This is a revised version of an article originally prepared for the National Trust’s White Cliffs of Dover volunteers’ intra-web site. Parts of this note assume some familiarity with the National Trust’s Fan Bay Deep Shelter. This is a system of Second World War tunnels situated between Dover and South Foreland and can be visited by taking a “hard hat” tour lead by enthusiastic and knowledgeable volunteers. Opening times can be found on the National Trust’s White Cliffs of Dover web site.
Short answer:
Flint formed in chalk after it was deposited on the seabed but before it was compressed into chalk. It started as a chalky ooze, accumulating from the remains of microscopic plankton shells on the seabed. Silica, dissolved in seawater along with some impurities, precipitated within permeable pathways in this sediment. These pathways were animal burrows, fracture planes caused by stresses in the seabed, or bedding planes, all of which resulted in a local increase in the permeability of the chalk, allowing the passage of the silica-bearing water.
Over time, the precipitated silica crystallised into flint, a micro-crystalline form of quartz, and took on the shape of the original cavities. Thus, flints are internal moulds of these cavities. The dark colour comes from the included impurities. The knobbly types are called burrow-form flints. The less common thin bands are called sheet flints and were formed along bedding planes and fracture surfaces.
Long answer:
Flint is a form of silica. Crystalline types include many well-known gemstones such as Rock Crystal (quartz), Amethyst, and Citrine. Some types of silica consist of microscopic crystals, such as Chalcedony and Chert. Some are non-crystalline, such as Opal. When found in chalk, Chert is usually called flint, and those found from Norfolk southwards are characteristically black or very dark grey. The black variety has a higher proportion of iron and carbon impurities.
Flint is a hard mineral, so how is it that it is found embedded in chalk, a soft white limestone? And why do flints occur in layers? Photo 1 shows typical flints at Langdon Cliff, near Dover.
Image reproduced by kind permission of the National Trust: https//www.nationaltrust.org.uk
Firstly, before looking at their possible origins, it may help understanding to consider some key features of the environment in which flint was formed – the chalk seabed.
Chalk was formed from coccoliths (microscopic shells) produced by a type of calcareous, single-celled algae called coccolithophores (a type of microscopic plankton). In the “hot house” climate of the Upper Cretaceous, about 101 to 66 million years ago, the seas were sufficiently warm to support blooms of coccolithophores. As they died, they produced a steady shower of calcareous debris and some organic remains, which accumulated as a slightly organic, chalky ooze. Over millions of years, this compacted under its own weight to form chalk.
The silica needed to form flints was dissolved in the water within the sediment (porewater). It originated from organic sources, particularly sponges, but also from Radiolaria and Diatoms (single-celled plankton), and perhaps from inorganic sources such as submarine volcanic eruptions. The organic content of the sediment derived from the bodies of animals and plants. This provided food for bacteria, which, after burial for some time, broke down the organic matter, releasing sulphur in the form of sulphates (sulphur combined with oxygen).
The mechanisms outlined below for the transformation of dissolved silica into flint are based on those described by Bromley & Ekdale (1983) and Clayton (1986). Both mechanisms require deoxygenated porewater in the chalk sediment. This usually occurs at a depth where normal (aerobic) bacteria, feeding on organic debris, deplete the oxygen to levels too low for aerobic function and where the depth is too far below the seabed for oxygenated water to permeate and replenish oxygen. At this depth, a primitive type of anaerobic bacteria called sulphate-reducing bacteria takes over, which can function in the absence of oxygen. The boundary between deoxygenating (reducing) and oxygenating conditions is the Redox Boundary. The depth to this boundary depends on the permeability of the chalk sediment and its organic content, but depths of 10 cm to 10 m below the seabed have been quoted. In the absence of other variables, the Redox Boundary would be parallel to the seabed, the source of the oxygenated seawater.
The Bromley model describes a situation in which silica is precipitated directly as small nodules that accrete to form flint. This mechanism more readily explains the formation of discrete flint fossils, such as shells and sea urchins (echinoids).
The Clayton model has a wider application and can explain the formation of most types of flint, but the process is more complex. In this model, silica passes through several intermediate phases before precipitating as micro-crystalline silica (flint). A detailed description of the chemical reactions is not included here but is provided in Clayton (1986).
The process begins at shallow depths within the chalk sediment, after deposition but before the sediment has fully compacted into solid chalk. The first requirement is that the silica dissolved in the sediment be precipitated. This was initiated by the anaerobic decomposition of organic matter below the Redox Boundary.
This well-known process occurs in many fine-grained sediments containing organic matter such as silts and clays. In these environments, anaerobic decomposition releases sulphur as a sulphide (sulphur without oxygen), which can form iron sulphide (FeS), known as pyrite or “Fool’s Gold,” in the resulting sediments. Since chalk sediments contain very little iron, the sulphur instead combines with hydrogen to produce hydrogen sulphide gas (H2S), which has a rotten egg smell. This gas bubbles upward through the sediment, and when it encounters oxygen-rich areas, it oxidizes into sulphuric acid (H2SO4).
This acid has two effects. It causes silica dissolved in the porewater to precipitate. It starts as a type of opal, which is silica with molecularly bound water, but with time and increasing pressure the water is driven off and the remaining silica eventually crystallises as flint. Secondly, the acid dissolves chalk, enlarging any permeable pathways in the chalk that preferentially contain a higher concentration of porewater.
Occasionally, iron is present in the chalk sediment, and you can sometimes see orange-brown iron staining concentrated on the underside of sheet flints. The position on the underside suggests that it is not a recent permeation from the surface but that it had risen from below in the chalk ooze at an early stage (see Photo 2).
The depth of the Redox Boundary influences the location of flint production. Under conditions of constant sedimentation, this boundary would rise uniformly through the sediment, maintaining the depth below the seabed at which oxygen depletion occurs. If this were the case, why isn’t flint found uniformly within the chalk as the Redox Boundary rises through it?
The flints in Photo 1 show the typical arrangement of flints in bands; they are not uniformly distributed. One suggestion is that variations in sedimentation produced permeable layers that provided pathways for silica-rich porewater to enter the sediment. This would explain flint bands of limited horizontal extent, but a key feature of many flint bands is that they can be traced over hundreds of kilometres (for example, from North Yorkshire to north-eastern France). Such widespread features are unlikely to be the result of sedimentary processes but point to a global influence. Climate variation is the most likely cause of global effects.
There are several ideas about how temperature could have affected the seabed sediments. The simplest is that the amount of plankton available to produce the chalk sediment varied with regular shifts between hot-house and more temperate conditions. A similar idea involves variation in the supply of silica in the seawater. Another idea involves the Redox Boundary and the time required to complete silica precipitation. The suggestion is that cooling would reduce plankton blooms and sediment accumulation, allowing the Redox Boundary to remain stationary long enough to complete the silica precipitation process.
So, the next question is what caused this global variation?
A current idea is based on the recognition that dates of flint bands show a periodicity of roughly 21,000 or 41,000 years, which is similar to the periodicity of two of the Milankovitch Cycles. Milankovitch Cycles are regular, long-term variations in the orbit and spin of the Earth around the Sun, which affect the amount of the Sun’s heat reaching the Earth. (For further details, see the section on Milankovitch Cycles in “Determining a timetable of glaciations”). It is suggested that flint formation coincided with the cooler parts of the Milankovitch Cycles, which caused the coccolithophore blooms to disappear, reducing the build-up of sediment on the seabed. This allowed the Redox Boundary to remain at the same depth below the seabed for long enough for the process of silica precipitation to complete, although the solid precipitate at this stage was a form of opal (silica with molecularly bound water). Restoration of normal hothouse temperatures restarted sedimentation, along with movement of the Redox Boundary up through the sediment until the next cooling period.
This may explain the regularity of flint bands on a larger scale, but there are irregularities in their orientation on a smaller scale. So what caused this?
The seabed was subject to episodes of tectonic (deep-seated) uplift and tilting, which caused faulting, slumping, and short-lived but vigorous tectonically induced currents that scoured the seabed (tsunamis?). These disruptions, in the otherwise uniformly stratified sediments, produced local variations in permeability, allowing accumulations of silica-rich porewater.
Examples that most convincingly do not conform to the regular banding are the steeply inclined sheet flints, which are sometimes cut off and displaced along horizontal fault planes. These inclined sheet flints have been interpreted as forming on fault or slump surfaces (R. Mortimore, 2011, Fig. 67; Clayton, 1986, Fig. 4.1c). It has also been suggested that the permeation of silica-rich fluid under pressure is likely to have been involved.
Flints that form on these surfaces are sheet flints. They are usually less than 50 mm thick but can be quite extensive. A good example of sheet flint can be seen in the National Trust’s Fan Bay Deep Shelter, shown in Photo 2. In this example, the sheet flint is parallel with the bedding and can be followed along the tunnel system. Here, the silica-rich water preferentially followed a more permeable discontinuity in the bedding; such discontinuities can be formed by lithological variations or bedding-plane fractures.
Image reproduced by kind permission of the National Trust: https//www.nationaltrust.org.uk
It is difficult to attribute the formation of the chalk lens in Photo 2 to simple sedimentation. The lens is enveloped by a surface that joins on each side of the lens; deposition of successive layers could not form such a surface. Some post-depositional process must have occurred to produce this feature. Mortimore (2019, Fig. 32) explains such lenses as chalk blocks that have been rolled and elongated by horizontal shear movements. But this block is extremely “streamlined”; it gives the appearance of being formed by flowing water. Perhaps it was produced by one of those tsunami currents flowing across the seabed?
A deviation: If you are visiting the Fan Bay tunnels, at the location of the lens structure in Photo 2, take the opportunity to view a rare example of a well-preserved fragment of fossil clam shell in the roof above your head. Follow this link to see Cladoceramus undulatoplicatus (Snowshoe Clam).
There were also biological processes that produced local variations in the chalk sediment after its initial deposition. Bottom-dwelling animals found the seabed an ideal habitat for burrowing for feeding or protection. Disused burrows became filled with fine debris, which was more permeable than the surrounding sediment. This local increase in permeability influenced the location and shape of the precipitating silica. The flints formed by this process are called “Burrow-Form Flints”. They are internal moulds of animal burrows, albeit enlarged and distorted by subsequent acid dissolution of the surrounding chalk. These are the commonest flints. They are the knobbly-shaped flints you may see used as building stones or come across in fields in the Chalk Downland of southern England.
An uneven concentration of decaying organic matter also altered the rate of oxygen depletion by anaerobic bacteria. For example, a large mass of decaying matter could have attracted a high concentration of bacteria with a high oxygen demand, resulting in local reducing conditions, even if it lay above the main Redox Boundary. In this situation, the Redox Boundary would have been a three-dimensional surface surrounding an anaerobic core.
This process may have been the origin of a third type of flint, the Paramoudra flints, sometimes called Pot Stones. These are roughly cylindrical, up to about 0.5 m in diameter and slightly taller. Giant Paramoudras are aggregates of flints that can be several metres in diameter. (The normal Paramoudras and Giant Paramoudras are probably different, and it might be appropriate to use the term Potstone for the normal Paramoudras.)
The normal Paramoudras have a hole through the vertical axis or, if not, a hard chalky core. Some can be pebble sized with a small central hole penetrating them. Photo 3 shows a slightly wave battered example of a Paramoudra (without a chalky core) on the beach at Ness Point southwest of St Margaret’s Bay. Photo 4 shows one in-situ in the cliffs at Pegwell Bay in north Kent showing how it protrudes from a flint band.
Giant Paramoudras can reach nine metres in height (as seen in cliffs) and up to two metres in diameter (Bromley et al., 1975; R. G. Bromley & Ekdale, 1986). Rather than being a single flint, they are aggregates of flints. When seen in section on cliff faces, they show an irregular shape, and when they pass through a flint band, the flint may extend into it.
The chalky core frequently contains a trace fossil, a tube or track of a creature, which Bromley et al. (1975) called Bathichnus (from Greek, deep trace). According to the Redox Boundary model, this creature would have been the source of the organic matter decomposing under anaerobic conditions, and the Redox Boundary would have been a vertical cylinder surrounding a decomposing core. Given that these Giant Paramoudra can be several metres tall and pass through chalk spanning several million years, the creature living in the central core would have had an inconceivably long lifespan. Therefore, the origin of these paramoudras must be different from that of other flints.
These were described as fossilised sponges in the 19th century, and Yeomans (2018) has spent much time refining this idea. Photo 5 shows the top of a partly submerged Giant Paramoudra on the shore at West Runton Beach, Norfolk. This example is about a metre and a half in diameter. It also shows a gap in the circumference, which Yeomans (2018) refers to as the “exhalent dip” found in most Paramoudras. These are always orientated in the same direction, which Yeomans attributes to the prevailing current, supporting the sponge origin rather than silica accretion.
Yeomans (2018) has identified a large Cretaceous seabed reef on the foreshore at West Runton, comprised of Giant Paramoudra flints. They show a north easterly alignment which he attributes to the effect of the prevailing sea-bed currents.
Andrew Coleman
Rev. 02/07/2026
References:
Bromley, G. R., Schulz, M.-G., & Peake, N. B. (1975). Paramoudras: Giant Flints, Long Burrows and the Early Diagenesis of Chalks. The Royal Danish Academy of Sciences and Letters., 20(10), 1–31.
Bromley, R. G., & Ekdale, A. A. (1983). Flint and fabric in the European chalk. In G. de G. Sieveking & M. B. Hart (Eds.), The scientific study of flint and chert. (pp. 71–87). Cambridge University Press.
Clayton, C. J. (1986). The chemical behaviour of flint formation in Upper Cretaceous chalks. In G. D. G. Sieveking & M. B. Hart (Eds.), The scientific behaviour of flint and chert (pp. 43–54).
Mortimore, R. (2011). A chalk revolution: What have we done to the Chalk of England? In Proceedings of the Geologists’ Association (Vol. 122, Issue 2, pp. 232–297). https://doi.org/10.1016/j.pgeola.2010.09.001
Mortimore, R. N. (2019). Late Cretaceous to Miocene and Quaternary deformation history of the Chalk: Channels, slumps, faults, folds and glacitectonics. Proceedings of the Geologists’ Association, 130(1), 27–65. https://doi.org/10.1016/j.pgeola.2018.01.004
Yeomans, R. (2018). Paramoudra: Observations on large flint structures from the Chalk (Upper Cretaceous) and flint formation. Proceedings of the Yorkshire Geological Society, 62(3), 210–216. https://doi.org/10.1144/pygs2017-005
Geomusings 