Lincolnshire's Jurassic and Cretaceous rocks are tilted gently to the east and their variable softness and hardness means they have a strong control on the county's large-scale landscape features.
In the east, the more resistant chalk of Cretaceous age forms the Lincolnshire Wolds, while further to the west the Middle Jurassic marine limestones form a raised plateau surface running northwards from Stamford. As this outcrop of limestones narrows to the north it gives rise to the Lincoln Edge, which falls away to the east, to the broad and low-lying Ancholme Valley underlain by Oxford Clay, and which broadens southwards to the peat- and silt-filled depression of the Fens.
To the west of the Lincoln Edge lies the Vale of Trent, which is underlain by Lower Jurassic clays. The landscape of the county has also been significantly influenced by the tremendous climatic changes and physical forces that have dominated the last two million years of geological time. During this period, known as the Ice Age, vast quantities of sediment were deposited over the land surface by the waxing and waning of glaciers and ice sheets.
The oldest rocks in Lincolnshire are of Triassic age (230-195 million years-old) and occur along the western edge of the county. Mudstones and shales (belonging respectively to the Mercia Mudstone and the Penarth Group) were deposited in a desert environment and a shallow sea, which penetrated into the area towards the end of the Triassic. All of these rocks are poorly exposed, being overlain by much younger sediments deposited by the ice sheets of the Quaternary Period.
Shallow marine conditions continued into the Jurassic (195-140 million years ago) with deposition of clays and thin limestones known as the Lias. The Lower Lias comprises shales (with some basal limestones) split by the Frodingham Ironstone. Around Scunthorpe, there are numerous quarries where the ironstone was quarried. Shales also dominate the Middle and Upper Lias and are poorly exposed apart from within a few brick and cement works. The Lias rocks are relatively soft and easily eroded and form the floor of the Vale of Trent.
The Lias is overlain by a series of marine limestones known as the Inferior Oolite and the Great Oolite. The lowest part of the sequence comprises the Northampton and Grantham Sands, which are sands and muds, deposited in a shallow marine and estuarine environment some 170 million years ago. The overlying Lincolnshire Limestone is a buff-yellow limestone, its outcrop being easily recognised by its use in buildings and as debris in ploughed fields. This relatively hard rock also gives rise to the relatively straight and abrupt west-facing escarpment overlooking the Vale of Trent.
The Great Oolite is divided into four horizons (the Rutland Limestone, Blisworth Limestone, Blisworth Clay and Cornbrash). These varied rocks were deposited in shallow seas, tidal mudflats, lagoons and large river deltas and contain many fossil shells such as small oysters and other bivalves.
The continuation of marine conditions led to deposition of the Kellaways Beds, comprising a sequence of muds and a sandstone band which contains fossil oysters. Deeper marine mudstones; the Oxford, West Walton, Ampthill and Kimmeridge Clays, dominate the remainder of the Jurassic sequence. These softer clays run through the central part of Lincolnshire and floor the Vale of Ancholme between the limestone ridge to the west and the Lincolnshire Wolds to the east.
The end of the Jurassic Period was marked by a global fall in sea-level and the retreat of the sea from much of the area now occupied by Lincolnshire. This led to the formation of and a period of erosion began. After an interval, the early Cretaceous sea invaded this land area from the east. The first sediments laid down in this sea are represented by the Spilsby Sand, which has a characteristic phosphate-rich nodule bed at its base. The overlying Claxby Ironstone is predominantly clay, with some layers rich in iron, hence the name.
The Lower and Upper Tealby Clays (split by the Tealby Limestone) overlie the ironstones followed by the Roach, comprising interbedded muds and sands. Outcrops of the Tealby series and Spilsby Sandstone occur across the southern part of the Wolds. Some villages within this area had their own brick works and small quarries for building stone.
A number of rock units were deposited following deposition of Spilsby sandstone, including the fully marine Carstone, which is an iron-rich gritty sandstone. The Carstone grades upward into the Red Chalk, comprising a pink limestone and brick red marl which are rich in fossil bivalves, ammonites and other marine creatures. A major phase of sea-level rise and deepening of the Cretaceous sea marked the beginning of the Upper Cretaceous. The almost pure limestone of the chalk, which forms the main bulk of the Lincolnshire Wolds, was deposited in this warm, sub-tropical sea.
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.
Around 450,000 years ago, a severe cold phase known as the Anglian glaciation led to an ice sheet spreading across East Anglia and most of Lincolnshire. All but the highest parts of the Wolds lay beneath a thick sheet of ice grinding slowly south, eroding massive quantities of chalk and other rocks, and depositing till (or boulder clay). After the Anglian, a further series of warm and cold phases occurred, but sediments deposited during these phases are poorly exposed in Lincolnshire. However, during the Ipswichian interglacial, a rising sea filled the eastern part of Lincolnshire forming a sea cliff (now degraded and partially buried with later glacial tills) along the eastern edge of the Wolds.
It began to get colder again around 115,000 years ago, marking the beginning of the Devensian glaciation. During this time, ice flowed along the Lincolnshire coast and through the entrance to The Wash and into the Fenland embayment. The ice sheet left a mantle of till, which is spread over Lincolnshire to the east of the Wolds. For 100,000 years the Wolds stood above the ice, experiencing severe tundra conditions, with snow and meltwaters seasonally cutting valleys into rocks that were permanently frozen below the surface. Most of the deep, dry valleys and the steep-sided, open-ended gorges, including the River Lud at Hubbards Hills, were formed at this time.
By 10,000 years ago the ice had melted and the ensuing rise in sea level created the North Sea and the inundation of east Lincolnshire. Marine-brackish sediments and freshwater peats were deposited on the till surface, forming the present-day flat landscape of the Lincolnshire coastal fringe and the Fens. The present-day coast of Lincolnshire is dominated by saltmarshes, coastal dunes and wide beaches.
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.