The geology of Warwickshire is extremely varied with the underlying rocks representing the last 600 million years of time.
However, due to the general ‘flatness’ of the county, exposures of the underlying geology are rare and are largely restricted to those found in quarries.
Rocks from the Permian, Triassic and Jurassic periods dominate much of Warwickshire, with the red clays and sandstones of the Mercia Mudstone (Triassic) forming the underlying geology of the northern part of the County and the marine clays of the Lower Jurassic (known as the Lias) occurring in the south. This is not the whole story, however, and much older rocks from the Precambrian, Cambrian and Ordovician periods occur in the north-east of the county around Nuneaton. Between Nuneaton and Tamworth, rocks belonging to the Carboniferous have been pushed up through the younger Mercia Mudstone and form the once economically important Warwickshire Coalfield.
Throughout the County, extensive deposits of clay, sand and gravel cover the underlying geology. These were deposited by ice sheets and rivers over the last 2 million years and belong to the Quaternary or Ice Age.
Between them, the rocks found within Warwickshire provide evidence for a diverse and often remarkable series of past environments, ranging from volcanic islands, swamps and tropical seas, to tundra-like conditions during the last ice age.
Warwickshire's oldest rocks (about 600 million years old) belong to the latest part of the Precambrian Period. The rocks are known as the Caldecote Volcanics and are well exposed in some quarries in the Nuneaton area, where they outcrop along the Hartshill Ridge. These originated as sheets of molten material injected under pressure into earlier rocks and provide evidence for violent volcanic eruptions along the edge of an ancient ocean.
Cambrian (570-510 million years ago) rocks form part of the Hartshill Ridge in the north-east of Warwickshire. Here they comprise conglomerates and sandstones that were deposited in a shallow marine environment following the invasion of the sea over the Precambrian landmass (Hartshill Quartzite Formation). These rocks have yielded the past traces of animal life (burrows and tracks of worm-like creatures), which constitute the earliest, commonly found, traces of animal life in the British Isles. Shales of Cambrian age also occur in the Nuneaton area (Stockingford Shales) and contain fossil brachiopods and trilobites which can be matched with very similar fossils that occur in Cambrian aged rocks in Newfoundland.
Rocks of Ordovician age (510-439 million years old), although not exposed at the surface, are represented by bodies of molten rock that were intruded into Cambrian shales. These bodies are exposed in the large aggregate quarries, such as that at Griff Hill on the Hartshill Ridge, where the baked contact between the igneous rock and the shales can be seen.
Carboniferous rocks (354-290 million years old) are represented by the Warwickshire Coalfield. The outcrop of this is roughly oval in outline and extends from Tamworth to Warwick. The Coal Measures were deposited in an extensive low-lying, swampy area of river deltas. 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 fireclay and ironstone. The overlying Upper Coal Measures, which forms the bulk of the surface outcrop, are mainly barren of workable coal seams and are largely composed of red marls, mudstones, siltstones and sandstones.
The Permian (248-290 million years ago) and Triassic (248-205 million years old) periods in Warwickshire are represented by red mudstones and sandstones that underlie much of the northern half of the county. These rocks were deposited under arid, desert conditions. The Permian sandstones represent the remnants of a vast area of sand dunes that extended across much of what is now southern England and the Midlands. The mudstones of the Triassic Mercia Mudstone Group probably represent wind-blown dust and intermittent river sediments that settled in shallow salt-lakes and sun-baked mudflats. At this time, the Precambrian rocks of the Hartshill ridge probably rose as a range of hills above the desert flats, but were weathered and eroded in the intense heat over millions of years. The distinctive red Permian and Triassic sandstones have been extensively used as building materials over many centuries.
Much of southern Warwickshire is underlain by rocks of Jurassic age (205-142 million years old). These were deposited as layers of mud and sand in the warm, tropical shallow sea which covered much of central England at this time. The oldest part of the Jurassic succession is termed the Lias and comprises clays, limestones and sands. The clays of the Lias tend to form lower-lying ground and give rise to heavy clay soils. These rocks have yielded the skeletons of marine reptiles (ichthyosaurs and plesiosaurs) and many other fossils can be found, such as ammonites, bullet-like belemnites (the internal shell of extinct squid-like animals) and bivalves. In the far south-east of the County, a 40m band of limestone, the Marlstone Rock, represents the Middle Lias, and forms the escarpment of Edge Hill. This bed of iron-rich, sandy limestone formed from sand deposited in a shallow current-swept coastal area. The iron may have originated from tropical soils on nearby Jurassic islands.
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.
Within Warwickshire evidence for the mix of glacial and warmer conditions is provided by deposits of clay, sand and gravel scattered throughout the county. The most extensive deposit is till (or boulder clay), which is formed in and beneath glaciers and ice-sheets and is often considered to be the typical deposit of glacial periods in northern Europe. Till covers much of the land surface in the north of the county and usually gives rise to heavy soils characterised by woodland and pasture. Significant amounts of gravel and sand present in the southern part of the county represent the remnants of past landscapes and the deposits of ancient river systems. The alternation of glacial to interglacial periods led to complex patterns of deposition and erosion, creating the river terrace systems that in many cases survive today, although in many cases the ancient rivers have been obliterated by the advance of glaciers. The highest terraces represent the oldest land surfaces from which the river has progressively cut-down through to the modern floodplain at the bottom of the valley. These river deposits often contain evidence for human occupation in the form of stone axes and choppers and also other fossils (e.g. mammal bones and snails) which allow the gravels to be dated to certain parts of the Ice Age.
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.