Sink holes and solution hollows

A solution pit or hollow exposed in the face of the Quidhampton Quarry at SU 11403151.  Like most others, it is more or less circular, with steeply sloping sides (not vertical) and an infill of sediments, with virtually no surface expression.

Having examined the published data again, I'm increasingly convinced that there is no reason at all for the seven "Larkhill" hollows called "Durrington Shafts" by Gaffney et al to be referred to as man-made features. In fact, the authors admit this themselves. They demonstrate in the text that the seven features (numbered iii, iv, 10D, 11D, 12D, 13D and v) are almost certainly natural solution features, being aligned along a shallow natural depression or dry valley running down towards a bend in the Avon Valley. They say this:

"That general presumption that the group of features north of Durrington Walls were natural in origin and, probably, solution features gains some support in the geological literature. Such features are relatively common on the chalk and the available mapping is likely to provide an underestimate of their actual distribution (Hopson et al. 2006, 215). Some of the features recorded north of Durrington are set within a slight valley trending west-east towards the Avon. While such a topographic situation can provide the conditions that can lead to the development of solution features, the southern group of anomalies does not align with any similar topographic feature, and actually crosses higher ground above dry valleys. Consequently, the origins of the southern group of anomalies as solution features or doline is less likely."

So they are not arguing against a natural origin for these seven northern features. And yet suddenly, in the next part of the paper, they are treated as essential or integral parts of the "Durrington Shafts" arc or circuit, and it is assumed from this point on that the seven pits are man-made.........

This is illogical and inconsistent, and seems to signal a switch from straightforward description and interpretation to a process of fitting evidence into a ruling hypothesis.

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Here are some relevant extracts from a Geological Survey Report:

Geology of the Salisbury Sheet Area

http://nora.nerc.ac.uk/id/eprint/7175/1/IR06011.pdf

Report on the Geology of Sheet 298 Salisbury
and its adjacent area.
A compilation of the results of the survey in Spring and Autumn 2003 and from the River Bourne survey of 1999
Internal Report IR/06/011

P M Hopson, A R Farrant, A J Newell, R J Marks, K A Booth,
L B Bateson, M A Woods, I P Wilkinson, J Brayson and D J Evans

Extracts:

A wide variety of solution features occur but only two, ‘buried’ and ‘subsidence’ sinkholes are common on the Chalk. The term sinkhole is interchangeable with the term doline, and can also be applied to surface features where a stream wholly or partially disappears underground. Buried sinkholes (as defined by Culshaw and Waltham, 1987) are typified by ‘pipe’ or cone-like cavities within the chalk (Plates 44 and 45), infilled by the overlying deposits that have subsided into the cavity as a result of dissolution. Most are circular or oval in plan and can be many metres deep, often bifurcating into several smaller ‘pipes’ at depth. They often have no surface expression and are commonly infilled with flinty gravelly clay derived from the superficial cover, usually clay-with-flints.

Subsidence sinkholes are closed surface depressions, usually either bowl, pipe or cone-like in shape. They can occur as isolated examples or as groups, often coalescing into large composite dolines. They can form rapidly as a dropout failure following the washing out of pre-existing infilled pipes. Most occur in covers of unconsolidated sediment between 1-10 m thick, such as the clay-with-flints and older head.

However, the main control on near surface solution features is the geomorphic setting and the presence/absence of an impermeable cover. An area of impermeable strata either adjacent or overlying the Chalk serves to concentrate recharge and hence dissolution at the contact between the two rock types. The highest density of sinkholes occurs around the margin of the overlying Palaeogene strata or around the clay-with-flints outcrop. Topography and drainage patterns affect the distribution of solution features. Dissolution is enhanced where underground drainage routes are concentrated such as along valley floors and at spring lines. Typically the chalk is far more weathered under valley floors than under interfluves. Topography also influences whether drainage from the Palaeogene outcrop flows onto or away from the chalk and thus influences the location of water recharge via stream sinks.

An understanding of the geomorphic evolution of an area is vital to identify potential areas of karst development that have little no surface expression today. This is especially the case for karst features formed under differing climatic conditions or relict karst formed prior to present topography. Where the present land surface is close to the sub-Palaeogene peneplane, solution features inherited from the former Palaeogene cover may still exist. For example, solution pipes may still exist below ground level in areas where a former clay-with flint or Palaeogene cover has now been eroded. Elsewhere, erosion and dissection has removed these relict solution features.

Karstic features are also known in the Purbeck strata in the Vale of Wardour, both in the Salisbury district and to the west on the Wincanton sheet. Many stream sinks have been noted around Tisbury and Sutton Mandeville, (Sparrow, 1975, 1976; and Clark and Waters, 2002), as well as a few small phreatic caves and resurgences. None of these have been traced to any resurgence (see above for details of sinks and resurgences observed during the survey).

9.4 Distribution of Solution Features

The distribution of observed solution features is shown on the 1:10 000 scale geological maps and on a small scale Figure 78 below. Many sinkholes have been ploughed in or landscaped so the distribution of solution features marked on the updated geological maps is certain to be an underestimate of the true density. Others have been worked as chalk pits and some ‘dolines’ may simply be small, degraded marl pits. Furthermore, many solution features such as the infilled ‘pipes’ often have no surface expression and cannot be identified by surface mapping.

The Chalk outcrop with the highest density of solution features is in the extreme south of the district, close to the Palaeogene outcrop, and through the central part of the district around the extensive clay- with-flints cover of the Great Ridge. Many of the dolines here have been landscaped or worked as pits. Some minor stream sinks occur along the margin of the overlying Palaeogene strata, but these are only intermittently active during wet weather. Areas of clay-with-flints exhibit high densities of solution features, notably the crest of Great Ridge, on the interfluve between the Lower Avon and the River Wylye, north of the Palaeogene outcrop east of Salisbury and on the interfluves to the north and south of the River Ebble. Here the present land surface is close to the sub-Palaeogene peneplane and both recent active and relict solution hollows derived from a former Palaeogene cover occur. Other outcrops of clay-with-flints are associated with dolines but elsewhere, the land surface has undergone greater dissection and these relict solution hollows have been eroded.

Minor solution features occur widely throughout the area, especially where there is a thin superficial cover, although many of these are likely to have been ploughed in and obliterated or worked as pits. Solution features (‘bourne holes’) can be expected to occur along the middle and upper reaches of the Bourne, Till and Chitterne Brook where significant recharge into the aquifer occurs. These may act as either sinks or springs depending on relative groundwater levels.

Details

The presence of these solution features is dependent on several variables including rock lithology, fracture style, geomorphic setting, geological structure and even anthropomorphic factors. The wide variety in chalk lithology (discussed above), fracture style, geological structure, flint content, porosity and fissure permeability significantly affects the style and degree of karst weathering, both at surface and underground.


Here are two previous posts:

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

Dissolution pipes on a buried chalk surface in the Chiltern Hills (Peter Worsley)

I'm also interested in the link between sarsen stones and chalk solution.

A section showing a solution pit beneath a large embedded sarsen on Fyfield Down -- from a booklet written by Mike Clark and published by NCC in 1976.

The diagram above shows a section cut alongside one of the recumbent large sarsens found in Clatford Bottom. It shows that the sarsen is embedded in a layer of brown flinty loam which extends for about a metre beneath the stone base, with combe rock beneath that, and then with almost a metre of strongly weathered chalk above largely unaltered chalk bedrock. The brown loam is presumably the periglacial material that has moved downslope, maybe carrying or rafting the sarsen along as it accumulated. But what interested Clark and his colleague was the evidence that in ten pits examined under and adjacent to recumbent sarsens, there was increased soil acidity as compared with soils where no sarsens were present. There was also a tendency for solution pipes to occur beneath sarsens of various sizes in the combe rock, penetrating into the weathered chalk beneath. One of these pipes can be seen in the illustration above. Conclusion: the presence of sarsens in one position for many thousands of years leads to enhanced solution in the regolith and rotten chalk beneath. This rotten chalk mat well be exploited for residual flint nodules or indeed for chalk debris if that is required for the building of embankments etc.

With evidence like this in the literature, it is surprising that Gaffney et al, in their new paper, have not considered the possibility that some at least of the pits they have examined in the vicinity of Durrington may actually be extraction pits from which sarsen stones have been taken.

See also:
Locating dissolution features in the Chalk
Matthews, M. C. et al.
Quarterly Journal of Engineering Geology and Hydrogeology(2000),33(2):125
http://dx.doi.org/10.1144/qjegh.33.2.125

A summary of terminology and the locations / origins of key features

The logical conclusion has to be that the "Larkhill group" of hollows or anomalies must be interpreted as natural features related to those shown above, unless powerful evidence to the contrary is provided. There is no such evidence.

Postscript

Here is another interesting article, on the collection of sarsens in the Avebury landscape.

https://eprints.soton.ac.uk/392645/1/Gillings%2520%2526%2520Pollard%2520CAJ%2520final.pdf

There are extraction pits and hollows all over the place in the Avebury district, most of them still identifiable but others infilled and maybe nowadays only traceable through detailed ground surveys.  Interestingly, the authors suggest that large recumbent sarsens, in their "original" positions, may have protected the underlying chalk from solution processes, leacing them perched of slight platforms or pedestals -- but that around the edges of the sarsen stones solution processes may have been enhanced.  This is a fertile field for somebody looking for a project.......