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Arrowhead and Angular Shapes

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  • Arrowhead and Angular Shapes


    Just about the commonest breakage shape for rocks which have natural cleavage planes is triangular, closely followed by trapezoidal and diamond shapes.

    Frequently people remark on the resemblance to an arrowhead or spearhead shape of some piece of rock they have found – normally some kind of rock that is quartz-rich, and often it’s a piece of quartzite or slate. They also comment on sharp edges, a highly pointed tip, symmetry that can’t possibly be natural and so on.

    Unfortunately, all of those characteristics are typical of rocks that have simply broken through natural causes and the commonest natural cause is frost shattering. Here’s a typical example of what can happen on an exposed rock face when mother nature gets going:

    [Picture by LarsBr on]

    This kind of breakage happens when water finds its way into the bedding planes of sedimentary (and to a lesser extent, metamorphic) rocks, or rocks which have a macro-crystalline structure. The planes and faces in such rocks are less weakly bonded such that when the water freezes during cold weather, the expansion fractures the rock. When water turns to ice it expands by about 10% in volume and that’s what creates the pressure which causes the fracturing. Symmetry is not at all uncommon since the nature of macro-crystallisation in rocks is that it usually follows a mathematical pattern with very precise angles. The angular pieces fall away and collect in a scree at the base of rock faces and may be carried away by water during periods of heavy rain or flood.

    Even igneous rocks with a macro-crystalline structure are prone to this kind of shattering. In these cases, the breakage may follow the crystal planes within the rock where the bonding is less strong. Here’s some frost-shattered granite:

    [pic by EA Fitz Patrick]

    The forces created by ice can be extremely powerful, splitting even very large rocks by a principle known as “wedging”:

    Glaciation produces even more extreme shattering and wider distribution of the fragments from either glacial movement or the sheer volume of melt-water after retreat of the glacier. Smoothing of the angular edges during water travel and tumbling may increase the resemblance to artefacts and create the impression of antiquity.

    It’s not always necessary for water to be involved in these processes. In desert environments when there is an extreme difference between hot daytime temperatures and cold frosty nights, the expansion and contraction of the rock itself can create internal pressures from thermal stress that the rock may not withstand. Here’s a shattered rock from the Mojave Desert in California which has experienced this:

    Last edited by painshill; 01-28-2016, 08:47 AM.
    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.

  • #2

    A “dreikanter” is a particular kind of “ventifact” – a rock modified by (thousands or millions of years of) wind erosion and scouring. That kind of erosion is caused by small abrasive particles such as sand, carried by the wind. Frequently, the wind is so directional that the abraded rock develops a “keel” which is parallel to the prevailing wind, plus what may look like faceting.

    Wiki entry here:

    Dreikanters are commonly found in desert and periglacial regions (the edges of glacial areas). They are well-reported from the Lake Michigan area, where they have a periglacial origin. That area is particularly noted for the form known as the “brazil nut”. This extract is from “”:

    “Pebbles and small stones in particular have a distinct form. While rocks polished by water movement tend to be flattened circles or ovals, and the stones smoothed in a glacial till tend to be irregularly shaped, ventifacts tend to form facets. As the wind blows on the stone, it smooths down a flattened surface on the top-windward side of the stone, leaving the rest of the stone untouched. Eventually, the stone is likely to be undermined as the wind erodes the ground beneath it, rotating to present another side to the wind. The result is a number of elongated facets intersecting along 'sharp' edges (not cutting sharp, just pointed). The shape is sometimes compared to that of a brazil nut. These stones may even be mistaken for an eroded man-made artifact.”

    Here’s one in the making – it would eventually have developed a smoother streamlined shape, given a longer scouring period:

    [pic from The Environmental Significance of Ventifacts - Jasper Knight]

    The degree of symmetry is to a large extent dependent on the shape of the original source rock. Depending on how far the polishing has progressed, you may be able to see small semicircular indentations in the surface with a decent magnifying glass.

    Dreikanters are well-documented from the beach dunes on the Lake Michigan shore, the Udden lag gravel of the Grand Sable section of Lake Superior in Michigan and the Trempealeau beds of eastern Wisconsin and northeastern Illinois.

    They can look remarkably like artefacts. These are all non-US examples formed in desert environments but the principles are exactly the same as for those found in periglacial environments in the Michigan area:

    [all pics by Gabor on the Discussfossils forum]
    Last edited by painshill; 11-05-2015, 04:34 PM.
    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.