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  • Cortex

    Here's a question.
    Can anyone give some insight into the formation of cortex? I am especially interested in obsidian as it's the material I find most.
    My crude understanding is that is forms sort of like oxidation (over time), and if that is the case, can we infer that a knapped object (that has had it's original cortex removed) can be dated by the extent that a cortex has reformed?
    Anyone?
    California

  • #2
    Very good question I'm not a geologist or scientist but my WAG about cortex is that it is formed during the cooling process of the molten rock, and the oxidation or weathering you are talking about would be what is considered the patina but I don't think technology exists to accurately determine an age as there would be too many factors that would affect the rate of weathering depending on where the material was located for example underground, underwater, out in direct sunlight or exposed to constant wind blowing dust and grit over it etc. etc. This is just my uneducated guess, I'm sure someone on this forum will have a more educated answer for you.

    Comment


    • tomf
      tomf commented
      Editing a comment
      There's a method called Hydration dating that measures the amount of water that an obsidian surface has absorbed. I guess 'cortex' is what you described and not what is measured. So, can we judge age by patina?

    • rock ON.
      rock ON. commented
      Editing a comment
      I think that knapped stone tools and such can only be somewhat accurately dated when they are found in context with organic materials that can be carbon dated. But I'm sure someone here will have an answer for you. I would be interested to know as well.

    • tomf
      tomf commented
      Editing a comment
      I think Hydration Dating is now an accepted method for dating - though it's hard to understand exactly how it works; does a stone in water absorb quicker than one in a field? - but not sure if it only works with obsidian or if it can be used on other material.

      Still context is very important when applicable.

      My finds are only in general context of geography. Northern California points have not been codified by type nearly as thoroughly as some others and as there have been people producing lithic tools around here for 12,000 years or more, it's very difficult to figure out what I've found.

      I have pet theories about how long it takes a creek to wear off flaking patterns and that bigger points might be older but it's mostly just guess work.

  • #3
    Here is some info on hydration dating

    California

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    • #4
      I have one point that was dated by University of Berkeley , Tom Hester worked with me on this project. Hester is working a project on paleo points all over America to see how far the obsidian was traveled... They took a tiny slice out of the artifact and I did not mind for science sake..They sent all the documentation to Hester, he then sent it to me. It’s is a interesting project to say the least ! There are a couple of paleo points that they have no idea where the source was! JJ
      Lubbock County Tx

      Comment


      • tomf
        tomf commented
        Editing a comment
        That is very interesting. I would love to pick that guys brains. He must know tons about Northern California.
        I have heard that California Obsidian made as far as Missouri but don't know when.
        Curious what analysis of your point revealed.

      • Lindenmeier-Man
        Lindenmeier-Man commented
        Editing a comment
        The lithic material came from Toledo Cerro quarry west over the mountain from the Trinity nuclear test site. 8000 B.C.

    • #5
      I suggest you use Google to search this very technical question. There is an abundance of info in the geological literature. As to how cortex formation differs on obsidian as compared to chert with which I'm most familiar I don't know. It is a weathering process but because chert and obsidian are formed by different processes maybe the cortex is formed somewhat differently, too.

      Comment


      • tomf
        tomf commented
        Editing a comment
        I have been reading all I can but it's nice to get a condensed version from someone who knows.
        And it's something to talk about.

        I think, as rock ON, said my terminology was wrong when I refered to cortex when I meant wear/patina/oxidization.
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