The Flints from Portsdown Hill

The flints of Portsdown Hill can clearly be seen in the quarry faces, the nearby fields, and the local gardens.

The following discussion covers:

• The principal derivation of flint
• Flint - a type of Chert
• The Form of Flint on Portsdown Hill
• Fossils of Flint and in Flint
• Secondary Derivations of Flint
• Where Portsdown flints are found today

The principal derivation of flint

The flint that is so obvious in seams in the Chalk is made of silica. Whilst oxygen is the most common element in the Earth's crust, silicon is the second most common - so the small occurrences of flint represent the most common components of the crust. The calcium and carbonate of the chalk are significant but less common in the Earth's crust's overall composition.

Most of the silica that makes up the flint nodules is biogenic. Although flint itself is an inorganic mineral, most of the silica derives from the internal skeletons of sponges in the form of spicules , which were common in the marine environment at that time.

The silica from these sponges dissolved and formed a fluid 'gel', contemporaneously with the deposition of the chalk. Pre-lithified chalk is a calcareous ooze on the sea floor, so this gel, a 'mobile, gelatinous, silica-based mass', flowed through it and was thus redistributed. Eventually, it accreted around nuclei of marine detritus, which could have been the remains of other sponges. As the gel stopped flowing, it dehydrated and hardened into the microscopic quartz crystals which constitute flint. This formation of flint nodules involved several stages of crystallisation, which are described below.

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Flint - a type of Chert

Flint is a term with a number of vague meanings, but it is indisputably a type of chert; while chert is 'cryptocrystalline silica' - that is, it is a rock consisting of a random mosaic of microscopically-small crystals of silica.

The quartz crystals that constitute the flint are only a few microns in diameter, and are interspersed with minute water-filled cavities. Because of this, those who work with flint today prefer 'quarry flint' to 'field flint', as the latter have had prolonged exposure to the air. Thus, they have dried and hardened and are less easy to work.

Chert itself occurs in a range of colours, from white through various shades of blue, grey and brown, to black. It often occurs as 'massive bedded deposits' - i.e. in continuous beds without form or structure. However, at Portsdown, we see flint in definite layers of nodules, and as fissure fillings.

Thus, 'flint' is a term variously used to refer to:

• 'Black chert' - which has the smallest crystals and highest proportion of silica
• The chert that occurs in chalk and marly limestone, usually in the form of nodules
• Some archaeologists even want 'flint' to refer only to the chert that has been made into tools and weapons - but that suggestion is unlikely to gain acceptance in the geological arena

Within this site, I use the term 'flint' to refer to all fine-grained siliceous chert found within the chalk.

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The Form of Flint on Portsdown Hill

Freshly-broken flints vary in colour from light to dark grey, and various tones of bluish-grey, brownish-grey, brownish-black and smoky black to black.
Flint is both hard and tough, meaning that it is resistant to both scratching, and breakage.
However, when it breaks, it does so with a characteristic conchoidal fracture. The resulting curved break resembles a scallop shell, so two breaks on either side of a flint leaves a razor-sharp edge between - a characteristic much used by Neolithic Man for tools and weapons.

Occasionally, broken flints can show "swirls" of a lighter grey, and may contain

• crystallised calcite,
• a white-cream coloured soft, powdery chalk, known as 'flint meal', or even
• a silica fossil

Flint with internal calcite

Flint generally appears in a variety of forms. Massively bedded chert is not seen at Portsdown, but both nodular flints, and sheet flints, are seen there.
The nodular flints may lie in a definite layer, in which they are known as tabular or semi-tabular, if the bed is less clearly defined. Alternatively, the nodules may be isolated.
The sheet flint lines and fills cracks, so follow the direction of any fissures in the chalk.

• Layer of flint nodules in chalk The most common are the flint nodules, which frequently have knobbly projections. These tend to lie in identifiable beds along the chalk, and typically vary in size from a few centimetres to 20cm.
  These often form around a nucleus of some sort, which could be a sponge skeleton or other marine detritus.
  Often, these preservation levels provide an additional stratigraphic tool for correlation.
  The image here shows a layer of such flints which are thus tabular, by virtue of their position, and nodular, by virtue of their form.
  This flints were almost certainly formed at the same time as the chalk was lithified, and clearly mark a depositional bed, or strata.
  
  
• Flattened flint nodules This image is of a layer of flattened flints.
  It appears to be a layer of nodules that have been subjected to pressure, so have been squashed. However, hardened flint cannot be squashed, so the flint probably formed during the compaction of the surrounding chalk, which is part of the lithification process. A semi-solid accumulation of silica gel had already gathered, but not yet hardened before compaction of the surrounding chalk began.
• Sheet flint lining a fracture Sheet flint forms after the lithification of the chalk. Here, the pre-flint silica gel has clearly filled a fissure that formed after the chalk had hardened, moved and cracked.
  Whilst the source of flint in this case may have been dissolved sponges, it is more likely to have been a result of later, post-marine processes.

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Flint nodules

The flint nodules that form in spaces in the chalk are often trace fossils; i.e. they fill the burrows that organisms have left in the seafloor. These are known as 'paramoudra' flints, and their formation is described below.
These sometimes-lobed nodules are often covered with a lime cortex, or coating, which appears to be the result of transitional chemistry between the flint and chalk. When freshly released from the chalk this cortex is pure white, but when exposed in a field setting, it can darken and turn pale-mid brown.
This colour change is due to the absorption of iron oxide and other chemicals into the silica crystal lattice from the surrounding soils and gravels.

Flint meal

When broken apart, some flints have been found to contain a soft, powdery chalk, known as 'flint meal'. This is only rarely found, and only ever in the Southern Chalks of the south coast and Norfolk. It is not found in the Northern Chalks of Yorkshire.
This powdery flint meal is rich in microfossils. There are more microfossils per unit weight in flint meal than in soft chalk, and their preservation is excellent. Furthermore, fossils from flint meal are much easier to process than from chalk. Ostracods and planktonic foraminifera are more common, and sponge spicules are preserved. Similar powder can sometimes be found inside hollow echinoids and sponges, but again this is very rare.
It is not certain what this flint meal represents, but reasonable suggestions have been made.
Many current research workers believe that flints form a few metres below the sediment-water interface at an early stage of diagenesis. If this is the case, then this flint meal must represent sediment trapped within the flint at that depth. It is thus chalk sediment that has been protected from most of the lithification processes that occur at greater depths, because it has been encased within the flint (or sponge or echinoid).
However; why is the microfauna is different, and why it only occurs in the black flints in the South and not in the grey Northern flints, is not known.

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Paramoudra Flint

Many of the nodular flints are termed 'paramoudra' flints. These are trace fossils of the burrows of an unknown organism. Because it is known only by its trace, both the trace and organism are given the same name, that of Bathicnus paramoudrae.

Paramoudra flints are often tabular and appear in definite layers. This indicates that they are marking a time and a surface that was particularly favoured by the organism. These same forms are seen throughout the Chalks of Europe. In some locations, the fossils of burrows can go from a few millimetres to about thirty centimetres in diameter and can reach a length of ten meters. Sometimes, a 'tube' of lithified chalk, one or two centimeters cross, can be found in the centre of a flint tube. Across the many outcrops of Europe, these tubes can be vertical, cylindrical, pyramidal, in the shape of shell, ringed, or horizontal. In certain cases, 'U' bends can be seen.

These flints are the traces of burrows, dug by an unknown organism(s), so are an indication of bioturbation. The penetration depth is sometimes surprisingly deep, up - or down - to 3 metres. There is difficulty in explaining the process of formation, and depth, of these burrows. Possibly, a sheath of silica gel formed around the burrow, which would have been used as a shelter for the animal. The habit of the animal is unknown; it could perhaps have been a filter-feeder, or a predator.

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Fossils of Flint and in Flint

As described above, flint can formed both during and after the lithification of the chalk matrix around it.
The earliest precipitations of flint occurred near organic remains, such as in burrow-fills and around decaying debris. Trace fossils of flint can often be found that hold the form of a burrow which was simply filled by the silica gel and formed paramoudra flints.
Occasionally, and more excitingly, uncompressed replacement fossils of organisms such as ammonites and echinoids in flint can be found. Submicroscopic studies of flint fossils show that they can form by replacing every detail of the organism at a sufficiently early stage to preserve original, undeformed textures, fabrics and soft tissues.
It is thus clear that the replacement of the organism by silica began early in the lithification of the chalk.

Replacement fossils can also be found within flints.
When the calcium-shelled animal died and settled to the sea floor, silica solution from the calcareous ooze leaked into the shell. Inside the calcite shell, it replaced the body of the creature, forming a silica model in its image.
At the same time, the shell of the organism could have have acted as a nucleus for the formation of a flint nodule around. In this case, the shell will be both surrounded by flint, and enclose a flint replacement fossil.
Ultimately, the calcareous shell surrounding the original organism will disintegrate; but it may already be enclosed and protected by a flint nodule that formed around it. Thus, the nodule with contain a loose fossil within, which may rattle if shaken and easily falls out whole when the flint is broken.

In areas of fossiliferous chalk, flint-filled fields are rich hunting grounds for replacement fossil echinoids - but not, unfortunately, in the fossil-poor Upper Chalk of Portsdown.

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Secondary derivations of Flint

Most of the silica that forms flint, as already mentioned, is derived from the spicules of sponges.
However; silica may also be derived from microscopic organisms (radiolarians) that live in shallow water, that also have shells of silica. On death, these shells sink to the sea floor, and with time and compression from the later sedimentation that covers them, the silica dissolves and resolidifies to form flint.

This, as with the dissolution of sponges, occurred at the same time as the deposition of the Chalk, during the Cretaceous.

However; another source of silica may be have been much later, during the Tertiary Period, when groundwater percolated down to and through the underlying porous, lithified Chalk.

Traces of very fine-grained quartz particles occur throughout the Chalk made from silica which did not dissolve and accrete to form nodules at the time of lithification.
However, the percolating groundwater of later times can dissolve these small crystals of silica. When this happened, the silica solution flowed through the porous and permeable chalk and recrystallised in any voids in the chalk where it could accumulate. These voids may be cracks in the rock which are the fissures that opened following lithification, uplift, and movement of the solid chalk matrix. This process thus gives rise to the sheet flints that line fissures and joints, and possibly also to some of the isolated nodular flints, if other spaces are made.
It is the presence of flint sheets along joints and faults that shows that some quartz was again mobile during the burial, folding and faulting of the lithified Chalk.

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Where Portsdown flints are found today

Flints are found, as mentioned, as nodules in tabular layers in the chalk, in sheets and as isolated nodules. It can can now be seen in the disused quarries on Portsdown Hill, such as Candy's Pit, Portsdown, and Downend, and all over the surface of the Hill.

It is also seen in the nearby fields, and, as any local gardener will assure you, it appears in the local gardens, too.

Furthermore and a little further away, the local beaches at Southsea and Stokes Bay at Gosport are all of flint pebbles. These have markedly different in appearance to the fresh flints of Portsdown. Firstly, they are smaller and more rounded, following millions of years of weathering and buffeting by the sea. However, they are also of a different colour, being more frequently a light brown, rather than the grey of the fresher flints. White, grey, black and red pebbles are also easily found.
Many of these pebbles often show colour-zoning, with the innermost area being grey whilst the outer is brown. This colour-change is due to the absorption of iron oxide and other minerals into the crystal lattice during the millions of years of weathering.

Flints can also be found in the walls and buildings, as discussed in Buildings.

The local museum in Fareham has a collection of Neolithic flint tools and weapons.

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The following section discusses sponges which provide most of the silica that forms the flint. Please read on, return Home, or use the navigation links on the left.

For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk