The geology of the West Midlands is dominated by the South Staffordshire Coalfield, the exploitation of which has contributed greatly to the industrial and economic development of the area.
Upper Carboniferous Coal Measures underlie the main conurbation of Wolverhampton, Walsall, West Bromwich and Dudley. Surrounding these shales, sandstones and mudstones are Triassic aged rocks which comprise red mudstones and sandstones. These underlie much of Birmingham and form the solid geology up to Sutton Coldfield. Within the main mass of the Coal Measures are a number of isolated outcrops of older Silurian rock. These shallow water limestones and shales contain a wide range of marine fossils and form the famous outcrops at Wren’s Nest and Dudley Castle Hill. There are also a number of igneous intrusions into the Coal Measures. Much of the area has been mantled in thick deposits of boulder clay and sands and gravel deposited by ice sheets and meltwaters during the Ice Ages of the last two million years.
The oldest rocks present in the area are of Silurian age (443 - 417 million years ago). These outcrop, at Dudley Castle Hill, Wren’s Nest Hill and Hurst Hill and also as an isolated block in east Walsall. Their presence here is due to folding and faulting of the rock succession which has led to the main Silurian succession of the Welsh Borders, being brought to the surface in this area.
The Silurian rocks of the West Midlands and the Welsh Borders have traditionally been split into three units on the basis of the type of rocks and the fossils they contain. In the West Midlands, rocks of the Middle Silurian Wenlock Series are represented by the Wenlock or Dudley Limestone. This shallow marine limestone contains a rich and varied suite of fossils including corals, brachiopods, crinoids (sea-lilies) and trilobites. The succeeding shales seen at Dudley belong to the Lower Ludlow Shales of the Ludlow Series. Like the underlying limestone, the shales are richly fossiliferous and yield many types of brachiopods and trilobites. Good displays of the fossils can be seen in Dudley Museum.
The Carboniferous Period (354-290 million years old) is represented by the Coal Measures of the South Staffordshire Coalfield. These rocks were deposited on a low-lying area of river deltas, feeding off an upland area to the north. 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 thick sequences of mudstone and shale. These underlie the Dudley-Walsall area. The Upper Coal Measures outcrop between Halesowen and West Bromwich and largely comprise marls, red sandy mudstones and grits. These sediments mark the oncoming of more continental conditions due to uplift of the land at the end of the Carboniferous Period.
Several intrusive masses of igneous rock (basalt or lava) occur in the Coal Measures of the West Midlands. These include outcrops at Rowley (the Rowley Hills), at Wednesfield and Pouk Hill near Walsall. At Rowley, the basalt, known as ‘Rowley Rag’ shows the characteristic columnar jointing. These originally molten rocks were probably intruded during a phase of uplift and mountain-building in the late Carboniferous or early Permian.
The Permian (290-248 million years ago) and Triassic (248-205 million years old) periods in the West Midlands are represented by red mudstones and sandstones that underlie the majority of Birmingham, Solihull and extend northwards through to Sutton Coldfield. They also form the prominent ridge of hills that extends northwards from Bushbury in Wolverhampton. These rocks were deposited under arid, desert conditions. The Lower Triassic sandstones of the Sherwood Sandstone Group occupy the north-east of Birmingham and comprise red, yellow, and brown sandstones that often show colour mottling. Pebbles are scattered through much of the sequence in central England and include the well known Chester Pebble Beds which underlie the area between Walsall and Sutton Coldfield. Where not mantled by thick, younger drift deposits, the pebble beds and sandstones give rise to free-draining soils, such as those supporting the heathland vegetation of Sutton Park. The smoothness and roundness of the pebbles within the Chester Pebble Beds indicates that they were transported by a large and powerful braided-river system, probably on the margin of an arid, desert mountain system.
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 alluvial plain. These sediments underlie the south-east Birmingham conurbation and, when weathered, give rise to the characteristic red soils of the area.
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 in the West Midlands. The most extensive deposit is till (or boulder clay), which is formed in and beneath glaciers and ice-sheets. Much of the West Midlands has been covered at least twice by the melting and refreezing of ice sheets over the past 1.5 million years. During the last glacial advance some 20,000 years ago, ice invaded from the Irish Sea area and deposited till, sands and gravels with large amounts of gravel and boulders derived from the Lake District and southern Scotland.
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.