The Stonehenge bluestone erratic boulders -- when were they emplaced?

What do Stonehenge and East Greenland have in common?  Not a lot, you might think ---- but it might just be that work done on this massive end moraine may help to solve the problem of when, and how, the Stonehenge bluestones were moved from West Wales to Salisbury Plain......

East Greenland  (Kjove Land) 1962.  On the delta terrace on the eastern flank of the massive Holger Dansker Briller end moraine, at the exit of a diffluent glacial trough. There are erratic boulders on the terrace, and even more on the ice-contact slope and on the inner moraine surface.

In the middle distance, the terminal moraine with its flattish delta terrace top.  Beyond it, hidden from view, are the two lakes of the Holger Danskes Briller.

The pretence that the bluestones at Stonehenge are all pillars has been promoted vigorously for many years, by many people who should know better.  As I have pointed out many times before, the great majority of the 43 bluestones are not pillars but slabs and boulders which look for all the world like an erratic assortment collected from near the front of a wasting glacier.  They are weathered and heavily abraded, with very few sharp edges -- suggesting that wherever they have come from, they have been collected or gathered up, and not quarried.  The members of the MPP "quarrying" team seem to be in complete denial about this, and never mention it in their papers......

https://brian-mountainman.blogspot.com/2016/08/the-stonehenge-boulders.html

Stone 37

Stone 39

There has now been much work on the origins of the stones, but not much work at all on  the amount of time that has elapsed since their weathered surfaces were first exposed to cosmogenic radiation.  all we can say at present is that most of the boulders and slabs have weathering crusts on them, suggesting that they have been exposed to the atmosphere for tens of thousands of years.  Surface sampling would be easy to do, and would sort out the dilemma, if only EH would allow it.  As it is, that august organization treats every stone as if it is a religious relic, too precious to touch, and it seems to be far more interested in mysteries and narratives than in hard science.

Cosmogenic dating methods have come on by leaps and bounds, and we are now in the "mature investigative phase" with thousands of cosmogenic dates in the bag and hundreds of studies which have gradually ironed out the inconsistencies which were at first puzzling.  This happens with all "new" scientific methods -- pollen analyses, C14 dating, amino acid dating, X-ray studies of rock surfaces and so forth. (To a large degree this explains the recent spat between me and David Nash over the "discovery" of the source of the Stonehenge sarsens.  He believes implicitly in the accuracy of his new techniques, and his interpretations, whereas I employ a degree of scepticism on the grounds that the methods are immature, and are bound to be improved as experience accumulates......)

Below I cite two quite important studies of erratic boulders on or near moraines, which have led to the same conclusion:  namely that boulders carried in glaciers tend to be modified sufficiently (even if they have not been carried very far) for any "inherited age" characteristics to be eliminated.  This means that the dating of surface almost always underestimates the real exposure age, with incomplete exposure due to post depositional shielding by (for example) vegetation, snow cover, or blown sand.

So let's get those bluestone boulders at Stonehenge sampled and measured. I am quite certain that the ages will come out at far in excess of 5,000 BP -- which is what they should be if they were quarried by our Neolithic ancestors.  I would estimate that the exposure ages on the boulders will be around 20,000 - 15,000 yrs BP, with some irregularities down to intermittent surface shielding.

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TWO INTERESTING PAPERS

Dating of raised marine and lacustrine deposits in east Greenland using beryllium-10 depth profiles and implications for estimates of subglacial erosion

BRENT M. GOEHRING, MEREDITH A. KELLY, JOERG M. SCHAEFER, ROBERT C. FINKEL and THOMAS V. LOWELL

JOURNAL OF QUATERNARY SCIENCE (2010)
DOI: 10.1002/jqs.1380

ABSTRACT: 

Here we combine 10Be depth profile techniques applied to late glacial ice-contact marine and lacustrine deltas, as well as boulder exposure dating of associated features in the Scoresby Sound region, east Greenland, to determine both the surface age and the magnitude of cosmogenic nuclide inheritance. Boulder ages from an ice-contact delta in northern Scoresby Sund show scatter typical of polar regions and yield an average age of 12.8 +/- 0.5 ka – about 2 ka older than both our average profile surface age of 10.9 +/- 0.7ka from three depth profiles and a radiocarbon-based estimate. On the other hand, boulder exposure ages from a set of moraines in southern Scoresby Sund show excellent internal consistency for polar regions and yield an average age of 11.6 0.2 ka. The profile surface age from a corresponding ice-contact delta is 8.1 +/- 0.9 ka, while a second delta yields an age of 10.0 +/- 0.4 ka. Measured 10Be inheritance concentrations from all depth profiles are internally consistent and are between 10% and 20% of the surface concentrations, suggesting a regional cosmogenic inheritance signal for the Scoresby Sound landscape. Based on the profile inheritance concentrations, we explore the first-order catchment-averaged bedrock erosion under the Greenland ice sheet, yielding estimates of total erosion during the last glacial cycle of the order of 2–30 m.

This is the sampled area on the Holger Danskes Briller end moraine / delta terrace -- which I described with my colleague David Sugden back in 1962..........

This is the spillway through which the eastern lake overflows, near the southern end of the moraine. One of our 1962 photos.

The area in which we worked in 1962.  The Holger Danskes Briller moraine is at the eastern end of the eastern lake, and is a relic of an important glacier stage dated to c 11,000 yrs BP.

From Sugden and John, 1965.  We dated the big Holger Danskes Briller moraine to 10,500 yrs BP -- which was not bad, given the limited resources and dating methods at our disposal.   We were, as it happens, about 500 years adrift with the dating.......  and the moraine is now deemed to be a classic indicator of the "Inner Milne Land Stage" in East Greenland.  But our levelling of the marine stillstand to 101m was pretty well spot on.  We measured the marine limit in this region at 134m.

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Too young or too old: Evaluating cosmogenic exposure dating based on an analysis of compiled boulder exposure ages
Jakob Heyman, Arjen P. Stroeven, Jonathan M. Harbor,Marc W. Caffee


Earth and Planetary Science Letters
Volume 302, Issues 1–2, 1 February 2011, Pages 71-80


https://www.sciencedirect.com/science/article/abs/pii/S0012821X10007478?via%3Dihub


ABSTRACT

Cosmogenic exposure dating has greatly enhanced our ability to define glacial chronologies spanning several global cold periods, and glacial boulder exposure ages are now routinely used to constrain deglaciation ages. However, exposure dating involves assumptions about the geological history of the sample that are difficult to test and yet may have a profound effect on the inferred age. Two principal geological factors yield erroneous inferred ages: exposure prior to glaciation (yielding exposure ages that are too old) and incomplete exposure due to post depositional shielding (yielding exposure ages that are too young).Here we show that incomplete exposure is more important than prior exposure, using data sets of glacial boulder Be exposure ages from the Tibetan Plateau (1420 boulders), Northern Hemisphere palaeo-ice sheets (631 boulders), and present-day glaciers (208 boulders). No boulders from present-day glaciers and few boulders from the palaeo-ice sheets have exposure ages significantly older than independently known deglaciation ages, indicating that prior exposure is of limited significance. Further, while a simple post-depositional landform degradation model can predict the exposure age distribution of boulders from the Tibetan Plateau, a prior exposure model fails,indicating that incomplete exposure is important. The large global dataset demonstrates that, in the absence of other evidence, glacial boulder exposure ages should be viewed as minimum limiting deglaciation ages.