Northamptonshire (including Milton Keynes)
The geology of Northamptonshire is mostly sedimentary rocks of the Jurassic Period, although this underlying geology is obscured by areas of sands, gravels and till (boulder clay) deposited during the Ice Age (Quaternary).
Jurassic limestone is a dominant rock type in Northamptonshire. This quarry was created to extract Northamptonshire Ironstone, which occurs beneath the limestones
Northamptonshire forms part of the Wolds landscapes, with the Northamptonshire Heights forming part of the great line of Jurassic limestone hills that stretch from Yorkshire to the Dorset Coast. This ‘upland’ area, which reaches heights of 700ft, forms the main watershed of Middle England, giving rise to the Cherwell, the Warwickshire Avon, Welland and the upper catchment of the Nene.
The upper Nene divides the Northamptonshire Heights to the north from the Cherwell/Ouse tablelands or 'Ironstone Wolds' to the south. Running along the southern side of the Welland Valley is the steep scarp slope of the Rockingham Forest ridge, which is formed from Jurassic limestones belonging to the Great Oolite, although the higher ground in this area is capped by till (boulder clay) which gives rise to heavy soils unattractive for cultivation.
Jurassic
The underlying geology of Northamptonshire belongs entirely to the Jurassic (195-140 million years ago). The oldest rocks exposed are of Lower Jurassic age and belong to the Lias, which is divided into three units, each with slightly varying characters. Overall, the Lias comprises greyish-blue clays with limestone and sandstone beds which come to the surface over a large area in the south-west and centre of Northamptonshire, around Banbury, Daventry and Market Harborough, although exposures are few.
These rocks are also exposed along rivers and stream courses near Towcester, Northampton, Wellingborough and Kettering, where erosion has removed overlying limestones. Certain beds within the Lias are of commercial importance, notably the marlstone or 'rock bed', at the top of the Middle Lias, which is used as a building stone, and parts of the Upper Lias which are excavated for brick-making.
Overlying the Lias is a varied sequence of limestones and clays of Middle Jurassic age which form the most significant component of Northamptonshire's geology. These belong to the Inferior Oolite and the Great Oolite. The lowest part of the sequence (the Northamptonshire Sand) outcrops around Northampton, Pitsford and Corby and contains the distinctive ironstone with which so many buildings in Northamptonshire and surrounding counties are constructed.
Historically, the Northamptonshire Sand has been an important source of iron ore, with from 9-12 ft of workable beds being present at locations such as Kettering, Northampton, Corby and Towcester. Resting upon the Northamptonshire Sand is the Lincolnshire Limestone, which in places can be thinly split and has been used extensively for roofing (eg Collyweston, near Stamford).
The succeeding clays and sandy limestones of the Great Oolite Group outcrop in a broad swathe through the central part of the Northamptonshire, giving rise to an undulating and characteristic landscape which contrasts strongly with the lower-lying pastureland developed on the Lias outcrop to the west and north-west. Collectively these rocks, which belong to the Rutland Formation, Blisworth Limestone and the Blisworth Clay, were deposited near to the shore of a shallow tropical sea. Some beds are very fossiliferous, notably the White Limestone, and yield the fossils of extinct bivalves, sea-urchins, brachiopods and occasionally ammonites.
In the eastern parts of the county west of Oundle, outcrops of the thin, sandy coloured limestones of the Cornbrash occur. These are overlain by the Oxford Clay, which outcrops south of the river Nene and form the south-eastern boundary of the county.
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.
The sands, gravels and till (boulder clay) found over areas of higher ground in Northamptonshire are usually attributed to a glacial period known as the Wolstonian (a Glacial period approximately 200,000-130,000 years ago). The till varies in thickness from as little as a few inches to as much as 120 ft in places, especially in eastern parts.
East of Northampton, the river Nene has a very broad valley which seems out of keeping with the size of the present day river. This was probably caused by the enormous amount of water that was released by melting ice during the Ice Ages. Substantial deposits of gravel were laid down in the valley by the glacial river system. These have been exploited for aggregate and other uses for many years and the flooded former workings now provide important habitat for waterfowl and other animals and plants.
Geological highlights:
- The area around Corby has always been rich in iron-ore which was excavated and worked before the Romans, who it is believed, from various finds, had an ironworks there during their occupation. Royal furnaces, or 'ferraria', were also set up at nearby Geddington and Gretton from the time of Edward the Confessor's reign to that of Henry III, and the Doomsday Book names the 'Manor of Corbei' as an iron producing centre. Corby had its own ironstone works in 1910, the plant being taken over by Stewarts and Lloyds in 1920, but it was not until 1933 that steel production began.
- In some locations, the ironstone deposits have been strip mined, resulting in deep linear quarries known as 'gullets', surrounded by extensive areas of spoil. Some of these former workings have been returned to agriculture, but others (e.g. Twywell Gullet, near Kettering) support limestone grassland rich in plant species such as orchids.
- William Smith, the ‘father of geology’ produced the first geological map of England and Wales in 1815. He died on 28th August 1839 in Northampton, whilst on his way to attend a meeting of the British Association in Birmingham. He is buried in St. Peter’s Church in Northampton.
- A number of hand axes forged by early man (of Lower and Middle Palaeolithic age, 2 million to 250,000 years ago) have been found during gravel quarrying in the Nene valley where individual pits have produced small assemblages of up to 10 artefacts. Most of these quarry finds are in a “rolled” condition, suggesting that hey have been transported some distance by the river.
- An interesting series of animal tracks and mammal bones have been recorded from a layer of mud beneath river gravels in a quarry at Little Houghton, Northampton. These have been interpreted as the remains of a waterhole that was used by animals during a warmer interval of the Ice Age, probably over 400,000 years ago.
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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