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.
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.