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.
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.
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.
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.
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.