Schmidt Hammer exposure dating (SHED) (sometimes called SHD) is another technique coming into its own -- particularly valuable because it is non-invasive, meaning it does no damage at all to the dated rock surfaces. Putting it crudely, the instrument measures a "rebound value" for each reading which is related to rock coherence or hardness. The principle is that as rocks are exposed to the atmosphere over time, weathering sets in, during which the surface is rotted or softened because certain crystals break down either quickly or slowly. Every rock type will have its own "rotting rate" which can be calibrated via many measurements. Ten readings should be taken from each sampled area, and an essential feature of the method is that there has to be a "control surface" of a known age, against which all other readings can be calibrated. The method is very quick and not all that accurate, but if carefully used it provides a useful guide to the relative ages of rock surfaces of interest -- and most researchers suggest that preliminary work using SHED can and should be followed up with more accurate cosmogenic dating methods.
The method does not seem to be suitable for sandstones or tuffs -- and maybe not for rhyolite either -- but for dolerites it should work well as long as internal lithological variations are taken into account.
Here is one of many papers that have been devoted to this method.
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Schmidt Hammer exposure dating (SHED): Establishment and implications for the retreat of the last British Ice Sheet
Author links open overlay panelM.D.Tomkins, J.M.Dortch, P.D.Hughes
https://doi.org/10.1016/j.quageo.2016.02.002
See also:
M.D. Tomkins, J.M. Dortch, P.D. Hughes. Erratum to: “Tomkins et al. (2016) Schmidt Hammer exposure dating (SHED): Establishment and implications for the retreat of the last British Ice Sheet” [Quat. Geochronol. 33 (2016) 46–60]
Quaternary Geochronology, Volume 38, March 2017, Pages 75-76
Highlights
Robust SHED calibration curve (R2 = 0.81, p = < 0.01) for granite surfaces.
SHED not appropriate for sandstones, quartz, gneiss, and tuffs.
SHED ages match precision for TCND.
Shap deglaciation occurred at 16.5 ± 0.5 ka.
SHED can provide a cost efficient alternative to TCND.
Abstract
Ninety-eight surfaces related to the retreat of the British Ice Sheet (BIS) and dated using Terrestrial Cosmogenic Nuclide Dating (TCND) were sampled using the Schmidt Hammer to expand on relative dating techniques and establish Schmidt Hammer exposure dating (SHED) as an effective method for dating glacial landforms in the UK. The BIS is an effective analogue for contemporary glacial systems but our understanding of its retreat under changing climate conditions is constrained by a limited number of dates obtained from existing methods (14C, OSL). These methods are restricted in their application to glacial environments and while TCND has addressed this to some degree, its cost and potential for outliers encourage the establishment of new techniques. SHED fulfils this requirement by providing a cost-efficient method for obtaining numerous direct ages that are of comparable accuracy and precision to TCND. A multi-lithology approach has established that many rock types are unsuitable for numerical dating. However, a robust calibration curve was generated (R2 = 0.81, p = < 0.01) for granite surfaces and applied to 31 undated granite erratics on Shap Fell, NW England. SHED indicates that BIS retreat occurred at 16.5 ± 0.5 ka, a conclusion which supports our current understanding of regional deglaciation and indicates that SHED can be a valuable and cost-effective geochronological tool.
7 comments:
This is very intriguing.
Would it be possible to look at a rock outcrop and by relative analysis tell the difference between the effects of Anglian and later glaciation, or between Neolithic and later quarrying?
Dave
You have to prepare the surface first with these (unless you happen to find a bit that is flat). So not entirely non-destructive (doubt English Heritage would approve at Stonehenge for instance). They're usually supplied with an abrasive stone for preparation. I can see an issue if the rock has sustained some sort of damage at sometime in the past. Rarely use mine these days.
Dave -- yes, that should be feasible. At that level of resolution, it should be possible to get rather good results. But if you were trying to decide whether a surface was 5,000 years old or 5,500 years old, you would have a problem, because the accuracy is not great enough.
Jon -- from what I gather, it's like smoothing the rock surface with a bit of sandpaper. They probably do more damage at Stonehenge when they remove bird shit or lichen.........
There are lots of variables that have to be taken into account. But I have been reading several studies, and the guidancxe is now pretty tight as to what needs to be done, how the samples should be taken, and how the results should be calibrated. looks quite impressive to me.
You mean you have a Schmidt hammer? Old mechanical style, or modern digital / electronic style?
Could you calibrate using a Neolithic polished axe from a known rock source and then go to that source to see what results you get?
Dave
Interesting thought, Dave! But from what I have read, the hammer gives quite an impact, and one might be rather concerned that the impact might destroy the axe! Those who do this sort of work say that the best control is from a completely fresh surface which is modern. The problem with a Neolithic axe is that you do not know that it has genuinely been subjected to 5,000 years of weathering -- it has probably been buried for most of that time, and then carefully protected since it was discovered. In contrast, a Neolithic stone monolith in an archaeological setting can be assumed to have been continuously exposed to the weather since it was erected, and would give a much better control.
Old mechanical style Brian. Been in the cupboard of 'useful bits of kit' for probably a couple of decades now. Must be a few years since last used as can't remember the last time I actually used it: probably would need recalibration now.
If surface isn't totally smooth, the end of the hammer will compress the surface locally as it impacts: that gives false readings. It's fine if already smooth. The suppliers give a small abrasive stone to do that with, but it's easier to use a mechanical power tool (also in the cupboard). Some of the rocks at Stonehenge could probably be done without an issue.
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