone which is quarried at Clee Hill near Ludlow. The localised occurrence of good quality coal, ironstone, limestone and timber in the Ironbridge area was a major factor in the birth of the Industrial Revolution.

South and West Shropshire
South and West Shropshire is made up a wide range of older rocks of Lower Palaeozoic and Devonian (Old Red Sandstone) age. The Lower Palaeozoic rocks (comprising the Cambrian, Ordovician and Silurian systems) represent the deposits of an ocean (the Iapetus Ocean) which formerly divided England and Scotland. Deeper water deposits to the west of the Church Stretton Fault were the site of extensive volcanic activity in the Ordovician. Thick limestones were deposited in the shallower tropical seas to the east of the Church Stretton Fault in the succeeding Silurian.

The Iapetus Ocean became progressively narrower throughout the Lower Palaeozoic, finally closing to form a great mountain range in the succeeding Devonian. Erosion of the mountains led to the accumulation of great thickness of sediment known as the Old Red Sandstone which have an extensive outcrop around the Clee Hills.

The county's older rocks contain several important horizons of harder rock suitable for use in road construction (roadstones). This includes Pre-Cambrian gritstone which is quarried at Bayston Hill and Haughmond Hill Quarries near Shrewsbury and Silurian limestone which is worked at Lea Quarry on Wenlock Edge. The West Shropshire mining district was also a leading producer of vein minerals including lead, zinc and barytes in the 19th and early 20th centuries.