Tempus fugit, even if we don’t know how fast…

Heat-treated silicates are a huge, untapped, chronometric resource in California archaeology. Elsewhere in the world Neanderthal sites are dated using thermoluminescence, so too, later Palaeolithic contexts. Burned ‘flints’ are rarely overlooked as a datable resource everywhere, it seems, but in North America, and especially in California, where it could promise accurate absolute dates with the added advantage of being comparable across sites within and between regions.
     For the most part California is bereft of stratified archaeological sites thanks to the ubiquitous and prolific burrowing rodents that populate the grasslands and parklands of the state, and which burrow, burrow, burrow their lives away. The worst culprits are members of the Geomyidae, but Sciuridae and others only make matters worse. The situation is so bad that California archaeologists have taken to sampling the temporal span of a given site using 10-cm deep, one by one meter shovel tests. Coring would be useless, as would any attempt to ‘sound’ the site using deeper test excavations. The ‘Mixmastering’ of site sediments in California is so profound and complete that everything from the earliest occupations to those of the proto- and peri-historic period have the same probability of occurring near the surface as they have of showing up at any other depth.  
     In California, the paucity of stratified sites obviates the utility of radiocarbon, except when organic artifacts can be directly dated. In addition, without verifiable stratification, lithic artifacts are only slightly useful as temporal markers. However, one material, obsidian, is used for a form of relative dating called the obsidian hydration method, whereby newly flaked surfaces begin to be suffused with water and develop a ‘rind’ that thickens with time and that afford archaeologists the possibility for intra-site and (less defensibly) inter-site comparison of the age of individual pieces of obsidian.
     Yet, however earnestly it is undertaken, however precisely the measurements are taken, and regardless of efforts to overcome problems of intersubjectivty, there is enough uncertainty in the hydration rate within a site and between obsidian from different sources, and enough of a question as to the actual burial context of individual pieces of obsidian that most workers will use it as a very gross temporal touchstone, and stick to using it for relative purposes. Those who seek to do more with it will do so, at best, provisionally.
     I’m surprised, shocked, really, that it appears no one has given thermoluminescence (TL) a try. It is, after all, an absolute technique that yields results in years before the present. Whether incidentally annealed in proximity to a long-burning fire or purposefully annealed using slow heat to improve its flakability, silicious materials other than obsidian would lend themselves to the well-known but untried (in California to my knowledge) technique. Unlike its sister, optically stimulated luminescence, TL is usually used to directly date an artifact, and not a component of the site’s sediments about which little is known of its depositional history. TL has been used in Europe and elsewhere and is the basis for the (presumably more-or-less) accurate dating of a number of important Palaeolithic sites.
     All that is needed is an eye for the effects of heat-treating, and a willingness to destroy the occasional formed lithic tool, and you’ll have a whole new ‘ball game’ in California archaeology. Take my word for it.

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