Landscape and sediment modification on Salisbury Plain

Sarsen litter on Fyfield Down. Here today, but yesterday somewhere else......

Some of the Stonehenge bluestone erratic boulders.  Here today, yesterday somewhere else, higher up.   Courtesy Simon Banton.

I have visited this topic on many previous occasions in this blog,  for example here: 

https://brian-mountainman.blogspot.com/2022/02/southern-england-where-is-glacial-limit.html

https://brian-mountainman.blogspot.com/2016/12/sarsens-and-solution-hollow-dilemma.html

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

https://brian-mountainman.blogspot.com/2022/02/salisbury-plain-no-traces-of-ice-action.html

My conviction that Salisbury Plain was glaciated a very long time ago is stronger than ever, and I have argued consistently that there may well have been a patchy cover of till (derived from the west) which has been destroyed and redistributed over the course of 45,000 years or more of continuous denudation.  Oh no, say the usual suspects, if there had been a glaciation you would expect to find a scatter or a trail of erratics across the chalk downland, patches of till and fluvioglacial deposits, and maybe even a terminal moraine or two..........  These people are slow on the uptake, and refuse to accept that there are indeed abundant erratics scattered across Salisbury Plain on the surface and in assorted sediiments, and that the bluestone monolith assemblage (including the Boles Barrow bluestone) is as fine a collection of glacial erratics as you are ever likely to see.

I have also explained in a recent post that the chalklands of Salisbury Plain provide an extremely aggressive geochemical environment in which glacial and other deposits are very readily destroyed:

https://brian-mountainman.blogspot.com/2026/08/salisbury-plain-serene-landscape-but.html

Murton and his colleagues have also made a persuasive case for the destructive operation of periglacial processes, especially when the chalk lands have been "solidified" by the presence of permafrost:

https://brian-mountainman.blogspot.com/2023/01/periglacial-britain.html

https://www.lyellcollection.org/doi/10.1144/EGSP28.5

During the Quaternary, these periglacial processes have perhaps been in operation for 25% of the time, with each glacial episode ushered in by a prolonged periglacial phase and each interglacial ushered in by a phase of rapid ice wastage and rapid warming.   The temperature graph shows a sort of "saw tooth" pattern.  

However, these geomorphological considerations are brought into even sharper focus when we consider the landscape history of Salisbury Plain. In brief, the Plain is not where it used to be (on the altitude chart) when it was last affected by glacier ice, so all of the deposits that were on the chalk surface must long since have disappeared.  Their calcareous components will have been dissolved away, and their quartz-rich components will have been disaggregated and redistributed.  Only in the most freakish or  favourable of circumstances can small fragments of these original sediments have survived in situ. (I use that phraseology because I don't like the word "impossible"..........)

So to what extent has the land surface been lowered during the last 450,000 years or so?  There are several basic calculations in the literature.  These can be tested one against another.

1.  In the assessments of chalk dissolution and land surface altitude reduction since the formation of the Paleogene silcrete duricrust that gave rise to the sarsen stone scatter, the figure of 30 - 100 m appears in the literature, dending on local circumstances.  Tectonic and regional climate changes have varied from place to place, but let's take a conservative figure to 50m of land surface lowering.  Initially the rate of surface lowering was around 2.5 mm per millennium.  Because of regional uplift and climate changes, the rate of lowering increased tom around 10mm per millennium from the beginning of the pliocene, accelerating again to 19.2mm per millennium during the Quaternary -- associated with violent swings in climate and  glacial / interglacial oscillations.  Since the onset of the Anglian glaciatiuon, surface lowering of between 7 and 14m has been suggested, with a widely accepted average calculation of 8.6m.

2.  Modern geomorphological studies suggest that in the UK, under current temperature and rainfall conditions, surface lowering on the chalk is around 15-30 mm per millennium.

3.  The Norber Erratics in northern England, left stranded on a limestone pavement at the end of the LGM, now stand on pedestals c 30 cm high.  That indicates baseline surface lowering at 20 - 25 mm per millennium. 

4.  Surface lowering in the Driftless Area (USA) has been measured at 10-15 mm per millennium, on much more resistant bedrock of Palaeozoic sandstones and dolomite.  The Quaternary rate of surface lowering on the chalk plateau of Normandy is estimated at around 20mm per millennium.

There is a very satisfying equivalence in all these figures.  The land surface of Salisbury Plain has been lowered by 8-10m over the last half million years.  The landscape as it was at the time of the Anglian Glaciation (or the Wolstonian, for that matter) is simply not there any more.  End of story.