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Tuesday, 14 December 2010

Salisbury Plain -- a periglacial landscape?



To what extent is Salisbury Plain a periglacial landscape, fashioned by frost-related processes during past glacial periods?  This question has received a fair bit of attention over the years, and the assumption has always been that this rolling chalk downland has NEVER been directly affected by glacier ice.  Geologists and geomorphologists including James Scourse and Chris Green have even gone so far as to say that it was IMPOSSIBLE for glacier ice to have extended this far into the interior of Southern England.  As I have argued, it is certainly not impossible from a glaciological point of view, and indeed the isostatic history of the area (insofar as we can unravel it) does suggest deep depression and recovery inexplicable except in terms of an ice cover.   The strange assemblage of more than 30 different rock-types in the "bluestone" collection also argues for the glacial deposition of either an erratic train or fan, or even a layer of till, in the vicinity.

I have always been intrigued by the "clay-with-flints" puzzle, and have wondered whether this catch-all term has been used for genuine periglacial slope deposits, residues from Tertiary or other rocks that once capped the chalk, and for ancient and denuded glacial deposits.  From the descriptions, it does seem rather variable.

Click on the title above to go to a previous post, from 9 Dec 2009.

Anyway, in an attempt to understand the history of Salisbury Plain a bit better, I came across these:


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(1)  Devensian periglacial influences on the development of spatially variable permeability in the Chalk of southeast England
P. L. Younger
Quarterly Journal of Engineering Geology and Hydrogeology; 1989; v. 22; issue.4; p. 343-354


In unconfined parts of the Chalk aquifer in southeast England, permeability generally varies laterally with the lowest permeabilities occurring beneath interfluve areas, and the highest beneath river valleys and dry valleys. Furthermore, the Chalk in the river valleys is normally in excellent hydraulic continuity with the overlying highly permeable Quaternary gravels. However, recent field investigations in the Thames Valley have demonstrated the existence of zones of anomalously low Chalk permeability associated with the development of thin discontinuous confining layers of low permeability ‘putty chalk’ at the gravel-chalk interface. Hitherto putty chalk in the Middle Thames Valley has mostly been reported from interfluve areas where it can occur as a periglacially frost-weathered mantle on the upper surface of the Chalk. The true extent and hydraulic significance of putty chalk in valley bottom positions is only now being realised. Existing models for the lateral variation in Chalk permeability cannot explain these new observations. A new model is therefore proposed in which it is envisaged that, during the Devensian, carbonate dissolution in perennial taliks (unfrozen zones) beneath the major channels of the braided palaeo-Thames caused the high-permeability zones, while permafrost beneath the interfluves restricted dissolution at those sites. Freeze-thaw action in seasonal taliks beneath minor channels would account for the formation of putty chalk at the gravel-chalk interface, and the persistence of permafrost beneath these seasonal taliks would lead to a restriction of dissolution, and thus to a zone of low permeability.

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(2)  Anon:    Environmental Statement -- Stonehenge Visitor centre -- Geology and Soils

9.3.1 The 1:50,000 Geological Survey Sheet 298 indicates the site to be underlain by Upper Chalk
of Cretaceous age. Along the southern and eastern sides of the site deposits of Valley Gravel
and Alluvium associated with the River Avon are indicated to be present.
9.3.2 The Upper Chalk comprises weak white fine grained limestone composed almost entirely of
foraminifra. Due to its weak/soft nature the Chalk is easily weathered and can range between
completely re-worked soil composed of chalk fragments to structured chalk rock. There are
also frequent bands of flints within the Chalk.
9.3.3 The valley gravels lie beneath the Alluvium and extend further up the valley sides and
represent river deposits of earlier times when the river was much larger. The Valley Gravels
generally comprise sandy gravels and gravelly sands including much reworked chalk and flint
in this area. In addition the valley gravels can vary significantly over short distances both
horizontally and vertically.
9.3.4 Alluvium is associated with the present river course and is typically highly variable in
composition ranging between gravels and clays often with organic matter and lenses of peat.
9.3.5 Often above the Chalk there is a thin cover of clay with flints which tend to mantle the
slightly uneven upper weathered surface of the Chalk. This material typically comprises
brown silty and sandy clays with abundant sub-angular to sub-rounded gravel and cobbles of
mostly flint.
9.3.6 The Chalk is a major aquifer and groundwater flows through the Chalk via interconnecting
fractures. It is likely that at this location there is direct continuity between groundwater in the
Chalk and the River Avon.

Geophysical Characteristics

9.3.7 Results of the geophysical survey are shown in Figures 9.1 to 9.4. The survey clearly shows
the area of a former borrow pit which was earlier identified by the archaeological magnetic
survey. The anomaly related to the borrow pit shows as a broad low resistivity area. The
shape of the resistivity anomaly related to the borrow pit is similar to that provided by the
magnetic survey.
9.3.8 The areas outside the borrow pit are divided into two zones. These are broad linear zones of
either relatively constant resistivity or zones characterised by rapidly varying resistivity in
which a number of enclosed oval or irregular resistivity anomalies are present.
9.3.9 These zones are most strongly differentiated at around 5m depth and generally reduce with
depth.
9.3.10 These zones of varying resistivity anomalies correspond to areas of deeper superficial
material identified by the physical investigation.
9.3.11 It is considered that these zones represent probably structurally controlled areas of enhanced
surface and subsurface solutioning which can be referred to as examples of doline fields.
Such features are common in areas of limestone (of which chalk is a type) which have
undergone solutional weathering or karstification. The lineation of these zones is clearly
identifiable as E-W to ENE-WSW. This trend is compatible with the tectonic discontinuity
patterns in the area.
9.3.12 Doline formation in the Chalk has been attributed to a number of mechanisms and different
shapes of features are possible. Common conical shaped rapidly tapering forms are
attributable to authigenic (A-type) solutioning where solution is carried out by rain falling
directly onto the chalk surface. Elongate forms are more common with allogenic (B-type)
solutioning by streams forming on the adjacent impermeable cover material and sinking as
they cross the boundary of the chalk.
9.3.13 A mixed mechanism of solutioning and modification under periglacial conditions has been
proposed by P. L. Younger in Devesian Periglacial Influences on development of Spatially
Variable Permeability in the Chalk of South East England (QJEG Vol 22 No 4 1989) for
forms which show a very tabular shape and broad flat base which are also common in the
South East. The base of these features has been related to the depth of permafrost and may
be up to 15m deep.
9.3.14 The depth of the base of the features found on the site is generally less than 5m although PH6
superficial material was encountered to a depth of about 8m. The base of a solution feature
was encountered in trial pit 4 at about 2.5m depth as a broad flattish base. This observation is
consistent with periglacial modification.
9.3.15 C. L. Edmonds in Towards the Prediction of Subsidence Risk on the Chalk Outcrop (QJEG
Vol 16 1983), has indicated that there is a low density of solution features on the Chalk of
Salisbury Plain. This observation has been queried on the basis that the low density of
development on the Plain compared with other Chalk areas has not afforded the same
opportunity for the discovery of solution features. It is also likely that variations in solution
feature density are possible on any area of chalk where differences of geology and structure
exist. Mixed A and B-type development is possible.
9.3.16 Edmonds (op.cit) suggests that this lack of solution features may be related to the Plain
having had much of its surface removed during the late glaciation. This suggestion is
consistent with the base of the periglacially modified solution features in general being less
than would be expected from P. L. Younger (op.cit) had a significant depth of surface been
removed subsequent to periglacial modification.
9.3.17 In areas outside the zones of significant anomalies small purely solutional pipes were evident
at the clay/chalk junction.
9.3.21 The chemistry of the groundwater has been analysed in one sample obtained from BH8. The
results of this indicate that the water has a pH value of 6.7 a temperature of 16.2°C,
conductivity of 780μS/cm a total hardness of 338mg/l and a saturation of -42mg/l.
9.3.22 In consideration of these results the temperature of the groundwater at this location was
unexpectedly high. In addition the water was undersaturated (-42mg/l Ca CO3). Borehole 8
from which the sample was obtained was within the borrow pit and it is likely there is a
pathway for surface water to travel to the groundwater easily in this area as the less pervious
9.3.18 A proposed model for karstification and subsequent modification is shown in Figure 9.5.
This shows the possible landscape changes since the later stages of the present glaciation. It
should be noted that the model is schematic and that no absolute time frame over which the
changes have occurred is implied. The model is however, consistent with published
variations in karstification known from other areas. Present day or more recent solution
activity is indicated by the development of small scale, A-type solution pipes present across
the investigation area.

16 comments:

Constantinos Ragazas said...

Thanks for your post, Brian.

I do appreciate all your efforts to dig up geological evidence that comes from field research of Salisbury Plain. What geologists have to say about Stonehenge is in my opinion far superior to what archeologists have been telling us.

There is much information contained in this post, but what immediately struck my attention is the following quote.

“... anomalously low Chalk permeability associated with the development of thin discontinuous confining layers of low permeability ‘putty chalk’ at the gravel-chalk interface.”

Consider also the following quote,

“... the temperature of the groundwater at this location was unexpectedly high.”

Brian, as someone who knows well the geomorphology of Stonehenge, does the first quote above in any way describe also the Stonehenge Layer 'gravel-chalk interface'? Does the chalk bedrock beneath the Stonehenge Layer exhibit such 'putty chalk' characteristics?

I say this because the Atkinson photos of the Stonehenge Layer that you posted several months ago do give that 'putty' appearance of the chalk bedrock. If so, this would explain how it was that these huge stones could be so securely implanted in the 'putty chalk' bedrock. Such implantation, of course, would be made more feasible if the stones where 'dropped from above'.(sorry, I couldn't resit without feeling muffled by your censorship of my posts).

Constantinos Ragazas

BRIAN JOHN said...

Kostas, I'll let your comment about censorship pass...!!

I can't really answer your questions because the authors of this report are rather vague. But I get the impression that the "putty chalk" is rather lower, particularly beneath gravel layers -- and that it therefore has nothing to do with the Stonehenge Layer (which is quite close to the surface).

As for the water temperature, I think it was for water from a borehole, and it might be a result of the very calcareous nature of the water -- or maybe something to do with geothermal warming? There are after all hot springs in Bath and in the Southampton Basin......

Can't be much help on this, I'm afraid.

Robert Langdon said...

Good Blog Brian.

At this rate I will need to add a couple of new chapters to my revised book using your supporting evidence.

RJL

BRIAN JOHN said...

Robert -- supporting evidence?!! I am intrigued to see how you will interpret widespread evidence of periglacial and solutional processes and slope accumulations and transform it into "evidence" for a Mesolithic submergence of the landscape.........

Robert Langdon said...

Water Brian - without the proof of water....i'm sunk!

You have shown that the Devensian Periglacial was much more extensive than current geologist believe.

Even you will admit that directly after the ice caps melted the 'dry valleys' in Wiltshire would have filled with water as the volume is so great that it would be impossible to be completely absorbed - the question we disagree on is HOW LONG did this process take!

RJL

BRIAN JOHN said...

I don't think there is any serious doubt among geomorphologists that Salisbury Plain was affected by permafrost and periglacial conditions for thousands of years during the Devensian. The only doubt is over glacial ice -- did it reach Salisbury Plain, and if so, for how long?

The dry valleys will never have been "filled with water". More likely, they were used intermittently for the transport of snowmelt water. The deeper ones may well have held seasonal snowbanks too, especially on shady north-facing slopes.

If the ice reached Stonehenge, and then melted rather rapidly, the pre-existing valleys would have been used by fluvio-glacial streams every year for hundreds of years as the ice melted and as the ice front retreated.

Robert Langdon said...

Absolutely right Brian.

Now take into account the Isostatic movement due to the ice then the water table will rise.

If it goes up by just 30m (next to nothing by isostatic standards) the said 'transport ice melt' would have nowhere to go and hence it floods.

This then becomes the river at 'Stonehenge Bottom' at a height of 95m which complements the archaeology.

So you see we are in agreement!

RJL

BRIAN JOHN said...

No way, Robert. We are most definitely NOT in agreement here. What do you mean by the "water table"? You seem to use that term to mean "relative sea level" -- and you want relative sea level to be at around 100m in the Stonehenge area in the Mesolithic. There is no evidence for that, no matter how much you talk about moats and docks -- and indeed the field evidence from all around the coasts mitigates against you. As I explained in my posts on the giant boulders, the signs are that isostatic depression seems to have been in the same order of magnitude as the eustatic sea-level fall.

Instead of relative sea-level being at +100m, as you want, it was probably at least as low as -10m and rising towards present dat MSL.

Robert Langdon said...

The water table is the level at which the groundwater pressure is equal to atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity.

However, saturated conditions may extend above the water table as surface tension holds water in some pores below atmospheric pressure individual points on the water table are typically measured as the elevation that the water rises to in a well screened in the shallow groundwater.

Inland rivers are dependent on the water table and if your trying to investigate the effects of isostatic transformation on the landscape it is the single factor that needs to be understood.

Sea levels as you point out are relative at best.

RJL

BRIAN JOHN said...

Still haven't got a clue what you are talking about. Water tables do not flood landscapes, and neither do they lead to dramatic isostatic depression. There is water present in all rocks at some depth or other -- and there will be an isostatic equilibrium until an additional load (in the form of sediment or ice) is added to the land surface. End of story.

Robert Langdon said...

The BGS have produced a very good slide show to allow people to understand processes leading to groundwater flooding in Chalk.

What you see is an 'idea' of the type of predicted models I have used to forecast the Prehistoric Water levels due to rises in the water table attributed to isostatic transformation.

www.groundwateruk.org/.../groundwater.../GWSW_Talk3_Williams.ppt


RJL

BRIAN JOHN said...

This is all very sensible, Robert. Of course you will get floods in chalk areas (as in any other areas) if the water table intersects with the surface following heavy periods of recharge, or if you get exceptional deluges that the local infiltration rate cannot cope with. You may get quite prolonged flooding of closed basins such as dolines in karst areas, or in areas where excess water cannot escape from an upper catchment because of a constriction downstream -- for example through a narrow gorge through a hill ridge. But you will NOT get total submergence of the whole landscape (including hill summits), in the manner you propose, since water will escape to the sea -- and since you do not have a mechanism for isostatic depression on the scale you propose.

" ........ rises in the water table attributed to isostatic transformation". What on earth does that mean? Sounds wonderful, but I think it's a meaningless phrase.

Robert Langdon said...

Tell that to the poor people in Pakistan.

The Indus River is less than 200km from the sea in some flooded areas. The flood was caused with a rainfall of just 7 to 10 inches of rain raised the water table a lot more as it gathered in certain regions.

Thats why access to the sea is immaterial, otherwise all the water would have drained by now.

The analogy I use in the book is to see Britain like a giant wet sponge - the water surrounding looks low but push your finger down just a little in to the sponge and it gets wet and leaves a puddle.

Now push down with palm of hand(ice action on Britain) water moves up (water table)and your hand gets wet. Now release sponge and it returns to original height (isostatic transformation)- but for a few seconds the water will sit on the surface of the sponge before getting absorbed.

In a sponge this process is quite quick, but on Britain it takes 1,000's of years but with the same results. In Pakistan if he water table stayed high(because of the rain and the lowering of the relative height of the land due to isostatic transformation) the flood water would remain for 1000's of years as at Stonehenge, until the water table lowered.

It's basic physics but not fully understood in geology it seems!

RJL

NB. Do yo think I should use the floods in Pakistan as yet another proof of hypothesis?

BRIAN JOHN said...

False analogy, Robert. The Indus has a vast floodplain (so called because it floods) with a very low gradient. No different in that respect from all the other great flood plains of the world. Excessive rain will cause it to flood, since river gradients are too low to evacuate the water efficiently. This is essentially a zone of fluvial sedimentation, not fluvial erosion -- as any GCSE geography student will tell you. So flooding and sediment release are entirely predictable -- that is why floodplains are so fertile, and why millions of people choose to live on them, in spite of the attendant risks.

The scenario you are talking about is entirely different -- an undulating landscape in Middle England with a long (millions of years) history of erosion in the chalk coombes and terrace formation in the middle reaches of the rivers.

Your sponge analogy is false too -- if the land is depressed, so is the water table. You cannot expect water tables to stay high just for the convenience of your theory! And if there was isostatic depression of Middle England, you would expect instant flooding of the land after ice retreat. There is absolutely no evidence for that either in the sedimentological record or anywhere else.

Show me some shorelines or some varved sediments or even delta sediment sets, and I might take your theory seriously. Never mind about all your "proofs" and your talk of "isostatic transformation". What I want is some simple geomorphology that makes sense -- of the sort I have seen in many parts of the world -- where shorelines, sediments and the biological record all tell the same story.



.

Robert Langdon said...

A flood plain - who would believe that! Is that something to do with the water table?

Sukkur is one of the many towns effected, interestingly its about the same height OD as Stonehenge - your theory on waterflow suggests that as its 90m high all the water should flow away quote 'But you will NOT get total submergence of the whole landscape (including hill summits), in the manner you propose, since water will escape to the sea --'

Well is exactly what we can see now, and this proof is not 'theoretical' science - the Pakistan floods are 'real' science, 90m up and no flow to the sea!! Why because the water level is also high.

This proves beyond doubt that you can have rivers above this precious relative sea height - in fact sea level has very little effect on water tables and so should be completely disregarded in the study of Stonehenge and it's Mesolithic Rivers!

So would Stonehenge flood - well Sukkur had 10 inches of rain that caused flooding for the last 3 months, just think what 1 - 2 km of water will do to the land! only a member of the flat earth society would argue against the post ice age floods.

And for evidence - well the river avon was once a flood plain just like sukkur T5 at 75m; T6 at 85m; T7 at 95m (stonehenge); T8 at 115m and finally T10 at 135M are flood plains identified by Maddy Geomorphology 33 2000. 167–181.

How did they date them, by archaeological finds from not the Avon but the Thames Flood plains and how did the archaeologist date those finds, thats easy by the Geology - best known as a 'Circular Argument' in philosophy.

A clear case of the Blind leading the Blind and a basis of so many wrong academic papers and articles - what a waste of our precious trees - and that's the real final story Brian!

RJL

BRIAN JOHN said...

I was asking you to look at the local topography -- I have never said that floods are impossible, well away from the sea, or that they may persist for some time. But to compare the Avon valley with the Indus flood-plain is absurd. I keep on asking you for evidence of this wonderful inundation, and you persistently fail to provide it apart from saying that it has something to do with water tables and isostatic readjustments. Fantasy world, Robert. What I want is evidence on the ground, verifiable, and repeated over many localities.

We are not going anywhere on this discussion -- you have clearly invested so much in your book and your wildly enthusiastic web site that it's too late to change anything now. Please go off your way, and I'll go mine.

History will eventually reveal which of us is right...