

Cambridgeshire’s Geology
The ‘Superficial’ geology
Of particular importance in Cambridgeshire is the Quaternary geology, comprising the deposits formed in the Holocene Epoch (the last 11,700 years), and those from the preceding Pleistocene Epoch, the oldest of which date back to the Anglian glaciation c.424,000 – 478,000 years ago. The former include the Holocene deposits now named (by the British Geological Survey as the Fenland Formation.
Shown as ‘Superficial’ geology on geological maps, the deposits of the Quaternary Period dominate the landscape over much of Cambridgeshire. Once regarded only in terms of the soil they form at the surface, we now have a greater understanding of their importance as geological units, often of considerable depth, under the soil. These deposits include: peat, silts, clays and marls of the low fens; alluvium, sands and gravels of the Nene, Ouse and Cam river valleys; gravels of the fen islands; and a mixture of glacial material on the chalk, sandstone and clay hills, including the widespread ‘boulder clay’ (till) and many ‘exotic’ cobbles originating from as far away as Scandinavia. In addition, during the dynamic climate changes of the Quaternary, geomorphological processes such as weathering and erosion have created many of the local landscape features.
The Bedrock geology
Underlying this relatively recent geology, or occurring at the surface where younger material has been eroded, is the bedrock, which, in Cambridgeshire, dates from the Cretaceous and Jurassic Periods. Any younger bedrock, if once present, has long disappeared and older rocks are only found at some depth.
Generally, the bedrock strata in East Anglia dip slightly to the south-east due to tectonic movements linked to formation of the Alps (that started by about 60 million years ago) and the sedimentary basin Therefore, in the south-east of Cambridgeshire, rocks from the Cretaceous Period (145-66 million years ago) occur at the surface whilst in the north-west they have been eroded, bringing older rocks of the Jurassic Period (201-145 million years ago) to the surface. The oldest Jurassic rocks in the county are found to the west of Peterborough and consist mostly of clays and limestones (with some sandstone), including the famous Barnack Stone (a limestone), used for the building of Peterborough and Ely Cathedrals. Further south-east, the Jurassic rocks consist mainly of clays of differing compositions, the most extensive being the Oxford Clay, which occurs over more than half of the west of the county; its Peterborough Member was well known for being easy to fire due to the organic-rich content and was therefore popular in the brick making industry (producing ‘Fletton’ bricks). The next youngest is the Ampthill Clay and younger still is the Kimmeridge Clay. These mudstones are also important for the fossils they contain, particularly those of plesiosaurs and other marine reptiles, with one site, at Ely, being designated an SSSI due to its fossil richness. The erosion of these relatively soft clays has resulted in the creation of the Fen Basin and their impermeable nature has been a key factor in the development of the water-logged conditions that created ‘fenland’. Also within the Upper Jurassic rocks is the limestone of the Corallian Group, mostly found around Upware and consisting, in part, of a coral reef. This relatively hard limestone forms a slight ridge rising above the fen edge between Upware and Wicken.
Overlying the youngest Jurassic rock is the oldest of the Cretaceous, a sandstone now called the Woburn Sands (previously called the Lower Greensand), better known in Bedfordshire where it forms the Greensand Ridge. As well as providing the sandy heaths around Gamlingay, it forms the ridge at Haddenham and the hill on which Ely Cathedral sits. Overlying it is the Gault Formation, here occurring as a band of clay stretching along the base of the Chalk escarpment and the south-eastern fen edge. A relatively thin band of Cambridge Greensand is present over most of the Gault in the county; classified as the lowest stratum of the Chalk, the Cambridge Greensand occurs only in southern Cambridgeshire and into Hertfordshire. It has played an important role in the history of the county in recent centuries due to exploitation of the many phosphatic nodules that it contained. Known as ‘coprolites’, they were used for fertiliser and their excavation trenches have left significant impact on the landscape in several areas. It is also highly fossiliferous and the remains of many fish and, notably, many Pterosaurs, were found in the diggings. The Chalk is the youngest bedrock in the county. Each of its strata has a different character dependent on the environment in which it formed: the depth of the seabed and distance from land being important factors along with the organic and non-organic content of the seawater. In particular, the different amounts of clay and silica present in each horizon affects the porosity and permeability of the Chalk; this has an influence on the landscape, including the location of the many ecologically important chalk springs. The variable physical properties of the Chalk strata also affect key ecological factors such as water retention and soil development.
As well as chalk and mineral springs, there are many other geological features formed by past and present geomorphological processes in the county. These include brecciated chalk, ice rafts, ‘patterned ground’ (from freezing and thawing), alluvial fans and larger landscape features such as river channels, dry valleys, floodplains, hill slopes and ridges. Many hidden features exist, only to be discovered when excavation takes place; examples are tunnel valleys (infilled by glacial deposits) and ice wedges.