Sometimes you live with something and regard it as normal, dull, quotidian, jejune, blah or maybe just meh. Then one day you suddenly get a moment of clarity and realise that actually, it is really weird. Subtly weird perhaps, but still very odd and mysterious. For me, chalk is like this. I live in Southern England and chalk is all around me, but I only recently got thinking about it properly. Accretionary Wedge #34 and its call for weirdness got me writing about it.
Chalk is a weird rock-type
Chalk is a very pure biogenic fine-grained limestone found across much of Western Europe. It is made up of marine dandruff, the hard bits of marine algae (coccolithophores) that have settled to the sea bed. Marine algae is nothing unusual of course, but chalk is made of nothing else. Really nothing, less than 3% of anything else, which makes a piece of chalk one of the dullest rock-types imaginable. No texture, no mineralogical variation, no structure, what a bore; when I moved ‘down South’ and first encountered it I was almost offended by its dullness. I’m a hard-rock man by preference, but even the Carboniferous sediments I grew up with have cross-bedding, variation in colour and other nice things that distract you from the fact that they are only sediments. Chalk is weirdly dull.
Flint is weird
On an outcrop scale chalk has some interest: it contains flint. Flint is chert that occurs in chalk, that is to say cryptocrystalline silica, black, shiny and with conchoidal fractures. It is extremely common, but no-one really knows how it forms. The silica doesn’t come from sand like in normal sediments, oh no, it is thought to come from sponge spicules, diatoms and other biological sources. It is formed somehow during diagenesis. Often it infills burrows or surrounds fossils, suggesting a role for micro-environments with unusual (weird?) chemistry that allow the silica to precipitate out as a gel. Sometimes soft-sediment deformation is seen to deform flints, so they are soft during early stages of diagenesis. Also flint sometimes infills early faults/fractures to form sheet flints. It can also directly replace chalk, rather than filling cavities. As a resistant erosional product, flint is ubiquitous in Southern England (it forms the gravel drive of my house) yet nobody really knows how it forms.
Flint is important in European archaeology. That conchoidal fracture means that hitting flints (knapping if you do it well) gives sharp edges: mammoth-killing sharp. Other rock-types with similar properties have a similar role elsewhere (e.g. obsidian in Central America). Flint tools are very important as they are common and easily preserved and so can be used to trace trade routes. Places such as Grimes Graves in Norfolk show how important they were 5000 years ago. Flint was so important that it was worth digging a 30ft vertical shaft with deer antlers down through the chalk to mine it.
Chalk is weirdly English
Chalk forms a distinctive landscape called downland that is the quintessential English green rolling landscape. Chalk is very homogeneous and so is a little like a blank slate on which other processes can act. Southern England was never glaciated, but was near the edge of the ice during glacial periods. It is therefore a good place to recognise periglacial landforms. One example is valleys within downland that don’t contain rivers (called ‘bottoms’) which are sometimes asymmetrical, with a shallow side and a steep side. The shallow side is usual the sunny side (South-facing) where more vigorous freeze-thaw broke-up the chalk and flattened the slope compared with the darker side.
During the dark days of World War Two, Britain’s fighting men’s morale was kept up by Dame Vera Lynn, the ‘Force’s sweetheart’, whose most famous song was “The White Cliffs of Dover”. The chalk cliffs of England’s south coast are an English icon. This is a bit odd however as they are at the far end of the country. Go to the wrong bit of Kent (the county Dover is in) and your mobile (cell) phone will pick up French base stations and connect you to a French mobile phone company. If you do actually move over the English Channel to France, what do you see? Large white chalk cliffs, which at the time Dame Vera was singing were covered with Nazis planning invasion. Incidentally, the nicest way to get to France from England is via the Channel Tunnel, which was drilled entirely through the chalk under the sea from England to France.
So how did white chalk cliffs become symbolic of England? Well, the song “The White Cliffs of Dover” was actually written by two Americans which might also explain why the song goes on about Bluebirds, which are not native to England.
Chalk downland has thin soils. If you strip off this soil you quickly get to chalk, brilliant white against the grass. This can be used to make large pictures on the slopes visible for miles. These images are generally very old and are often of horses but can be, er, other things.
What is the origin or meaning of these figures? Nobody really knows.
Chalk is geologically weird
Chalk is the dominant rock-type of late Cretaceous Europe. The simple picture is that high sea-levels meant that the Europe was ‘drowned’ meaning that large areas were far from land. No land means no terrestrial sediment (sand or mud) so, debris from marine algae could slowly build up undiluted to great thicknesses (over 1km of chalk in the North Sea). The traditional picture was that chalk was chalk was chalk. Nothing happened, there was nothing to correlate stratigraphically and thicknesses were relatively constant. A recent paper by Rory Mortimore (in Volume 122, Issue 2 of the Proceedings of the Geologists’ Association) gives a good overview of modern knowledge about the Chalk. Detailed correlation across a wide area is now established. A lot is based on bio-stratigraphy and the correlation of volcanic ash-bands (marls, in chalk terminology), which is familiar from many sedimentary sequences. However features such as a flint bands are also useful in correlating different sections. I can understand that the rapid evolution of bivalves or an unusually large eruption might create useful time markers, but why should bands of flint also do so? Weird.
The bulk of the paper makes a nice case for the importance of tectonics for controlling sedimentation. Since the rock is very uniform, evidence for this is subtle, but convincing. It is interesting that a rock-type associated with unusual eustatic conditions (an exceptionally high global sea-level during a period of high CO2 (greenhouse conditions)) should show extensive evidence for tectonic control.
The final weirdness is the question: why do large deposits of chalk exist only in Europe? To get chalk you need zero terrestrial sediment and a carbonate compensation depth that is not too shallow (or your marine dandruff gets dissolved before reaching the bottom). Terrestrial sediment is not there because of high sea-levels; these are globally high sea-levels, so why is chalk not a global phenomena? In particular why does the North American epicontinental seaway not contain lots of chalk? It was at a similar latitude, so there is unlikely to be any climatic difference to explain the lack of chalk. There is some chalk in Kansas, it seems, but nothing on the same scale as in Europe. Was there just more mud and sand around, diluting the chalk and turning it into marls, or slightly calcareous sandstone? I’d love to hear a knowledgeable answer to my question and what would be really weird is if none of you good folks could supply it.