The following has been written for LDHS by Dr Peter Friend. We are most grateful to him for his contribution.
The Natural Landscape of Longstanton and District
Peter Friend, 11 Nov. 2008, with additions and modification 3 Sept 2009.
This note considers the natural landscape of an area that I have been calling the Cottenham Flatlands. This extends from St. Ives in the west, to the River Cam in the east, and from the River Great Ouse in the north, to the A14 trunk road in the south.
Although the Flatlands appear rather featureless, at first sight, careful survey shows that they contain subtle ridges and hollows, and that their soils vary from muddy to sandy and gravely. In fact it’s network of roads and villages have very clearly grown in historic times on these subtle features in its natural landscape. This note summarises the episodes that have produced these features.
The formation of the Bed-rock foundations. Underlying and shaping the landscape is a foundation of relatively hard, very old, bedrock that has been covered by a surface blanketof much younger, softer sediment and soil. In the Flatlands the bedrock ranges from 160 to 90 million years in age, and was formed during the Jurassic and Cretaceous periods of geological time. All of this bedrock was deposited as sediments in seas that extended right across much of England, which had a totally different geography of land and sea in those days. The sediments that underlie the area range from mudstones (locally called the Oxford, Ampthill and Kimmeridge Clays, and the Gault Clay), limestones (called the Elsworth and St Ives Rocks), sandstones (the Lower Greensand), and Chalk (fine-grained sediment of calcareous algae).
Bedrock fossils. Historical accounts record the discovery of much fossil material from a scatter of localities in the bed-rock of this area and its surroundings, but these discoveries are rather rare now unless special excavations are being made. A recent interesting discovery has come from the excavation of a balancing pond on Home Farm, near the reservoir on the Longstanton-Over road, where numerous fragments of large ammonites were collected. These have been provisionally identified (in the Sedgwick Museum of Earth Sciences, University of Cambridge) as belonging to the genus Perisphinctes, and possibly also the related genus Decipia. These ammonites are typical of the Ampthill Clay of the upper Oxfordian age division of the Jurassic, dating from about 158 million years ago. This identification fits well with the mapping work carried out by the British Geological Survey.
Tertiary Earth Movements and Erosion.Near the start of Tertiary (Cenozoic) times, 66 million years ago, conditions changed drastically, movements in the Earth, probably related to the opening of the Atlantic Ocean, tilted Britain, raising the land in the west, and causing sinking in the Present North Sea area. During this episode, which is continuing today, the present pattern of land and sea has developed, initiating the erosion of a pattern of river valleys. The valleys of the Great Ouse and the Cam, bordering the Flatlands, were first formed during this period.
The Ice Age.Climate change has also been a key factor in the making of the landscape of this area. Over the last 2.6 million years, the Earth has cooled overall and been subjected to a regime of very cold climate phases, separated by short warmer periods, one of which has lasted over the last 10,000 years. These climate fluctuations have often caused accelerated erosion and river movement, so that the more resistant bedrock (eg. the limestones, sandstones and Chalk) tend to have been picked out to form ridges and hills, and the mudstones of the area have been eroded, lowering the landscape towards its present, Flatlands style.
About 0.5 million years ago, a particularly fierce cold phase, known as the Anglian, covered the whole of the Flatlands with an ice-sheet that moved from the north over the Chalk hills to the south-east as far as areas now occupied by the northern margins Greater London. The whole of the Flatlands was covered by a layer of boulder-clay, containing bedrock mudstones, with pebbles and boulders of limestones, sandstones and Chalk.
After the Anglian cold phase, the ice melted and river erosion and slope collapse began again. Isolated plateaus of boulder clay are preserved, often with steep slopes, such as those south of the A14 in the Elsworth and Madingley areas, and at Houghton near St Ives.
River movement events.Since about 450 thousand years ago, when the great Anglian ice-sheet was melting, none of the later Ice Age ice sheets has reached as far south as the Cottenham Flatlands, although the last ice-sheet came southwards as far as the Norfolk coast area, only some 20,000 years ago. But during this period the Flatlands have seen many changes in climate and the pattern of rivers.
About 300,000 years ago, a belt of sand and gravel was deposited by a river which can be labelled the ancestral Cam. At that time the River Cam flowed north-east wards, parallel to the Chalk Edge and then emerging, near Cambridge between hills represented now by the Gog Magog and Madingley Chalk hills. Downstream from these hills, the river became more free to migrate and swung westerly to flow out along what is now the Huntingdon Road (Cambridge) and on out via Girton, Oakington, Longstanton, Willingham and Over. The elevation of this ancestral river was almost 20 meters higher than the present Cam ( as represented by Castle Hill, Cambridge), because the landscape generally had not been lowered by erosion as much as now. Subsequent lowering tended to leave this belt of sands and gravels as a ridge, and eventually villages developed on the better drained land.
Later rivers, between 200,000 and 50,000 years occupied different and lower courses, often spreading gravels on their floodplains as the rivers changed their courses. The extent of these gravels provides a measure of the extent of these ancient river floodplains. Gravel pits, left by the extraction of the gravels for road and building works are obvious around St Ives, Earith etc, where they are increasingly used for recreation and as nature reserves. Many of these pits have revealed the deposits of warm and cold climate episodes, often marked by characteristic fossil bones.
As the climate warmed over the last 10,000 years, some of the river deposits were replaced by fen deposits, marking the flooding of the lower parts of the rivers by peats and lake deposits, formed as sea-level rose.
