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.