Cambridgeshire (including City of Peterborough)
Cambridgeshire is notable for its flatness, a feature largely attributed to its underlying geology and climatic events over the past two million years.
The major influence on the physical character of the county comes from the underlying Jurassic and Cretaceous rocks which tilt gently to the south, resulting in the oldest rocks occurring in the north-west and progressively younger rocks to the south-east. However, this relatively simple picture is complicated by the extensive clays, gravels and sands laid down over the past 250,000 years, which cover much of the surface area of Cambridgeshire.
Jurassic
The oldest rocks exposed in Cambridgeshire are limestones and clays of the Jurassic Period (195-140 million years ago) and include rocks belonging to the Cornbrash, Great Oolite, Lincolnshire Limestone and Northampton Sand. These rocks form the undulating low hills to the west of Peterborough. 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. Some of the limestones, in particular the Lincolnshire Limestone, were quarried for building stone and former quarries are prominent in the landscape and now provide important wildlife habitat (e.g. Barnack Hills and Holes National Nature Reserve which was first quarried by the Romans).
The succeeding shaly mudstones of the Oxford Clay underlie much of northern and north-western Cambridgeshire. The Oxford Clay is ideal for making bricks as it contains just enough fossil plant matter to make it self-firing once ignited. The chimneys and brick kilns at Whittlesey and Orton to the east and south of Peterborough are prominent landmarks. The Oxford Clay was deposited in a relatively deep sub-tropical ocean and is particularly rich in fossils, especially ammonites, belemnites and swimming reptiles whose skeletons were buried in the mud on the sea floor when they died. Peterborough Museum has an excellent display of these fossils.
To the south-east, the Oxford Clay is overlain by limestones and clays belonging to a sequence of rock known as the Corallian. These Middle Jurassic rocks were deposited in a shallow tropical sea and exposures at locations such as Upware yield many species of ammonites, sea-urchins, corals and bivalves. The final part of the Jurassic succession in Cambridgeshire is represented by the Kimmeridge Clay.
Cretaceous
A variety of Cretaceous rocks form the southern part of Cambridgeshire. The Lower Greensand, Gault Clay and Cambridge Greensand form a relatively narrow strip of land stretching from south-west of Cambridge to Soham in the north-east. The Greensand effectively delineates the southern boundary to the Fens, whilst the Chalk of the Gog Magog Hills to the east of Cambridge forms the highest part of the County, rising to 400ft.
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.
Deposits from this period of time, laid down over the past 250,000 years, partly obscure the Jurassic and Cretaceous geology. Boulder clay or till, which now covers large parts of Cambridgeshire in the west as well as the Cretaceous chalk uplands of the south, was deposited by a large ice-sheet. Following the retreat of the ice sheet (some 15,000 years ago) estuarine silts and clays and freshwater peats were deposited across the low-lying Fens.
Geological Highlights:
- At Alwalton near Peterborough there is a medieval quarry from which was extracted a bed of blackish limestone (from the Blisworth Limestone, Great Oolite), rich in fossil oysters. This is known as the as Alwalton Marble and was used for monumental work such as the tomb of Abbot Benedict in Peterborough Cathedral. Blocks of it can still be seen in the bank of the river Nene.
- The Lincolnshire Limestone has been used since ancient times to build houses and farms. Many old churches, including Peterborough and Ely cathedrals, were built of the shelly limestone quarried at Barnack (including Barnack Hills and Holes National Nature Reserve). Another type of the Lincolnshire Limestone, splits in frosty weather, forming thin layers that are used as roofing slates. Named after the village, Collyweston Slates have been used to roof many buildings in the villages surrounding Peterborough.
- Over the years, the Oxford Clay brickpits near Peterborough have yielded large numbers of marine fossils including the huge fish Leedsichthys and numerous types of ammonite. However, it is for the well-preserved marine reptiles that it is most famous. Skeletons of several species of ichthyosaur, including Ophthalmosaurus have been found, as well as the plesiosaurs, Cryptoclidus and Liopleurodon (at 25m the largest carnivore known). Rarer are the remains of dinosaurs whose carcasses drifted out to sea. Peterborough Museum has an excellent exhibit of the fossils from the brickpits.
- At the bottom of the Greensand chalk is the Chalk Marl, (also known as the Cambridge Greensand), with a layer containing phosphatic nodules (known as coprolites) at its base probably derived from the underlying Gault Clay. The nodules represent the phosphatised remains (teeth, bones and claws) of dinosaurs, marine reptiles and other marine organisms and the fossilised remnants of faecal material. In the second half of the 19th century the mining of the coprolite seam and the production of phosphate for use as a fertiliser became a major industry in the County. The Cambridge Corn Exchange, Fulbourn Hospital and St. John's New Chapel were just some of the buildings constructed using profits from this industry. The diggings ceased by 1894 due to the Quarries Act, the exhaustion of the seam and competition from overseas, notably America.
- The single obvious factor uniting the Fens is the low-lying, level terrain. With the exception of the Isle of Ely, which reaches above 20m (being located on an outlying block of Kimmeridge Clay), levels rarely pass the 10m contour, and typically vary by little more than one or two metres over many miles. Much of the land is below sea level and relies on pumped drainage and the control of sluices at high and low tides to maintain its agricultural viability.
- Beds of peat and silt, with the remains of trees, occur across the Fens. Radiocarbon dates of 4300-4600 years ago have been reported for the remains of oak trees (known locally as 'bog oaks') growing on the original land surface prior to burial by the onset of peat formation. Fen peat, deposited over many centuries, consists of the partially decomposed remains of the plants of wet fens and has been the basis of a thriving arable economy for more than a century.
- Gravel workings near Peterborough and Cambridge have yielded many fossil mammal bones from sediments deposited by rivers during a warm period of the Ice Age (known as the Ipswichian Interglacial, approximately 120,000 years ago). These include the remains of mammoth, rhinoceros and straight-tusked elephant. Peterborough Museum tells the story of the discovery and lives of these animals.
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.
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