The Geology beneath the Chilterns Countryside

The Chiltern Hills are formed by an outcrop of chalk, overlain by clay-with-flints, on the north-western side of the London basin, which stretch across southern England from the Goring Gap in Oxfordshire to near Hitchin in Hertfordshire. The chalk strata have been tilted to create a dip slope that falls gently towards the south-east. To the north-west, the hills end abruptly at a steep escarpment, which overlooks the Vale of Aylesbury.

The geology of the Chiltern Hills
The geology of the Chiltern Hills

The Chiltern range is one of several ranges of chalk hills in southern England and northern France. The chalk rock started forming around 145 million years ago in shallow sub-tropical seas that were far from land. Around 65 million years ago, these soft sedimentary rocks started to be compressed and uplifted under huge tectonic forces, and they emerged from the sea. Subsequent weathering and erosion have shaped the gently rolling landscape.

Chiltern Escarpment at Coombe Hill
Chiltern Escarpment at Coombe Hill

In the beginning…

Scientific evidence tells us that Planet Earth is about 4,540 million years old. We are going to skip through most of that time by missing out the first 4,390 million years.  Over this vast, unimaginable period, land and oceans have formed, life had begun and it had evolved into a myriad of plants and animals, ranging from single cell organisms to giant dinosaurs. We pick up this story just 150 million years ago.

Towards the end of the Jurassic period, the part of the earth’s surface that is now the Chiltern Countryside was about 35 degrees north of the Equator, (about where Tunisia is today). It was flooded by a shallow, warm sub-tropical sea. The planet was teeming with life. Larger mammals had yet to evolve, but there were land plants, many insects, reptiles, marine dinosaurs, a few early birds and many marine organisms. Nothing was like any of the wildlife you will see in the Chilterns today.

Geological timescale for the Mesozoic and Cenozoic
Geological timescale for the Mesozoic and Cenozoic

The Cretaceous period

For reasons unknown, around 145 million years ago, there was a quick and dramatic climatic change, bringing the Jurassic period to a close; the Cretaceous period had begun.

From the first 40 million years of the Cretaceous period, our region became predominantly land. It may have been on a low-lying, semi-arid island, but nothing remains to tell us what it was like. However around 105 to 95 million years ago, the Earth was warming. Sea levels were rising rapidly and our region disappeared beneath the waves. Initially, there was land nearby and the types of marine sedimentary rocks that formed were composed of the materials you would expect to find off any modern shoreline: sands in the shallow water and clays in the deeper water. These are now represented by the Gault Clay and the Upper Greensand.

The global warming continued, and by 100 million years ago the sea level had risen so much that any remaining nearby land was submerged. The sea water was relatively shallow (no more than about 300 metres deep) and it was probably crystal clear.

The Cretaceous sea
The Cretaceous sea

On the ocean bed, layers of chalk gradually developed from soft calcareous ooze, which was largely composed of disc-shaped calcite plates called coccoliths: the remains of millions of microscopic plankton. Every handful of chalk rock contains millions of these tiny fossils, which were produced in warm sea waters by coccolithophores, a type of planktonic algae. The chalk is up to 600 metres thick, so to produce so much rock from these tiny remains, evidently took a very long time, indeed geological evidence shows that the clear, warm chalk seas remained over our region for over 24 million years.

Fossil sea urchins
Fossil sea urchins

Although most of the chalk is composed of micro-fossils, occasionally it is possible to find larger remains of ammonites, belemnites, echinoids (sea urchins), sea sponges, brachiopods, bivalves, gastropods (sea snails) and even sharks’ teeth. Micraster is one of the best known of all Cretaceous echinoid fossils found in chalk, and it has formed the basis for classic micro-evolutionary studies. Specimens, which have been found in their thousands, show gradual evolutionary transitions upwards through 150 metres of chalk strata representing a 10 million year period.

Higher up the food chain, there would have been sharks and plesiosaurs. Fossils of these are rare and usually consist of vertebrae (parts of the animals’ backbones).

Near apocalypse

Around 66 million years ago the Cretaceous period ended abruptly in a catastrophic event in which between 70% and 80% of all living species were wiped out. The best-known victims were the non-avian dinosaurs. (Avian dinosaurs still live today in the Chilterns; we call them birds). Many other animal groups were also lost forever, including pterosaurs, ammonites and the spectacular mosasaurs and plesiosaurs. Plants and plankton were also decimated.

Tyrannosaurus Rex - one of the last Cretaceous dinosaurs
Tyrannosaurus Rex - one of the last Cretaceous dinosaurs

The Cretaceous-Palaeogene (K-Pg) mass extinction is famously associated with a large astronomical impact in the Gulf of Mexico, where evidence shows a huge asteroid or comet slammed into the Earth.  However, a closer look at the fossil record reveals that many groups, including dinosaurs, were already on the decline when the asteroid struck. Whether the impact caused, or simply contributed to the mass extinctions, is still hotly debated. Many palaeontologists think that the story is more complex. As well as the catastrophic impact, the mass extinction resulted from a combination of drifting continents, rapid climate change and intense volcanic activity that rocked the planet. Even today, the resulting basalt lava flows in India, known as the Deccan Traps, cover an area almost as big as France.  The volcanic activity poisoned the atmosphere, causing large global temperature fluctuations and corrosive acid rain. As plants perished, many herbivores and their predators would have starved to death.

The Palaeogene period - emerging from the ocean

About 50 million years ago, under the forces of plate tectonics, the African continent began to converge with the Eurasian plate on which our region stands. These convergent movements between the tectonic plates began early in the Cretaceous, but the major phases of mountain building, called the Alpine Orogeny, continued through the Palaeocene and Eocene epochs of the Palaeogene period. At the same time, a hitherto insignificant group of animals called mammals was rapidly evolving and diversifying.

Geology Map of South East England, the English Channel and part of France
Geology Map of South East England, the English Channel and part of France

To the south, the Alpine Orogeny created vast mountain ranges across central Europe, including the Alps and the Pyrenees.  However, it was also responsible for the shaping of the London Basin syncline and the Weald anticline, leading to the development of the North Downs, the South Downs and the Chiltern Hills. The folding of the rocks was not a sudden event, so marine, then estuarine, and finally river sediments, continued to collect in the synclines (the downfolds) of the London and Hampshire Basins, as the new ranges of chalk hills were forced upwards. Up to 320 metres of sediments were deposited in the London Basin during this time, but the Chilterns became dry land once again. Chalk is a soft rock, but it resists erosion more than the even softer clays and sandstones of Southern England, so it now forms ranges of low hills.

Rivers that formed on the uplifted areas began to erode the hills, leading to the deposition of further sediments in the basins of southern England, including the Lambeth Group of gravels, sands, silts and clays. At this time, the English Channel consisted of a river delta with mud flats and deposits of sand.

A few words about the flint stones

Flint is a common, local building material, which originates in chalk. Flint is a hard form of the mineral quartz and it occurs as banded nodules of various sizes and shapes, particularly in the Seaford Chalk Formation (Upper Chalk). Inside each nodule, the flint is usually dark grey, black, green or brown in colour, and it often has a glassy or waxy appearance. A thin layer on the outside of the nodules is usually white and a rougher texture.

Chalk cliffs in Étretat, Normandy showing bands of flint nodules
Chalk cliffs in Étretat, Normandy showing bands of flint nodules

The exact way in which flint was formed is not clear, but it is thought that it occurred as a result of chemical changes, long after the chalk was deposited on the seabed. Flint is a form of silica, the origins of which were the silica-rich organisms such as diatoms (microscopic algae), other plankton and marine sponges. When these creatures died, their silica skeletons dissolved in the water near the sea floor. The deep, warm waters of the chalk sea became locally saturated with silica, which was re-precipitated, giving rise to flints and replacing some of the chalk, which had dissolved away. Initially, this was a sticky jelly-like substance but, over a very short period, the gel solidified into the very hard flint we find today. Often fossilized remains of sea urchins and sponges can be found in the cores of flint nodules.

Flint nodule
Flint nodule

Flint has been used in the Chilterns since the early Stone Age for making tools such as axe-heads, cutting tools and arrow heads. A later use of flint was in the flintlock mechanism used primarily in flintlock firearms but also used on dedicated fire-starting tools.

Lower Palaeolithic (early Stone Age) hand axe from the Thames Valley near Maidenhead
Lower Palaeolithic (early Stone Age) hand axe

Flint nodules are hard, but as a building material, they have limitations because of their irregular shapes and sizes. This is why flint of often replaced by a more manageable stone or brick for the corners of buildings, and for window surrounds. Sometimes the flint nodules are ‘knapped’ (shaped) into more regular blocks, but this is a highly skilled and costly process, so it is reserved for finer buildings such as churches.

Knapped flint nodules in church wall at St Michael and All Angels Church, Hughenden
Knapped flint nodules in church wall at St Michael and All Angels Church, Hughenden

Other local rocks also have economic uses. The clay-rich chalk marls of the Pit Chalk Formation (Lower chalk) has been used to make cement, and younger Palaeogene clays have been widely used locally to make bricks, such as this used to build Bradenham House around 1670.

Bradenham House was built with red bricks and tiles made from local clay
Bradenham House was built with red bricks and tiles made from local clay

Sarsens, puddingstones and clay-with-flints

Sarsen stones are sandstone blocks found in places on the Chilterns. They are the post-glacial remains of a cap of hard sandstone that once covered much of southern England. This dense, hard rock is made from sand grains bound together by silica cement, making it silicified sandstone. This is thought to have formed during the Neogene and Pleistocene periods, from the weathering and silicification of Upper Palaeocene sediments. 

Sarsen stone on Bradenham village green
Sarsen stone at Bradenham

Puddingstones were formed in a similar way alongside the sarsens; they are silica-cemented conglomerates composed of rounded river pebbles and cobbles (usually derived from flint) with a matrix of fine sand and silica cement. The rounded pebbles together with the sharp contrast in colour give this type of conglomerate the appearance of a Christmas pudding.

Puddingstone in woodland near Bradenham
Puddingstone in woodland near Bradenham

Across many Chiltern hilltops, there is a layer or a rock up to 4 metres thick, which is aptly named clay-with-flints. This rock formation is a residual deposit formed partly by the solution of chalk, leaving the insoluble flint fragment behind, and partly by a process called cryoturbation, by which layers of soft sediments were churned by freezing and thawing during the Ice Age. This affected both the Cretaceous chalk and the marine, estuarine and river sediments of the Palaeogene, which lay on top of the chalk. The clay-with-flints is unstratified (no strata or layers) and heterogeneous (particles of different sizes, from clay to flint nodules, are all mixed together). The dominant appearance is orange-brown and red-brown sandy clay, with numerous nodules and rounded pebbles of flint.  Nodular flints are derived from the Chalk, and rounded flints, sand and clay from Palaeogene formations.

Temporary excavation showing clay-with-flints near Andover
Clay-with-flints

In and out of the freezer

The Ice Age, known more exactly as the Pleistocene epoch, started 2.6 million years ago. It consisted of a series of cold ‘glacials’ and warm ‘interglacials’. The last glacial ended only 11,700 years ago. The Chilterns were never covered by ice, but at its maximum southern extent during the Anglian Stage, the ice was nearby in the Vale of Aylesbury, resting against the Chiltern escarpment at Coombe Hill and Ivinghoe Beacon. Although glaciers did not cover the Chilterns, the hills were a tundra area, and its surface was severely disturbed by the action of frost and ice. During the Pleistocene, we find the first evidence of humans living in the Chilterns. Extensive flint-working sites, dating from the early Palaeolithic period (125,000 - 70,000 BCE) were unearthed in clay pits at Caddington near Luton in the 1890s. Mixed in with the flints were elephant and rhinoceros bones.

The Ice Age in the Chiltern Countryside
The Ice Age in the Chiltern Countryside

Dry valleys

With a handful of exceptions, almost all valleys in the Chilterns have no streams or rivers. Although they are dry today, towards the end of each glacial period they were sites of eroding surface streams. Under the tundra conditions, the water came from melting snow in the spring and from summer rainfall. However, it was unable to penetrate the permanently frozen ground (permafrost), which had made the chalk rock impermeable. Unable to enter the natural pores and fractures in the chalk rock, the surface water was forced to continue flowing over the surface, forming river channels and allowing valleys to be cut down rapidly into the chalk.  As soon as the climate warmed in each interglacial, the permanently frozen ground thawed, and the normal drainage system of the Chalk resumed, with most water flowing beneath the surface again.

Large dry valley at Bradenham
Large dry valley at Bradenham

Some of the large dry valleys, such as the Bradenham Valley, may have been cut during the Anglian Stage of the Pleistocene by seasonal meltwater flowing directly from the huge ice sheets that rested against the northern edge of the Chilterns at around 450,000 years ago. 

Springs and chalk streams

A handful of valleys have been cut so deeply into the chalk hills that water leaks out of the valley sides to form small chalk streams such as the river Wye in West Wycombe Park or the stream at Hughenden Manor. Chalk is an aquifer, so it is able to soak up and hold the rainwater. Rainwater moves down through pores, along the bedding planes, and through cracks called fissures, often taking several months before it re-emerges at the surface in the form of springs that feed the chalk streams. Since groundwater levels in the chalk rock vary according to rainfall and season, chalk streams are naturally intermittent in their flow. Sometimes, the streams dry up completely for many months at a time.

The chalk stream at Hughenden Manor
The chalk stream at Hughenden Manor

During the winter, when rainfall is heavy and able to percolate through the chalk, the aquifer will be well topped up. The head of the stream may move up the valley as the water table rises. In summer, little rainfall percolates into the chalk as it is mostly taken up by plants and lost through evaporation. The water table drops and the head of the stream moves down the valley, leaving the top section of the valley dry. This section is called a winterbourne, because it usually flows after the winter rains.

Winterbourne streams have their own special wildlife which is adapted to cope with intermittent flows. Abstraction of drinking water from the chalk aquifer through boreholes has artificially lowered the water table, leading to some chalk streams flowing less frequently, even in the winter months.

Common red darter
Common red darter

Chalk streams are globally rare habitats, confined to North West Europe and to the UK in particular. In fact, of the 200 or so chalk streams in the world, over 160 are found in England. Chalk streams are important habitats for wildlife and support a broad range of plants and animals.  They are home to some of our most threatened plants and animals, such as the water vole and brown trout. They also supported thriving industries in the past, such as the many water-powered mills that used to dominate the Wye Valley. In the Domesday Book (1086) there are 20 mills mentioned on the River Wye. By 1815, there were 34 mills between West Wycombe and Bourne End.

A small mill in West Wycombe Park combined corn and saw milling for the West Wycombe Estate
A small mill in West Wycombe Park combined corn and saw milling for the West Wycombe Estate

Valley head and alluvium

During the Pleistocene glacials, angular chalky gravel accumulated in the bottom of many dry valleys. These rocks were formed from material accumulating as a result of downslope movements including landslides, debris flows, solifluction, soil creep and hill wash. Many of these processes are linked to the freezing and thawing of water held in the deposits, causing the original near-horizontal layers to be churned and mixed: a process called cryoturbation. The material is sometimes called Coombe rock.

Weathered chalk exposed at Beacon Hill near Coombe Hill
Weathered chalk exposed at Beacon Hill near Coombe Hill

The most recent deposits of all are still being laid down by the chalk streams. These deposits are referred to as alluvium or fluvial deposits. These unconsolidated ‘rocks’ are formed by rivers depositing sands, silt and gravel in channels to form river terrace deposits. Fine silt and clay from overbank flooding are added to the floodplain alluvium.