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<The Chalk of Portsdown Hill
Portsdown Hill, when seen from the harbour in the south, or when driving, cycling, walking or flying over, clearly shows as a ridge of pure white chalk. Quarry faces, numerous pits, the road verges, as well as bare fields, grassland and scrubland all display the white rock which underlies the countryside.
The chalk is biogenic, being composed of the remains of calcitic organisms. It was deposited during the Cretaceous Period.
The following discussion covers:
- Introduction to Portsdown Chalk within the UK
- "chalk" v "Chalk"
- Chalk - a type of limestone
- What makes Chalk?
- The varying hardness of Chalk
- The lack of fossils in Chalk
- The environment indicated by the chalk
- Worldwide Cretaceous Chalk
Introduction to Portsdown Chalk within the UK
The Chalk ridge of Portsdown runs for about 10km from west to east, from Fareham to Bedhampton. Only a thin layer of soil covers the chalk, which has a depth greater than 200metres.
This same chalk is exposed across great swathes of Britain, from the South-East up to Yorkshire, and across much of Western Europe. It has been calculated that the total amount of Chalk in onshore Britain, both outcropping and lying below younger beds, is about 21 000km3, which is of course a tiny proportion of that that underlies the North Sea and much of Europe. The extent of the Chalk outcrop in Britain can be seen as bright green on the map within these pages.
Its importance within the study of geology cannot be over emphasised. Indeed, Thomas Henry Huxley wrote in his "On a piece of Chalk" in 1868:
What is this wide-spread component of the surface of the earth? and whence did it come?....I weigh my words well when I assert, that the man who should know the true history of the bit of chalk which every carpenter carries about in his breeches-pocket, though ignorant of all other history, is likely, if he will think his knowledge out to its ultimate results, to have a truer, and therefore a better, conception of this wonderful universe, and of man's relation to it, than the most learned student who is deep-read in the records of humanity and ignorant of those of Nature.
He rightly asserts that A great chapter of the history of the world is written in the chalk - and most of the history of Portsdown Hill is there.
"chalk" v "Chalk"
Within some contexts, the word 'chalk' is often spelled with a capital 'C'. This has a specific and separate meaning.
The rock 'chalk' is a fine-grained, white limestone, made principally from the biogenic calcite tests of coccoliths and foraminiferas.
The name 'Chalk' refers specifically to the late Cretaceous deposits of Western Europe, which are composed predominantly but not exclusively of this rock, chalk.
Chalk - a type of limestone
Chalk is a type of limestone, limestone is a type of carbonate rock, and carbonate rock is almost always a type of sedimentary rock.
Sedimentary rocks
Most of the outcropping rock that makes the surface of Britain is composed of sedimentary
rock.
Almost all sedimentary rocks result from depositions made by, and under, water - whether fresh-water lake, river, coastal, or marine.
Although soils can lithify and desert sands may consolidate to form sandstone, nearly all sedimentary rocks result from deposition in water.
Limestone Carbonate rocks
A carbonate rock is one whose main rock-forming component is CO3 (carbon with oxygen). The rock that is chalk is a limestone, and is calcium carbonate, CaCO3.
There are many distinct types of limestone outcrop in Britain of various ages, of which the three most important are:
- Carboniferous limestones from the Peak District, Mendips and Yorkshire Dales, which are usually well-cemented, tough, grey rocks.
- Jurassic limestones, seen mainly along the length of the Cotswolds. These are generally pale brown and much softer than Carboniferous.
- Cretaceous limestones, which are all varieties of chalk, and dominate the scenery of south-east England.
Both the Carboniferous and Cretaceous limestones are biogenic, although they result from different processes and are different in age. The Jurassic rock is not biogenic but is instead a chemical precipitate, which resulted from the crystallisation of dissolved carbonate around nuclei in the warm seas. This latter is often termed an 'oolitic limestone', as the particles are roughly spheroid.
Chemically, all forms of limestone are the same as the lining seen blocking pipes and appliances in chalk regions. As Huxley said, the fur on the inside of a tea-kettle is carbonate of lime; and, for anything chemistry tells us to the contrary, the chalk might be a kind of gigantic fur upon the bottom of the earth-kettle, which is kept pretty hot below.
He knew, however, that the origin of the chalk was different - that it is composed of the skeletal remains of billions of tiny organisms, of a type that still survive in today's oceans.
What makes Chalk?
Chalk is "a soft, friable, fine-grained, finely-porous, very pure, white, limestone". It is biogenic, and consists mainly of the calcite shells, skeletons and tests of organisms, most of which are microscopic.
Every metre of today's chalk represents about 100 000 years of deposition of these skeletons, which individually are microscopic in size.
The many hundreds of metres of chalk beds seen in the South of England thus represent tens of millions of years of deposition.
The pure white chalk of Portsdown and Southern England may be 98-99% carbonate, and most is over 90%. However, the 'grey' chalks of Northern England (see Chalk formations) may have up to 20% impurities, such as clay and marl.
The purity of Southern Chalk gives it its clean colour, and the high-reflectivity indicate its fine-grained nature.
Chalk consists mainly of the calcite skeletons and tests of microscopic organisms, and there may be around 80 different species of organism remains represented in a single sample of chalk.
However, most of the calcite remains that form the chalk is composed of coccoliths, which can form up to 80% of the rock.
A coccolith is a single calcite plate from the microscopic organism the coccolithophore - of which there is more in the page about Coccoliths.
Coccoliths in Chalk
The term 'coccolith' was coined by our friend Huxley back in 1868, who examined dredgings from the Atlantic ocean-floor of his day. He had already identified a large component of the sea floor mud as Globigerina. He knew that chalk was partially composed of Globigerina, then further discovered:
In working over the soundings collected ... I was surprised to find that many of what I have called the "granules" of that mud were not, as one might have been tempted to think at first, the mere powder and waste of Globigerinæ, but that they had a definite form and size. I termed these bodies "coccoliths," and doubted their organic nature. Dr. Wallich ... added the interesting discovery that, not unfrequently, bodies similar to these "coccoliths" were aggregated together into spheroids, which he termed "coccospheres." So far as we knew, these bodies, the nature of which is extremely puzzling and problematical, were peculiar to the Atlantic soundings. But, a few years ago, .... a careful examination of the chalk by means of thin sections and otherwise, observed, .... that much of its granular basis possesses a definite form. Comparing these formed particles with those in the Atlantic soundings, he found the two to be identical; and thus proved that the chalk, like the surroundings, contains these mysterious coccoliths and coccospheres. Here was a further and most interesting confirmation, from internal evidence, of the essential identity of the chalk with modern deep-sea mud. Globigerinæ, coccoliths, and coccospheres are found as the chief constituents of both, and testify to the general similarity of the conditions under which both have been formed.
The fragments of coccoliths are tiny - an original plate of maybe 10 microns in diameter produces fragments often less than 1 micron in diameter. Considering that that the wave length of red light is 0.72 microns, the diameter of a complete coccolith plate is barely 10-15 times of times that length - so it cannot be clearly detected by optical microscope.
Although complete plates or distinct fragments are sometimes preserved, it is more likely that physical degradation and compaction breaks them down further. Then, the lithification into chalk results in overgrowths and recrystallisation, which obscures the original morphology.
Other components of chalk
Along with the coccoliths, the remainder of the chalk is a collection of the skeletons of foraminifera (another microscopic organism), and fragments of the shells of bivalves, ostracods, echinoderms and polyzoa.
Radiolarians, diatoms, and other siliceous organisms are also incorporated, and are the main source of the silica that is eventually precipitated as flint nodules.
Conditions of chalk formation
The micro-organisms that make chalk were pelagic; that is, they lived within the sea's water column, rather than on the sea bed.
On death, their remains rained down to the sea floor. If this was neither too deep nor too cold, the calcite remains collected there, where they formed a 'calcareous ooze' - a water-laden sea-floor deposit of the calcitic skeletons of pelagic organisms.
(Below a certain depth and temperature, calcite dissolves into the water so does not reach the sea floor. However, given the high temperatures and the shallow seas of the Cretaceous Period, vast amounts of calcite did accumulate on the sea floors during that time.)
The planktonic foraminifera skeletons are larger that coccoliths, but still microscopic - between 25 - 100 µm. Mixed together with the tiny coccoliths, the result is a very fine-grained, but poorly-sorted sediment.
With settling, compaction and lithification, this poorly sorted sediment of microscopic remains leads to chalk - a very fine-grained, but poorly-sorted rock.
The poor sorting indicates calm conditions of deposition, as turbulence and varying energy conditions are the prime agent of sorting. The chalk is in fact so fine-grained it was once believed to be amorphous, but is now known to be cryptocrystalline.
The fine grains, and disc-like shape of the coccoliths, give the rock an exceptionally large surface area per unit volume, which makes it relatively chemically reactive. It is therefore a particularly useful form of carbonate in manufacturing )
The varying hardness of chalk
Chalk in the north of England is much harder than in the south. It is not absolutely clear why this is the case, and it is probably due to a combination of reasons. Some of these are suggested here.
Hardness may result from chemical differences, or from processes that occurred during, and after, the lithification of the chalk.
Chemical and mineralogical hardness
One possible contributory factor got the overall greater hardness of the chalk in the south relates to the fact that there are far more 'hardgrounds' in the south.
Hardgrounds are layers a few centimetres thick, with the degree of chemical cementation decreasing downwards. The cement may be of a variety of chemicals, including aragonite, magnesian calcite, and calcite. Furthermore, these different chemicals may also take the form of different minerals. One such that effects chalk hardness is micrite, which is a particularly fine-grained form of calcite. Micrite may result from biogenic processes, so be present in varying quantities in the calcareous ooze that forms the chalk.
Thus, the ooze that lead to the formation of chalk that formed the hardgrounds may have originally had a greater proportion of micrite in it than did the ooze that formed the softer chalks. When this micrite was deposited between the larger organic particles, it may have recrystallised around the larger fragments, producing a tougher rock.
Physical Processes
Hardgrounds need not have resulted from a different chemical composition of the ooze. They could, instead, have resulted from the hardening of the chalk due to physical conditions and processes. This could have been for a number of reasons.
For example, the hardness of the chalk may have been the result of 'pressure dissolution' which would have required no additional micrite particles for cementation.
Erosional surfaces
Although some hardgrounds of the chalk may result from submarine cementation processes at the time of deposition - either chemical or physical - they may also result from erosion. Following uplift, sea floor sediments would be exposed to air and the effects of weathering. This would have eroded the top, softer layers of sediments, and allowed chemical changes such as oxidation and other effects of atmospheric exposure to alter the rock.
On becoming covered in sea again, this new, hardened surface would have provided a secure base for the attachment of some organisms, before again becoming covered in soft sediments.
Hardgrounds of Downend Quarry
The hardgrounds of the Cretaceous are often bio-turbated with organic encrustations.
One such could once be seen at the Down End quarry, at the west end of the Portsdown ridge, near Fareham. This is about the only fossiliferous outcrop of chalk at Portsdown. Unfortunately, this quarry is now being used as a landfill site, so this rich but rare fossiliferous bed is buried under tonnes of rubbish. Specimens from it can, however, be seen in the collection room of the Gosport Museum, by arrangement. [Get and insert a photo of the Gosport specimen]
The lack of fossils in chalk
Although some chalk beds, such as Culver Cliff on the Isle of Wight, are very fossiliferous, chalk is often fossil-poor. This is especially so of the younger 'Upper Chalk' beds, and is because the chalk is so pure. However; this lack of fossil evidence does not represent a lack of marine life - the seas were veritably teeming with life
At the time of deposition of the calcareous ooze on the sea floor, along with the many shells of macro-organisms within it, there was little clay or other sediments being washed into the sea to mix with this accumulation.
The high sea levels of the time meant there was little exposed land, so little terrigenous debris entered the sea. (In fact, much of the non-marine debris that did get deposited in the waters was not terrigenous, but air-borne volcanic dust.)
Clay and marl, and other such land-derived sediments, usually mix with sea floor sediments and fill the spaces between particles. This then blocks the free flow of water through the sediments.
Many Chalks contain a high proportion of these impurities. Northern Chalks all contain a lot of clay, and in Southern Chalks, many layers of marl can often be seen. This impermeable sediment would have prevented circulation of water in the ooze, thus 'sealing off' the organic remains from fresh oxygen, and preventing decomposition - so promoting preservation.
However, with much of the Southern Chalk, and Portsdown Chalk in particular, there was very little clay and marl.
The ooze thus maintained its porosity and permeability for a long while, so oxygen-rich seawater was able to circulate. This allowed and promoted the decomposition of the dead organisms by aerobic bacteria.
Furthermore, the chemical composition of the chalk probably lacked micrite and other minerals that give hardness the rock at lithification, further preventing the preservation of calcite fossils.
[I don't really know if it is the lack of clay, or micrite, or both, or what, that means I have no fossils in Portsdown]
An additional reason for the paucity of fossils in chalk, is that many of the organisms had tests and shells of aragonite, which is a less stable polymorph of calcite. Fossils of these organisms (which were diverse at the time) have, with time, dissolved and disappeared. Thus, the few fossils that are found show a bias towards the calcite-shelled organisms.
The environment indicated by the chalk
Much about the conditions of deposition can be told by examining sedimentary rocks, and this is particularly true of biogenic rocks that retain evidence of the organisms that formed them.
The fossil content of the chalk of North-Western Europe, particularly the abundance and diversity of the planktonic foraminifera and coccoliths, indicate tropical temperatures of 20°C or more. (This figure has since been confirmed by other geo-scientific means, such as oxygen isotope analysis).
The fossil assemblage also indicates normal marine salinity, rather than estuarial or others non-marine setting.
The chalk is very pure, with little clay and quartz in it. As mentioned above, these minor components were probably not land-derived, but were airborne and of volcanic origin. This suggests high sea-levels, with little and low-relief exposed land to provide dust and debris.
The environment in which the chalk beds were deposited is gone into in more detail in the Cretaceous Environment pages.
Worldwide Cretaceous Chalk
Although chalk is associated worldwide with the straits of Dover in South East England, it occurs around the world, and is closely related to that of North-Western Europe.
Twelve different stages of Cretaceous Chalk have been identified across Europe, of which the youngest is the Maastrichtian, named for its outcrop in South East Holland. This stage that has been totally eroded in England, where the youngest chalks are of the Campanian stage.
Cretaceous Chalk also outcrops in the USA, in Texas (the Austin chalk formation), Alabama, Kansas and Nebraska, and small amounts in other states.
The following section discusses coccoliths, the tiny calcite plates that form most of the chalk. 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