Sarsens and the solution hollow dilemma

A few months ago I did a post on the 'honeycomb' characteristics of the solid chalk surface beneath Stonehenge -- or at least, beneath the bits that we know about.   I referred to the pits, hollows, sockets and solution features which are almost universally interpreted by archaeologists as being man-made -- the assumption being that the bluestones -- and maybe the sarsens as well -- have been moved about from one setting to another during the various "stone phases" of the monument.   This has been remarked upon by Anthony Johnson in his book on Stonehenge, and by many others as well.

Anthony Johnson says on page 157:  "... the maze of holes, hollows, abandoned settings and residual stone fragments in the centre of the monument has left the underlying chalk resembling a very badly mauled Swiss cheese which, compounded by extensive disturbance from rabbit burrows, has made the disentangling of the various periods of activity almost impossible, leading Hawley to conclude:  "I frankly confess that I have no explanation to offer in elucidation of this tangle, and I doubt whether anybody will ever be able to explain it satisfactorily."

I wondered whether some of the pits might actually be "extraction pits" from which embedded monoliths had been collected -- and I cited Nick Snashall, who is quite convinced that she can tell exactly which pits are man-made and which ones are not........  

https://brian-mountainman.blogspot.co.uk/2016/08/extraction-pits-solution-hollows-post.html

We have discussed the nature of the  chalk bedrock surface in other posts, including many on the so-called "periglacial stripes" within the Avenue near Stonehenge.  Here is another of my posts:

https://brian-mountainman.blogspot.co.uk/2013/04/the-chalk-surface-grooves-everywhere.html

I would hazard a guess that at least a part of the pitted surface we see in the image above is natural, revealed when the archaeologists carefully scraped away everything rotten or loose.

Some more guidance on the nature of the bedrock / regolith interface is found in the booklet on Fyfield Down written by Mike Clark and published by NCC in 1976.  This is a revealing and perhaps surprising diagram from page 16:

 It shows a section cut alongside one of the recumbent large sarsens found in Clatford Bottom.  It shows that the sarsen is embedded in a layer of brown flinty  loam which extends for about a metre beneath the stone base, with combe rock beneath that, and then with almost a metre of strongly weathered chalk above largely unaltered chalk bedrock.  The brown loam is presumably the periglacial material that has moved downslope, maybe carrying or rafting the sarsen along as it accumulated.  But what interested Clark and his colleague was the evidence that in ten pits examined under and adjacent to recumbent sarsens, there was increased soil acidity as compared with soils where no sarsens were present.  There was also a tendency for solution pipes to occur beneath sarsens of various sizes in the combe rock, penetrating into the weathered chalk beneath.  One of these pipes can be seen in the illustration above.  Conclusion:  the presence of sarsens in one position for many thousands of years leads to enhanced solution in the regolith and rotten chalk beneath.
  
Result: a highly irregular and pitted bedrock surface.

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Fast forward to another paper, kindly brought to my attention by Alex.  This one is from 1996, by Julian Murton.  Details:

Julian B. Murton:  Near-Surface Brecciation of Chalk, Isle of Thanet, South-East England - a Comparison with Ice-Rich Brecciated Bedrocks in Canada and Spitsbergen
Permafrost and Periglacial Processes,Vol. 7: 153-164 (1996)

ABSTRACT

Chalk on the Isle of Thanet, Kent, is brecciated to depths of a few metres beneath the ground surface. The brecciation commonly comprises (i) an undeformed layer of angular, platy blocks more or less parallel to the surface overlain by (ii) a deformed layer containing small open folds, typically with vertical axial planes. Above the brecciated chalk is an involuted layer (-0.5 to 2.0 m thick) of chalk diamicton and brickearth.
By analogy with brecciated ice-rich limestones, arkoses and shales in areas of continuous permafrost in Arctic Canada and Spitsbergen, it is suggested that brecciation of the Chalk resulted primarily from ice segregation in perennially frozen bedrock, and repeated segrega- tion formed an ice-rich layer just beneath the former permafrost table. Subsequent thaw consolidation of this layer is thought to have formed an involuted layer through soft-sediment deformation.
Three implications arise from this study: (i) near-surface brecciation of the Chalk probably took place during conditions of continuous permafrost; (ii) the growth and thaw of the ice-rich layer in chalk was probably an important element in the geomorphological evolution of the English Chalklands, heaving and brecciating the Chalk during permafrost conditions, and deforming or redepositing the overburden during periods of active layer deepening; and (iii) repeated ice segregation near the top of permafrost may have brecciated other bedrocks in the British Isles.

The paper illustrates how, under conditions of continuous permafrost such as those which prevailed for lengthy periods on Salisbury Plain, brecciation of the bedrock combined with frost-heave processes led to the creation of both involutions and a highly complex bedrock - combe rock interface.
  
Result: a highly irregular and pitted bedrock surface.

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Fast forward to another paper brought to our attention by Alex. This one is from a group of Polish researchers, published in 2012:

Radosław Dobrowolski, Andrzej Bieganowski, Przemysław Mroczek and Magdalena Ryżak:
Role of Periglacial Processes in Epikarst Morphogenesis: A Case Study from Chełm Chalk Quarry, Lublin Upland, Eastern Poland
Permafrost and Periglac. Process., 23: 251–266 (2012) 
DOI: 10.1002/ppp.1750

ABSTRACT

Pocket forms several decimetres in diameter, 0.5–1.5 m deep and infilled mainly with glaciogenic sands, silts and clays of Saalian age are commonly developed on the top of the karstified chalk massif of the Lublin Upland, eastern Poland. Analysis of lithofacies, particle-size distribution and micromorphology of three pocket infills in the Chełm chalk quarry reveals a prominent clay cortex between the host chalk and the glaciogenic infill and suggests that periglacial processes have played a considerable role in the formation of the pockets and in the redistribution of their primary glaciogenic infill. A conceptual model for epikarst morphogenesis for the chalk karst of Lublin Upland is proposed, involving three stages. Stage I: In the absence of permafrost, precipitation water infiltrates unconsolidated glaciogenic deposits. Stage II: Periglacial transformation with underlying permafrost. Primary cryoturbation structures became protokarst forms, and then epikarst forms. Stage III: Degradation of the permafrost, with increased carbonate dissolution and development of a clay karst cortex.

This is a highly technical paper, and a convincing one, arguing that karstification and the creation of deep "pockets" can occur in chalk landscapes under a periglacial / permafrost regime, with physical and chemical (solutional) processes operating together.  We are talking about pockets here, and not solutional rills or runnels.

Result: a highly irregular and pitted bedrock surface.

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 I suspect that if you were to let loose groups of archaeologists with brushes, pans, buckets and trowels in any of the areas described in these three research publications, they would eventually reveal a chalk surface looking very much like the one shown from Stonehenge, at the top of this post.