Rocks

This section describes the various rock types seen throughout the county, some of which are nearly 200 million years old. They are successively overlain by younger and younger rocks, although this process was neither continuous nor passive. Instead, geological history records time periods when rocks were gently folded and uplifted before being worn away by water and ice. Of particular significance are those deposits that accumulated over the last two million years, during periods of marked climatic changes known collectively as the Quaternary ‘Ice Age’. These deposits are often quite thin, but their influence on the landscape is profound and they generally provide the parent material on which soils develop.

The rocks of Bedfordshire, indeed of southern Britain as a whole, are of sedimentary origin. They formed in ancient seas as layers of sand and mud on the sea bed. This process of settling under gravity imparts their chief characteristic which is layering or bedding. It is generally assumed that bedding is horizontal at the time of deposition but rocks are often tilted and folded by later movements of the Earth's crust. This brings the rocks towards the Earth's surface, often many millions of years after they were initially deposited. There they are subject to the interminable processes of weathering and erosion that fashion them into landscapes. What results is a pattern of sedimentary rocks at or near the present land surface that is best illustrated on a bedrock geological map. Of course, the oldest rocks remain deeply buried and are known only from borehole or geophysical records, but by combining all this information it is instructive to visualize the entire sequence in a cross-section - effectively a vertical slice through the countryside.

JURASSIC

The Upper Lias is the oldest exposed rock in Bedfordshire, comprising a series of silty muds and muddy limestones that were laid down in the sea around 190 million years ago (Ma). This sea was warm, well-oxygenated and teemed with life, including abundant ammonites. Uplift and subsequent erosion means there is a time gap between this and the overlying Inferior Oolite, rocks that were deposited around 175 Ma. The Inferior Oolite contains tiny balls of calcite (calcium carbonate) that tell us they formed in a warm, shallow sea. Waves rolled the particles backwards and forwards on the sea floor, giving them an even coating of calcite. Both these ancient formations crop out in tiny areas on either side of the River Ouse.

Oolitic limestone, Pavenham

Oolitic limestone, Pavenham

The Great Oolite is a much harder rock, again an oolitic limestone formed in warm, shallow waters, but containing far less clay. Ammonites, sea urchins and bivalves are common fossils in this rock. It can be found at the surface along the valley sides of the River Ouse between Bromham and Milton Ernest, and also between Harrold and Felmersham. The Great Oolite is a good building stone and many of the north Bedfordshire villages provide excellent examples of its use.

The Cornbrash is a sequence of thin bedded rubbly limestone containing lots of completely smashed fossil shells, the result of strong wave action near a shoreline about 170 Ma. In Roman times it was occasionally used as a roofing material, but it is not a good building stone. However, it does make a fertile, well-drained soil (hence the name Cornbrash - ‘a site where good corn is grown’).

Amoeboceras from the Ampthill Clay

Amoeboceras from the Ampthill Clay

The Oxford Clay and West Walton & Ampthill Clay sequence records a deepening of the Upper Jurassic sea. The water column supported frequent algal blooms which, on death, resulted in a mass of organic matter accumulating on the sea floor. Where there was little or no oxygen available to oxidise it, the matter was preserved as rich organic layers within the clay. These organic compounds are the reason for the excellent firing quality of the local brick-making clays. The flat landscape of Marston Vale makes it impossible to see where one type of clay ends and another begins, but most of northern Bedfordshire is underlain by these clays.

Some folding, uplift and erosion occurred after the deposition of the Upper Jurassic clays. Such events are common throughout geological history and they punctuate the depositional cycle, either causing rocks not to be deposited locally or eroding those that were deposited.

CRETACEOUS

Greensand Ridge view

A view from the Greensand Ridge

The Greensand Ridge is an outcrop of the Woburn Sands Formation, a geological unit named after the the village of Woburn Sands, near Milton Keynes. These sands were laid down in a seaway around 120 Ma, during Early Cretaceous times. This was an exciting episode in Bedfordshire’s geological history, for having been dry land for about 30 million years, the area was then flooded again by the sea. A narrow tidal channel running southwest from the Wash, across Bedfordshire, and onward to the Isle of Wight, formed as a result of rising sea level caused by global warming.

Another unconformity separates the Woburn Sands succession from the overlying Gault Clay. The Gault Clay represents a return to deep, open water conditions as the climate continued to warm and southern England was progressively submerged beneath the sea. Thick sequences of bluish grey clays were deposited and they contain abundant marine fossils such as ammonites and bivalves. Towards the top of the clay sequence the Upper Greensand is distinguished by thin beds of sandstone that contain glauconite, a green mineral that imparts a distinct colouration to the rock. Both of these rock types are very poorly exposed in Bedfordshire because they are generally covered by glacial deposits, but they form the low-lying clay vale that runs from Wrestlingworth to Leighton Buzzard.

Totternhoe Stone

Totternhoe Stone

The Chalk is an unusual and distinctive rock that produces the best of Bedfordshire’s scenery. It is a particularly pure organic limestone made largely of the remains of microscopically small calcareous algae and yet it produces the thickest rock sequence in the county. The lower parts of the sequence are quite rich in clay but the upper parts are practically 100% calcite, suggesting that neighbouring landmasses were progressively submerged and contributed little detrital sediment into the sea. Occasional harder bands of limestone punctuate the succession and produce the subtle breaks of slope that make up the Chiltern escarpment; these are identified as the Totternhoe Stone (T) within the Lower Chalk, the Melbourn Rock (M) at the base of the Middle Chalk, and the Chalk Rock (C) at the base of the Upper Chalk. In addition, horizons of flint nodules and siliceous fossils occur commonly within the Upper Chalk.

TERTIARY & QUATERNARY

Towards the end of Cretaceous times, around 65 Ma, uplift affected Britain and the Jurassic and Cretaceous rocks were gently folded. Most of the British Isles became land, although parts of southern Britain were occasionally flooded by the sea. In Bedfordshire the Early Tertiary deposits probably comprised thin sands and clays laid down by rivers that drained eastwards across a low-lying marshland. Historically, a small remnant of that deposit was recognised in the vicinity of Slip End, near Luton, but it was extracted for brick making in the 19th century and is no longer evident. Nor is there any remaining evidence of younger Tertiary deposits because this was a period of ongoing uplift during which deep chemical weathering occurred under a warm and humid climatic regime. At the same time major river systems began to erode deeply into the underlying Chalk.

The legacy of the Quaternary period in Bedfordshire is out of all proportion to its duration. Lasting only about two million years, this dynamic time period is characterised by a number of alternating cold (glacial) and warm (interglacial) climatic phases, with the cold ones being far more long-lasting. The glacial episode with the greatest impact in this area is known as the Anglian and it produced an ice sheet about 450 thousand years ago that covered the entire county with the exception of the highest Chalk hills south of Luton. Other cold episodes before and after the Anglian did not produce permanent ice sheets in Bedfordshire; instead the conditions were periglacial, resulting in frozen ground that was subject to seasonal melting, and creating ephemeral rivers and unstable slopes.

As Quaternary sediments are relatively thin and blanket the underlying bedrock they are often referred to as superficial deposits. Their distribution is complicated by the fact they occur in a variety of topographic settings. For example, glacial deposits may infill a pre-existing river valley and suppress the topography, or accumulate on the high ground between river valleys (interfluves) and accentuate it. The principal types of superficial deposit in Bedfordshire are outlined below.

The oldest deposits are Clay-with-flints and they occur as a dissected capping overlying the dip-slope of the Upper Chalk outcrop around Luton. Generally less than 10 metres thick, they are composed of orange-brown or red-brown clays and sandy clays containing abundant flint nodules and pebbles. Clay-with-flints probably formed through the in-situ alteration of Late Cretaceous and Early Tertiary deposits by soil-forming and periglacial processes over a very long time period before the Anglian glaciation.

Subsequently, as the Anglian ice sheet advanced and retreated it deposited thick sequences of blue-grey clay containing clasts of chalk, flint, quartzite, some Jurassic limestone and reworked Jurassic fossils. Historically this type of deposit was called ‘boulder clay’ although now the term glacial till is preferred. The clay and silt content of the till is derived almost entirely from local erosion of the Oxford Clay. In contrast, many of the coarser clasts come from further afield; the chalk and flint originate from the Lincolnshire Wolds and East Anglia, the quartzite pebbles from the Midlands, and occasional ‘exotic’ rocks have been transported by the ice sheet from as far away as Scandinavia.

Glacial till is the most extensive of the superficial deposits in Bedfordshire. Only the highest chalk hills are free from of a cover of till and this is because the Anglian ice sheet flowed around them, occupying the low ground and exploiting natural ‘gaps’ such as that at Hitchin. The till thickness is highly variable, partly reflecting the undulating nature of the pre-Anglian landscape, but also the magnitude of subsequent erosion and weathering. Where the till has not been eroded by subsequent periglacial and fluvial processes it may be tens of metres thick, as on the undulating, elevated plateau north of Bedford. Isolated remnants of till also contribute significantly to the subtle topography of the county by forming low hills within the clay vales, as at Pulloxhill and Tebworth.

Glaciofluvial sands and gravels, Broom Quarry

Glaciofluvial sands and gravels
Broom Quarry

Sporadic, discontinuous bodies of sand and gravel occur beneath, within and on top of the till and these are interpreted as glaciofluvial deposits, dumped by meltwater streams flowing through and around the margins of the Anglian ice sheet. These two glacial deposits are distinguished on the map, but the complex relationship between them is more clearly expressed in the schematic cross-sections.

Thin but extensive sand and gravel deposits that occur along the valley of the Great Ouse and its tributaries - the Ivel, Flit and Ouzel - are regarded as river terrace deposits. They are typically recognised in the field as a series of flattish ‘benches’ or terraces in the valley sides and grouped according to their elevation above the present-day floodplain. Terraces represent the eroded remnants of older floodplain deposits and they formed in several stages during the 400 thousand years since the retreat of the Anglian ice. As the rivers shifted laterally they reworked and eroded the Anglian till and glaciofluvial deposits, progressively creating as much as 50 metres of valley topography. Fossils found in the terrace deposits indicate that this lowering of the local landscape occurred under a range of climatic conditions that varied from warm, through cool temperate to cold.

Head deposits, Clophill

Head deposits, Clophill

Head is a heterogeneous deposit derived from mass downslope movement of material mainly under periglacial conditions. Most head deposits are believed to have originated during the latest (Devensian) glacial episode that lasted from about 115-10 thousand years ago, compounded since then by soil creep and hill wash during the current temperate climate. The composition of head closely reflects the upslope deposits from which it has formed; thus the Woburn Sands Formation gives rise to head comprising sandy silts and clays with clasts of ironstone and varying amounts of flint gravel.

Head has not been mapped systematically across the county but it can be assumed that a thin veneer is widely present, particularly on the lower slopes and floors of valleys. Thicker and older (pre-Anglian) head deposits occur in the dry valleys or coombes of the Chalk escarpment. Here material from the Clay-with-flints has spread downhill and been preserved as a chalky loam in the valley bottoms because they lay beyond the erosive limit of the Anglian ice sheet.

The present-day floodplains of the rivers are underlain by alluvium comprising silt and clay reworked from earlier superficial deposits and the bedrock. In the main river valleys the alluvium is up to four metres thick and occupies channels cut into the lowest river terrace deposit. The alluvium may contain a significant amount of organic material, including lenses of peat, as recognised in the River Flit near Ampthill.