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Sunday, 19 December 2010

Why is Salisbury Plain different?

 The lower map (with a rather inaccurate Devensian ice limit shown on it) shows chalkland patterns (black dots), ice wedges (inverted triangles), ice wedge polygons (diamonds), involutions (squares) and stone polygons and stripes (circles).  (Richard West, 1968)

What interests me most from the info contained in the last couple of posts is this:  Why are the western downs (including Salisbury Plain) so different, in terms of their superficial geology and geomorphology, from those of eastern England? 

During the Pleistocene the Chalk to the north and east of Buckinghamshire was directly eroded by ice sheets in most parts of the Eastern region, and its surface layer was disturbed by frost action. Today the Chalk is mantled by a veneer of superficial  deposits such as glacial till (sometimes called the "Chalky boulder-clay) and erosional residues such as clay-with-flints.  There are also many traces of periglacial action (involutions, frost heave features, patterned ground including polygons, and even fossil pingos.)  These attest to a prolonged period of permafrost which had a dramatic effect on the ground surface.  There are even thermokarst features, associated with surface collapse following the melting of subsurface ice masses.  There are also more frequent solution features including sink holes, dolines and ponds.

The lower map above shows a high concentration of these permafrost features in Eastern England -- for example, in Norfolk and Kent.  On the other hand these features are very sparse in Wiltshire, on  the Downs.   Why?  If all of these chalk areas lay outside the limit of Devensian ice, we might expect similar traces of prolonged permafrost on all of them.  So were the abundant features in the east down to the presence there of chalky till and other sediments above the chalk -- in contrast to the relatively thin soils of the Wiltshire Downs?  Or is the difference down to the TYPE of permafrost?  Could it have been "wet" permafrost in the east, with higher water tables and a permafrost table close to the ground surface, as distinct from "dry" permafrost in the west?  One might speculate that a lower water table in the west, maybe because of the higher altitude of the Wiltshire Downs,  led to less activity in the surface zone of seasonal thawing and refreezing -- leading in turn to the formation of far fewer permafrost features.......

Something to think about.


Constantinos Ragazas said...


You write,

“One might speculate that a lower water table in the west, maybe because of the higher altitude of the Wiltshire Downs,  led to less activity in the surface zone of seasonal thawing and refreezing -- leading in turn to the formation of far fewer permafrost features.......”

How would seasonal thawing/refreezing and periglacial action be different for open and exposed land surfaces as compared to land surfaces under the cover of ice as with a solidly frozen lake?

Just an interesting curiosity!


Robert Langdon said...

Nice to see that 'water tables' are being recognised as identifying ice coverage.

A your maps show no 'neat' lines of dots, triangles, squares, polygons or circles - clearly anything could have happened during the last ice age and therefore the ice sheet could have expanded to anywhere in reality.

The two clearly identifiable processes we can measure though is 'Isostatic rebound' and the 'water table'. We know that Britain in it's entirety is 'rebounding' either up or down - an indication the entire ice sheet covered the land (permafrost would not have this effect).

And the relatively high water table - both East and West. But your blog:

"One might speculate that a lower water table in the west, maybe because of the higher altitude of the Wiltshire Downs.."

Is an incorrect assumption as Pyecombe (as we have shown in a previous blog on the South Downs - which is missing from your first map?) is 109m OD with a water table level of 71m -81m OD. Which is very similar to Stonehenge which is 95M with water level of 55m - 71m.

Or high on the downs, you have Clifford Hill at 245 with water level 130m - 150m OD comparable with the Devil's Dyke at 195m with water level of 90m - 100m OD


BRIAN JOHN said...

I know where your thoughts are going on this one, Kostas! Since periglacial activity of all sorts depends upon the presence of an "active" layer above the permafrost table, obviously if a land surface was to be snow-covered or ice-covered throughout the year then the ground would either remain frozen or unfrozen, and different processes would come into play. A lake bed, either with bottom water or solid ice in contact with the bed, would also inhibit periglacial processes.

BRIAN JOHN said...

Robert, I don't understand what you mean when you refer to "water tables being recognised as identifying ice coverage". Water tables do nothing of the sort.

By "last ice age" I assume you mean the last glaciation (Devensian)? It could not have expanded to "anywhere" -- it would still have to obey glaciological laws, conform to the best modelling that the glaciologists can do, and leave evidence that conforms with the evidence from palaeobotany, climate studies, sedimentology etc.

I fear that you have got it all wrong on isostatic readjustments -- the process is much more subtle than you insist, and no two parts of the UK will have the same isostatic history -- as I have tried to point out.

Again, I don't know what you are talking about re water tables. Water tables are partly dependent upon topography. I could find you water tables at literally every level between 150m and sea-level in Southern England -- which proves precisely nothing.

Constantinos Ragazas said...

Thanks for your response Brian!

I know that you know my thinking about Stonehenge. I respect your rights and will not elaborate on that in your blog. My question, however, was meant to be an 'interesting curiosity' independent and apart from my 'smart ice theory'. I hope you see it this way too.

In your reply you seem to agree that such periglacial evidence will be different for land under the cover of ice then if that land was exposed. I have a follow up question for you.

Are the periglacial markings of Salisbury Plain (different as you argue in your post from eastern England) consistent with what the land would have looked like if under the cover of ice?

You need not agree with my theory in order to answer this. It's just factual and draws from your vast knowledge and expertise on geomorphology and on Stonehenge in particular.


BRIAN JOHN said...

I wish I knew Salisbury plain better -- but the Military occupy most of it, and a number of geomorphologists have bemoaned the lack of decent exposures which will allow interpretation of the landscape history. But there are, as I understand it, examples of involutions and frost heave features -- they are good indicators of permafrost.

I'll put up another post on this.....