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Adventures in Silcrete: "It's flint Jim, but not as you know it!"

Something that everyone who works in the archaeology of deep prehistory has to get to grips with is the technology of stone tools, or lithics. This includes thinking about the ways in which people made their tools, which techniques they chose to use, etc. It also means that Palaeolithic archaeologists, alongside needing to know stuff about climatology, palaeontology, and ecology, need to delve into the science of geology. People in prehistory might not have understood the origins of different kinds of rocks, but they certainly appreciated the diversity in stone qualities, not only between very different rock types but also within geological/mineral categories.


These two Neandertal tools that I studied for my PhD, called handaxes, are both very finely worked, but made from completely different rocks. The one on the left (Castle Lane, Bournemouth) is made from Cretaceous flint found in the south and east of Britain, and the one on the right (Coygan Cave) from rhyolite, a volcanic stone from Wales. The material I'm working on in France is made from a rock that is related to flint, but a rather different beast. Read on to find out more about the strange stuff that is silcrete!

We have ample evidence that people, even right at the very dawn of stone tool technology more than 2 million years ago, were making very particular choices about which types of rocks to use (here's a free pdf on this). We also see more and more as time goes by that people chose to treat tools made on various stones quite differently. Almost always they preferred stone types that have particular mechanical properties meaning they break in a predictable way. And when they used less high quality stones, it was as a result of needing to balance other factors, such as how far away a stone source was- poorer quality but closer rocks might be preferred for some kinds of tasks. My own PhD research confirmed that Neandertals in Britain, for example, were very selective in the kinds of stone that they chose to carry long distances: only the highest quality stone type was moved, and only in the form of tools such as handaxes or scrapers, which were flexible and had potential for long use-lives through their capacity to be re-sharpened.

Flint handaxe (southern province) from eastern England

Coming from a British background, my understanding of stone tools (lithics) has a particular context and history. As with most archaeologists from this part of the world, I use the term "flints" casually to mean knapped (that is, humanly-worked) stone tools. However, not every lithic that people made in the past was actually made of flint. This, in British usage, refers to a stone that formed within Cretaceous deposits, typically chalk. Most people have some idea what flint looks like- matte to glossy, fine textured, dark stone which is often translucent at the edges. Yet there is variation even within this category- flints from Cretaceous deposits in Yorkshire and the East Midlands are often opaque and have a slightly different texture.
For British researchers, we understand that geologically, flint is a kind of chert, but when we talk about chert we mean something that comes from Carboniferous or Cretaceous Greensand deposits, which is not the same as flint. But North American lithic researchers have a wider use of the term, and for them chert includes things that for British people would probably be called flint. Similarly, French researchers use silex to mean flint, as well as cherts, and other types of geochemically similar stones.

Northern flint handaxe, actually found in eastern England but likely to have been sourced from glacially transported block

So far, so confusing? I felt the same upon arriving in France for my postdoc, which is centred on being trained in a technique to identify secondary sources of flint/silex- those which are not the actual outcrops of rock, but things like slope deposits and river gravels (read my posts here and here on my project, including more on secondary sources). Here the understanding of silex is very much wider, and in order to get my head around the material I'd be studying, it was time to get back to basics. I'm talking elemental basics: the geology behind flint, silex and the wider siliceous family.

So... that word I just snuck in- siliceous (sounds a bit like delicious). Essentially it means a type of rock that has a high silica content. Silica? It's like a flavour of silicon, which you can't get more fundamental than, seeing as it's a chemical element- Si, with an atomic number of 14.

 University of Nottingham video series The Periodic Table- Silicon

You've maybe heard the term "Silicon Valley", referring to the computing industry in California- silicon is integral to the late 20th/21st century technology of microprocessors, which run in a vast number of applications and objects we use every day. Silicon is a common element, being a major component by mass of the Earth's crust (almost 30%). But it's not often found in pure elemental form; much more commonly it occurs as silicates (minerals that contain silicon, oxygen and reactive metals, such as mica, the shiny little plates you sometimes see in granite rocks). Together these account for more like 90% of the crust, and are also common elsewhere in the rocky bodies of the solar system. This is because silicon and oxygen were common elements created in the supernova explosion that produced our solar system's proto-planetary disk (check out this great Jodcast on planet-building).

Proto-planetary disks within the Orion Nebula. Image: by C.R. O'Dell/Rice University; NASA [Public domain], via Wikimedia Commons
Silicon together with oxygen forms silica (Silicon dioxide), a compound that occurs in various mineral forms or polymorphs which the same chemical formula but different crystalline structures. Quartz is one extremely common polymorph of silica, and other forms with can occur naturally depending on heat and pressure. For example lechatelerite forms when lightning strikes sand, coesite forms in ultra-high pressure metamorphic contexts, stishovite is most commonly associated with very high pressures and temperatures created in meteorite impact craters, and seifertite is only found in Martian and lunar meteorites that have undergone the intensity of hurtling at high speed through Earth's atmosphere and slamming into the surface. 
Silica can also be produced and used within biological settings, perhaps you remember that old Star Trek episode with a strange blob-like lifeform that was not Carbon-based, and instead was Silicon-based? There are in fact many organisms that use silica within their natural structures, for example the radiolarians, which are single-celled marine creatures (see this lovely blog post by @ferwen), and some plants, including many grasses produce silica phytoliths.

Fulgurites: formed by lightning strikes melting sand, they are made of lechatelerite. Image: By Ji-Elle (Own work) [CC-BY-SA-3.0-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Coming back from our geological excusion through silicon and silica, how does this relate to stone tools? Very simply, flint and chert are really forms of quartz- and the French term "silex" clearly shows the link to silica. So they are derived forms of silica, and can be referred to as siliceous rock, that is, a sedimentary stone mostly made of silica. You will have heard of gems like jasper, chalcedony, opal... these are all other forms of silica: chalcedony is made from quartz and moganite, the latter is itself a kind of quartz with a different crystalline structure. Flint, chert, jasper, and chalcedony make excellent tools because their internal structures produce predictable fracturing mechanics- essentially when you hit it in a certain way, you can be fairly sure you will get a flake of a reasonable size.

Although there is usually water involved, the formation of different siliceous rocks is quite variable, with some like flint forming in marine sedimentary contexts, where the organic remains of tiny creatures made of silica (e.g. the radiolarians) fell to the sea-floor, dissolved and re-crystallized- a process of silicification. Sometimes flints took the form of tabular surfaces, sometimes the silica coalesced around small hard objects like shells, and sometimes it happened within the burrows of other creatures, forming long tubular nodules. Other siliceous rocks such as opal forms underground in rock fissures: when water containing high amounts of silica evaporates, it leaves behind a film of silica that builds up very slowly. And yet other kinds of formation occur, such as around hydrothermal vents under the sea.

Beds of flint visible in chalk cliffs on the Isle of Wight. Kayakers give scale! Image: Barry Deakin [CC-BY-SA-2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons

And finally we reach silcrete: as the name suggests, this is another kind of rock whose formation involved silica. Here however, instead of there being a pure lump or nodule, silcrete is a kind of naturally cemented stone, where silica acts to knit together pre-existing sediments. Often these are quartzites or sandstones, with a filling of chalcedony or opal. Silcretes are themselves quite complex and form in different contexts and types of environments, but one major type is pedogenic. This is a situation where silicification of surface or near-surface soils occurs due to the infiltration and evaporation silica-rich water. We actually have a type of silcrete called ganisters in Britain, which formed within Carboniferous deposits, often at the base of coal measures. They are thought to have been created from the accumulation of plant silicas dissolving and reforming within ancient soils- they therefore represent an ancient landsurface. The contexts of silcrete formation in the past, and the few present-day examples (e.g. in the Okovango Delta, Botswana) suggest that they primarily formed in hot climates. Another type of place this can happen is at the margins of lakes, and this is the precise palaeo-landscape context for the silcrete source I am studying for my postdoc.

Carboniferous silcrete formation or ganister, Waddens Bay, Nova Scotia. Image: By Rygel, M.C. (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Because, yes! People in the Palaeolithic sometimes used silcrete to make tools. Recent research shows that very early heat-treatment of stone tools (to improve their knapping quality) was being applied to silcretes, and more recently, there is evidence of very long distance transport of this stone type. Both these examples are from South Africa; in my case, the silcretes are just one of the many not-initially-inspiring siliceous raw materials available within the southern Massif Central, where I am working.
The silcrete source I will be looking at is Saint-Pierre-Eynac, east of Le Puy-en-Velay, and specifically the Palaeolithic. The history of the Massif Central is highly complex geologically, and included a lot of volcanism (which I've posted about here). At Saint-Pierre-Eynac, following a very explosive type of volcanic eruption where magmas meet water underground (phreatomagmatic), a crater was produced, and in this a lake was formed. This took place during the Miocene period, around 15 million years ago, when the palaeo-geography of Europe itself was looking a little unfamiliar, never mind the local topography.

Europe around 13 Million years ago during the Miocene. Image: copyright Ron Blakey, NAU Geology, used with permission for non-profit/education context
At the St Pierre-Eynac maar, or crater lake, silcretes were formed, probably at the lake margins or maybe in the centre within deep sediments. The formation process here however isn't very well understood as the crater has mostly now eroded away and there has been subsequent tilting of rocks, but it may be related to faulting. The outcrops also seem to have been affected by earth movements, as you can see fine cracks in the stone that has itself filled with more silica. At one time it was thought that the after-effects of the volcanic explosion itself created the silcrete in a hydro-thermal context, but the silex expert I'm working with (Paul Fernandes) has shown this can't have been the case. This sounds like quite a dramatic geological history, but if you visit the silcrete source now, it's a strangely quiet and out-of-the-way place, despite being well-known to geologists in the region for a long time. 
I'll be writing about our site visit and plans for research in more detail soon, as well as my stay with Paul Fernandes where I had my first experience with looking at lots of types of flint and silcrete using microscopes. In the meantime, here are some photos of samples we took from the site to give you a taste of what this silcrete looks like close up- however it's very variable as a whole and doesn't all look like this, which will become clear in my next project post.


Cobble I found on path leading up to first outcrop; silicification is clear, i.e. it kind of looks like flint, but the small mineral inclusions are obvious

Close up of cobble showing the flint-like fracturing pattern (the silicification is actually chalcedony), with mineral inclusions. The fine line running across the middle is a crack filled with more silicification.


Block from below outcrop, showing variation in structure across one piece, with greater amount of silicification at left, and more of the pre-existing matrix at right.

Close up of the silicified end of the block abovem showing a lot of veining


When we were visiting the site, Peter Bindon found this flake in the road cut along the side of the hill where the outcrops are. It is certainly humanly made, this is the ventral face or inner surface; the other side is the dorsal which faced outwards on the original block. The white colour may be alteration or original to the outcrop, but you can see there is more chalcedony and fewer inclusions in this piece.






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