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The Grainy Structure of Hornstone

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  • The Grainy Structure of Hornstone

    Every now and then I find debitage in which flakes have peeled away like an onion. This summer, I found a piece in which one of these “curled” flakes was touched up. It is cool piece as within 5 feet I found what appears to be debitage from the same nodule. Notice how the direction of the grain creates stripes.

  • #2
    Non-knapper here read something about a “bending” form of flaking. Maybe this example!

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    • Whippoorwill
      Whippoorwill commented
      Editing a comment
      Is there a link you can post? I am interested in learning more.

    • Cecilia
      Cecilia commented
      Editing a comment
      Will try. It’s either from old magazine called The International Journal of Fracture (really!) or from a two volume doctoral paper on knapping quartz written for University of Ireland (?, I think). There is a post I called “Appreciation .....” with a little more info re sources I made when I acknowledged how ignorant I was, and how gracious members had been.
      Last edited by Cecilia; 10-29-2019, 02:57 PM.

  • #3
    What exactly is this rock and where did it originate from? One of the problems with American ‘nicknames’ and colloquial names for rocks (especially among knappers) is that they have little respect for geological precision or any universal alignment in agreed names.

    In the US, ‘hornstone’ often colloquially refers to rocks such as Wyandotte Chert (and various other names), found principally in Indiana and Kentucky. It can be visibly or invisibly banded, but it’s a sedimentary rock that usually occurs largely in nodular form and the banding is most usually a result of compression/relaxation as the quartz nodules crystallized. Curvature from conchoidal fracturing that follows the curvature of the nodule is a common feature.

    By contrast, in the geological world, ‘hornstone’ is the alternative name for ‘hornfels’, which covers a range of generally fine-grained metamorphic rocks derived from contact heating by igneous intrusion into any of: sandstones, shales, slates, limestones and clays. Although a hornfels may exhibit visible banding from bedding planes of its original parent, it breaks as readily across those bands as it does along them because the metamorphism has destroyed any alignment it once had and toughened the overall structure. Any banding is no longer a structural feature of the rock that determines how it might break… it’s little more than mineral colouration.

    Both are knappable, but have very different behaviour and produce different types of fracture.
    I keep six honest serving-men (they taught me all I knew); Their names are What and Why and When and How and Where and Who.

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    • Whippoorwill
      Whippoorwill commented
      Editing a comment
      Thanks for your contribution. The image below is from The Whippoorwill Stone Manufacturing Site in South Logan County Kentucky. Is is possible to identify from the image?

  • #4

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    • #5
      With apologies that I have had to make some generalisations to avoid giving you an even longer answer, this may help (a bit... or not).

      That appears to be nodular chert, which is a sedimentary rock, rather than hornstone (hornfels) which is a metamorphic rock. Incidentally, you can often identify a rock as metamorphic by rapping it smartly with something like the wooden handle of a hammer. It’s not 100% reliable, but metamorphic rocks usually produce a distinctive short ‘chink’ sound, as opposed to a more resonant ‘chinggg’ sound.

      As I said earlier, the ‘non-geological’ names given to rocks (especially in the US) often derive from ‘similarities’ of appearance or properties but where there is no true geological connection. Things like ‘Indiana Hornstone’ (also found in Kentucky) and related rocks with various names are a case in point. Geologists of course also accept these colloquial names, but prefer them to be qualified by a locality and they aren’t regarded as a substitute name for the proper geological name. For example, many rocks referred to as ‘California Jade’ aren’t true jades at all and ‘Herkimer Diamonds’ aren’t diamonds, or even composed of carbon. In other cases, some of these colloquial naming terms are generally avoided by geologists because they lack precision about composition or mode of formation. ‘Jasper’ is a good example of that and widely used by non-geologists to describe all manner of rocks that aren’t really related to one another. Some names are just completely wrong. Granite Mountain in Utah isn’t composed of granite, for example.

      The term ‘flint’ is also widely used in America with a degree of vagueness that doesn’t properly distinguish it from ‘chert’. Mostly in the US the term ‘flint’ is used to refer to ‘high quality’ chert but the true geological distinction is that flint forms in chalk deposits or marls and chert forms in limestone deposits (and that in itself is an indirect reason why flint is usually a ‘better quality’ version of chert. There is only one material in the USA that truly qualifies geologically as ‘flint’ produced in a chalk environment.

      Sedimentary Rocks

      If you consult reference sources, they focus on sedimentary rocks being formed from deposition in layers and the consequent bedding planes that can usually be seen if there have been multiple depositions. Those kinds of rocks of course split readily along the alignment of the bedding planes. However, that’s not how sedimentary rocks such as chert nodules form. Chert forms within a sedimentary matrix such as limestone, but doesn’t necessarily have any structural orientation which derives from that. It can form as ‘tabular’ chert, in a distinct sheet layer of its own and will tend to fracture along that layer, but isn’t usually orientated as far as its own crystallography is concerned. Nodular chert even less so. It’s not formed in layers, but arises from silica-rich organic material accumulating in voids within its host rock and then solidifying as a mass, sometimes progressively around a nucleus
      Even if you can see concentric banding in a nodule, it doesn’t mean that there is any structural division between the bands. That’s how a ‘concretion’ forms, and is the defining difference between a nodule and a concretion… but it isn’t how chert usually forms, although ‘chert-rich’ concretions are certainly known. Any banding seen in chert nodules usually arises from fluctuations in temperature or pH, mineral diffusion or compression/relaxation during solidification, but represents non-structural features that don’t influence the way it’s going to split or fracture.

      Knappability and Fracture

      After glasses such as obsidian, the next best knappable rocks are flints, followed by cherts because they are composed of relatively high-purity cryptocrystalline or microcrystalline quartz. That means they will fracture conchoidally but, by definition, conchoidal fractures do not follow any natural planes of separation. The shape of the broken surface is controlled only by the stresses applied, and not by any preferred orientation of the material. When it fractures under impact, what you get are curved surfaces, often slightly concave, with concentric or radiating ripple fractures that are a consequence of the shock wave travelling almost uniformly through the material. The more uniform the material, the more rounded the breakage surfaces. And if the material was a rounded nodule to begin with then anything split from it may well have additional curvature arising from that, constrained by the shape of the nodule itself.

      There will be a test on Thursday.

      I keep six honest serving-men (they taught me all I knew); Their names are What and Why and When and How and Where and Who.

      Comment


      • Cecilia
        Cecilia commented
        Editing a comment
        Again, welcome back!

      • Cecilia
        Cecilia commented
        Editing a comment
        Painshill, I referenced you twice in a long Sept post “Appreciation....”, once as a possessive noun, and once as part of hyphenated multi-word
        adjective.

    • #6
      Woops, I missed an exam for the first time in my life. LOL

      In all seriousness, thanks Painhill for taking the time to give such a great description of chert. Had I been armed with this knowledge I would have named the thread “The Microscopic Structure of St. Genevieve Chert”. The manufacturing site is located in Kentucky’s far western Mississippian outcrop in Logan County near Tennessee state line. With a abundance of St. Genevieve Chert, the region has many of these types of sites.

      I have a few questions. Can nodules discharged naturally and become part of karst geography or are they always quarried? Can microscopic chert analysis trace materials to specific regions? If so, is there a lithic material database used the map lithic transportation thru history?

      I am amazed at the lack of archeology resources out there and hope we can make difference.

      Painshill, Thanks so much for your contributions to this post. chinggggg!
      Last edited by Whippoorwill; 10-27-2019, 11:05 AM.

      Comment


      • Ron Kelley
        Ron Kelley commented
        Editing a comment
        This is an extremely political remark. DON'T DO IT!!!

    • #7
      Comment noted and revised accordingly. Sorry if I offended anyone.
      Last edited by Whippoorwill; 10-27-2019, 11:11 AM.

      Comment


      • #8
        Originally posted by Whippoorwill View Post
        Woops, I missed an exam for the first time in my life. LOL

        In all seriousness, thanks Painhill for taking the time to give such a great description of chert. Had I been armed with this knowledge I would have named the thread “The Microscopic Structure of St. Genevieve Chert”. The manufacturing site is located in Kentucky’s far western Mississippian outcrop in Logan County near Tennessee state line. With a abundance of St. Genevieve Chert, the region has many of these types of sites.

        I have a few questions. Can nodules discharged naturally and become part of karst geography or are they always quarried? Can microscopic chert analysis trace materials to specific regions? If so, is there a lithic material database used the map lithic transportation thru history?

        I am amazed at the lack of archeology resources out there and hope we can make difference.

        Painshill, Thanks so much for your contributions to this post. chinggggg!

        It might be easier to answer the first question if it was clear why you were asking. Nevertheless, a few pointers.

        If we wanted to keep a completely clean sheet with the Geology Police then most properly we use the terms ‘nodule’ for small individually-formed pieces and the word ‘lens’ for larger pieces (because they often have a fat middle section and thin edges… like a magnifying lens in cross section). Such formations are said to be ‘lenticular’ deposits. When it occurs in massive continuous sheets or layers, we call it ‘tabular’. Even when we see nodules or lenses, they are broadly representative of stratified layers… just not continuous ones.

        Given that chert is a high silica rock which has generally good resistance to erosion from weathering and it mostly occurs in high carbonate limestone deposits which erode easily, it’s inevitably the case that chert can and will be discharged from its host formation. The discharge can accumulate at the base of exposed cliff faces in the manner of ‘scree’ and it may or may not be broken as a result of falling. Equally, it can simply be exposed as a result of the rocks above weathering away, including by percolating water. It can be water carried elsewhere and may become rounded due to tumbling. It may remain in situ, get re-buried, and become reformed into a conglomerate if rounded, or a breccia if broken.

        St. Genevieve chert (more correctly Ste. Genevieve chert) sits principally in a distinct layer at the base of the Ste. Geneiveive Limestone and on top of the St. Louis Limestone. Both are Mississipian in age and much of the chert in the Ste. Genevieve is reworked material that came originally from the St. Louis and has reformed as a thick brecciated layer in a newer limestone matrix.

        These strata are semi-continuous or overlapping with other strata formed at similar times under similar geological conditions, both across Kentucky and into other states. They each have their own chert deposits under different regional names according to the formations in which they sit and can be easily confused… at least on simple visual observation.

        That leads us to the second question. Is there a lithic material database used to map lithic transportation? Not exactly. Identification of cherts such that they can be tracked to specific sources is tricky. At its simplest, because the silica in chert has an organic microfossil origin, it can be useful to examine it microscopically for what are know as ‘index fossils’. Those are organisms or remnants of them known to occur in certain combinations only at particular locations, in particular environments and/or during certain time periods. Occasionally chert will contain fragments of larger organisms (crinoids, bryozoans, brachiopods, corals and such) which can be identified more readily.

        At a more sophisticated level, chemical analysis can help match chert to its source using a technique called CODA (Compositional Data Analysis). More recently, non-destructive techniques such as VNIR (visible/near-infrared) and FTIR (Fourier Transform Infrared) spectroscopy have been used. More recently still, even more sophisticated techniques such as LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry). Although these techniques are non-destructive, they need a freshly broken unweathered surface, so that’s not always desirable when identifying the lithic for an artefact, as opposed to unworked material. Otherwise, the reference database needs to include experimental data from specimens with different degrees of weathering to take account of the possible chemical and optical differences created by weathering and patination.

        The kinds of databases that do exist are not generally in the public domain and mostly relate to particular research groups with a specific interest… such as establishing the extent of lithic trade between native populations. There’s nothing that could be said to be a nationwide database of lithic source characteristics and it would be a monumental task to build one.

        I have sent you a PM with a pdf attachment which is a summary of a research paper relevant to your question that you may find interesting. I didn’t want to upload it here on the forum since it came out of my personal library and I can’t remember where I originally obtained it. Didn’t want to tread on anyone’s copyright, so please also respect that.
        I keep six honest serving-men (they taught me all I knew); Their names are What and Why and When and How and Where and Who.

        Comment


        • #9
          I can attest to Hornstones pain in the...rear workability. Pretty stone, I just need practice with it.
          "The education of a man is never completed until he dies." Robert E. Lee

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