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Monday, 13 December 2010

Periglacial effects on Salisbury Plain

I came across Mike Parker Pearson's references to those strange "grooves" or "striations" in the surface of the chalk, discovered during the Bluehenge excavations.  They were blithely described as "periglacial" -- but I would dearly like to know more about them.  On the matter of the lowering of the ground surface during Pleistocene / Holocene times, I also discovered this interesting little piece on the Geol Soc web site -- to whom acknowledgement is due.
 

Q: I am trying to ascertain the effect of acid rain on Salisbury Plain - particularly the loss of ground levels as the chalk dissolves. Is there anyone who can give me a rough estimate of the amount of ground level loss since the Neolithic period?

From Mr John Drews (August 2009)


Reply by Prof. Rory Mortimore

A simple question with a difficult answer! Yes dissolving of the Chalk happens all the time as we know from the “hard” water we get in areas taking their supply from it. How much this dissolving process has actually lowered the Chalk landscape is questionable.

For me, the most telling evidence is the fact that everywhere we dig into the Chalk with trial pits and boreholes (e.g. for the A303 investigations around Stonehenge and Winterborne Stoke on the south side of Salisbury Plain and quarry exposures around the edge of Salisbury Plain at Westbury, Stoke Hill etc.) we find weathering structures created during the last Ice Age – features typical of the frozen ground around the fringes of ice sheets. These are called “periglacial” weathering features.

The periglacially degraded top layer of Chalk, and the underlying periglacial fracture patterns, are hardly disturbed from the time they were produced - most probably during and at the end of the last major cold period 14-12 k yrs ago (with two “mini” ice ages since). Similarly, the downslope wasting of periglacial material by the action of freeze-thaw, solifluction (soil creep) and meltwater indicates that most of the landscape predates the last major cold period and is, therefore, much older than the Neolithic (New Stone Age – beginning c. 9500 BCE).

Over the last 12,000 years dissolving the Chalk by rainfall or groundwater flow in the Chalk has enhanced “karstic” features (landforms caused by dissolution of soluble limestone rocks) in some areas and we know where there are areas of active karst today, leading to ground collapse. These areas are in special geological settings (e.g. along feather edges of Palaeogene deposits on the Chalk in particular tectonic settings) and are the result of localised processes.

There is therefore no current evidence to show large-scale downwasting of the Chalk landscape by acid rain.

Despite all this work I cannot find reliable figures illustrating the quantity of dissolved ‘chalk’ entering rivers and supply networks (most of this dissolution would come from fissures etc rather than the near ground surface chalk, so it would not lower the landscape significantly). Unlike coastal cliffs, where it is possible to measure the retreat over time and see at first hand the processes involved, we do not have the same control on the general inland landscape of the Downs and Salisbury Plain. The era of satellite surveying techniques may change this.

Further reading


This list of references is not comprehensive – indicative only.

See Sparks 1960 page 160 in a summary of previous work up to that time, discussing rainfall and/or humic acid as a dissolving process and Price, Downing & Edmunds, 1993, p.49-50 on the evolution of acidic groundwater in the Chalk: there have been many other studies on Chalk landscape evolution since Sparks including the Soil Survey (Catt, 1986; Hodgson et al., 1967, 1974); DKC Jones (1971;1999) and studies of the hydrogeochemistry of the Chalk groundwaters (the most comprehensive data set is in the British Geological Survey publication, The Chalk aquifer of the South Downs, 1999 pages 55-63; and good discussions in Price, Downing & Edmunds, 1993.

  • J.A. Catt. 1986. The nature, origin and geomorphological significance of clay-with-flints. The scientific study of flint and chert: Proceedings of the Fourth International Flint Symposium, Brighton, April, 1983. In: G. De G. Sieveking & M.B. Hart Eds. Cambridge University Press.
  • R.A. Downing, M. Price & G.P. Jones. 1993. The Hydrogeology of the Chalk of North-West Europe. Oxford Science Publications. Clarendon Press.
  • J.A. Hodgson, J.H. Rayner & J.A. Catt. 1974. The geomorphological significance of Clay-with-flints on the South Downs. Transactions of the Institute of British Geographers, 61, 119-129.
  • D.K.C. Jones. 1971. Southeast and Southern England. Methuen: London.
  • D.K.C. Jones. 1999.
  • H.K.Jones & N.S. Robins (Eds). 1999. The Chalk aquifer of the South Downs. Keyworth, Nottingham, British Geological Survey
  • Price, R.A. Downing & W.M. Edmunds. 1993. The Chalk as an aquifer. In: R.A. Downing, M. Price & G.P. Jones. 1993. The Hydrogeology of the Chalk of North-West Europe. Oxford Science Publications. Clarendon Press. Pp. 35-58.
  • B.W. Sparks 1960. Geomorphology. Longmans.

6 comments:

Tony said...

For the sake of clarification, I would just point out that in your first sentence, you refer to coming across "Mike PP's references to strange "grooves" or "striations" in the surface of the chalk, discovered during the Bluehenge excavations."

These were discovered during the 2008 and 2009 excavations of The Avenue where it leaves The Heel Stone beyond Stonehenge, on the northern side of the A344, whence The Avenue heads downhill in a NE direction. Very deep natural gulleys occur in the centre of The Avenue, running the whole length down to The Avenue's "elbow", at which point it turns roughly E. (This is, of course, a long way from where The Avenue meets the river Avon, near "Bluestonehenge".

Mike Parker Pearson has said that a pair of natural ridges occur either side of a the gulleys. He stated that a geomorphologist had confirmed these features were natural.

The line of gulleys and ridges just happens to be on the Solstice line. Furthermore, it is possible that The Heelstone may have been in situ,recumbent,naturally.

I believe the geomorphologist's name was Charlie French, and he was part of the Stonehenge Riverside Project's team.

I have visited the site and MPP pointed these geomorphological features out to a group of us visitors during excavation. He has also described them at 2 lectures in Wiltshire on 11/11/08 and 10/10/09,as has his colleague Josh Pollard on a separate occasion.

No doubt this has been written up in Mike Pitt's "British Archaeology" magazine in greater detail, but I have not been subscribing long enough to have this.

The Stonehenge Riverside Project lead archaeologists wonder if this natural geomorphological feature (pre-dating The Avenue itself) was the prime reason that Stonehenge was built precisely where it was, i.e. with the Solstice Line leading away from the (naturally positioned?)
Heel Stone in a NE direction down the slope towards Stonehenge Bottom.

BRIAN JOHN said...

Thanks for that clarification, Tony. I had recalled reading other bits and pieces about these grooves, but a Google search proved fruitless. I'll try to follow this up.

Don't know Charlie French -- he's obviously one of the younger generation....!

Sounds a bit far-fetched that Stonehenge should be located just where some natural (?) grooves happened to lie. But the story of Stonehenge is full of far-fetched ideas.

I'm intrigued by the thought that the Heelstone might be in its naturally occurring position. Has anybody ever excavated under it, just to check?

Tony said...

I think you'll be interested in reading about aforesaid Charly French on the University of Cambridge's website at:-

www.arch.cam.ac.uk/~caif2/-

He is Reader in Geoarchaeology at the Laboratory for Geoarchaeology.

Mike Pitts relates in his "Hengeworld" (2000) that he excavated in the vicinity of The Heelstone in 1979, and found a huge pit indicating that the Heelstone originally had a partner.

Anthony Johnson (in "Solving Stonehenge", 2008,pages 120-1, thinks "It is not impossible that....the Heelstone was set up close to the position in which it originally lay, perhaps long before any other stones were brought to Stonehenge." In 1922 Hawley found a 'crater-shaped' hole measuring 1.5m in diameter and 1.37m deep approximately 7 metres south of the heelstone, which he thought may once have held a 'large natural stone'.

Anthony Johnson also stresses that the bulky and rounded form of the Heelstone is entirely unmanageable in comparison with the other Stonehenge sarcens, lacking the flat tabular surfaces that would have allowed it to have been readily conveyed on rollers.

Johnson also writes about other isolated sarsen monoliths in the wider Stonehenge landscape, such as the Bulford Stone. Another, the Cuckoo Stone, has been th subject of investigation by the Stonehenge Riverside Project in recent years.

Constantinos Ragazas said...

Tony,

In some sections of my paper “The un-Henging of Stonehenge” I propose explanations for The Heelstone and for the orientation of The Avenue consistent with the facts as you describe in your comment above.

Interestingly, the same theory can also explain the natural ridges and gulleys at the center of The Avenue. I am encouraged that such geomorphology is confirmed now to be natural, as my theory has claimed all along. Not wishing to upset Brian by going too deeply into these explanations here at his blog which he controls and sometimes blocks, let me suggest that you read these in my paper. A simple google search of the exact title will link you to it.

Kostas

BRIAN JOHN said...

Kostas, you make me sound like some power-crazed evil manipulator of men's minds......... but I'll let that pass, and hope that most readers think my attempts to keep the discussions within reasonable bounds are OK.

Men's minds ..... hmmm. That is interesting. Nearly all those who contribute their thoughts on this blog are men. I wonder why?

Robert Langdon said...

Ah!! you need an Archaeologist.

The Heelstone is probably not natural as a segment from the stone to the South East to the Avenue Ditch was cut in 1953 known as C36.

The two most interesting aspects are that excavation is that the heelstone has packing of flints to the excavated Heelstone edge - which indicates a hole was dug and then backfilled.

The other feature of interest is the 10m moat/ditch that is 0.5m wide and 1m deep surrounds it - This is a similar construction to the two Station Stones on the Aubery circle ring, with the same moats.

This would indicate that these three stones and moats were added after the bluestone/aubrey circle was abandoned.

The Avenue - as it does not line up with the Heelstone or its moat is even later addition - which would indicate the 'stripes' had no influence in Stonehenge's construction.

Hope that helps!

RJL