Why a landslip is unlikely at the present day White Horse, but how it might have happened with enough rainfall in the past.
(Parts one and three are here and here).
In the first part of this multipost I explained why I originally thought that the Uffington White Horse might be a landslip. However, is a landslip honestly a realistic idea? As far as I can tell it’s never been suggested before for the White Horse, and perhaps there’s a very good reason for that. So in this post I want both to highlight the problems involved in postulating a landslip at Uffington, and to see whether those problems are insurmountable or not.
As a general rule, landslips are encouraged by three factors: increasing slope steepness, weakened or slippery subsoil, and increasing ground saturation by water. I’ll deal with each of these factors in turn:
Slope steepness
Slope steepness encourages landslip simply due to the effects of gravity.
Our horse is located, appropriately, on the edge of White Horse Hill, the highest point in the Berkshire Downs. It lies just above a break in slope at the head of a small but deep, NW facing valley, or coombe, called The Manger. This valley cuts the north facing chalk escarpment of the Berkshire Downs on the south side of the Thames Valley.
A second, similar coombe (here called “Britchcombe”) trends NE just to the east of The Manger. The heads of these two valleys have probably the steepest slopes in all of the Berkshire Downs, reaching almost 40° in places.
Now breaks in slope on chalk escarpments are normally quite gentle and smooth as a result of a process known as ‘soil creep’. This causes the surface soil to inch down slope as the soil expands and contracts in alternating wet and dry conditions. What makes the breaks in slope around The Manger surprising is just how sharp they are despite the effects of soil creep, and this in itself is curious.
The subsoil
Landslips tend to be more likely on loose soils or in weak, broken rocks. They are much more likely when these overlie clay layers which allow the layers above to slide as the clay can act as a slip plane.
The Uffington Horse lies in a landscape of thin ‘rendzina’ soil overlying chalk. Chalk is a weak rock with many fissures and cavities. Nevertheless, historically this has not produced great landslips, except on the coast where the undercutting of chalk cliffs leads to their failure, for example on the two sides of the English Channel/Manche, at Dover and Wissant.
Only in places where there is a sufficiently shallow, slippery clay layer underlying the chalk is it easy for landslips to occur inland. This is not the case below the White Horse. The Jurassic clay layers beneath it are too deep to have allowed them to encourage slippage in the chalk. This is an important negative point against the possibility of a landslip as a cause of the formation of the White Horse.
Water saturation
Water saturation can decrease the friction in soil or rock, allowing things to slide better by filling the spaces in the rock or soil with water and pushing the grains apart. In this, chalk is not the best thing to produce a landslip.
Chalk, a kind of limestone, is notorious for its high permeability, a result of the interconnected holes between individual grains, the numerous fissures where the chalk has been broken by gentle folding in the distant past, and the cavities that form in it when rain dissolves the carbonate that makes it up.
This means that it’s actually quite hard to build up enough water pressure to cause slope failure in chalk. Even though heavy rainfall has been implicated in increasing the chances of failure in chalk cliffs in France, it would take a lot more water to cause failure in the more gentle slopes of the Berkshire Downs.
Where does this leave us?
Under present conditions, the evidence is against a major landslip at Uffington. However, at this point I need to launch into a digression, because we need to explain how The Manger, the valley at the top of which Uffington White Horse sits, formed.
(Please feel free to skip this section if your short time on this world is too precious.)
A small digression: What formed the Manger?
The Manger and Britchcombe are both examples of “escarpment coombes”, a type of valley which cuts many chalk escarpments in southern England.
Valleys normally form through the action of liquid water or ice. The escarpment coombes of the Berkshire Downs have never contained glaciers, so liquid water must have been important in their formation. However, they currently contain no rivers, and even the spring line is at 110m above sea level, tens of metres below most of the valley floor. This lack of water is in part due to the high permeability of both the surface soil and of the underlying chalk.
In the past these valleys cannot have been so dry. In order to form, either the water table must have been much higher, or the ground was less permeable due, for example, to its being frozen (permafrost) during cold phases. For each of these scenarios there are two further possibilities: either the water flowed along the entire length of the valley as a river, cutting down as it went, or it emerged as a spring at the base of the escarpment, undercutting the rocks above (‘groundwater-sapping’) leading to localised collapse of the escarpment and cutting back of a valley.
This has led essentially to four different views of coombe formation:
1) Rivers cutting down when the water table was higher (perhaps in the Tertiary period).
2) Seasonal rivers (e.g. spring meltwater resulting from snow melt) cutting down when the ground was frozen (during glacial phases of the Quaternary).
3) Springs eroding and undercutting the escarpment back when the water table was higher (perhaps Tertiary again).
4) Springs eroding and undercutting the escarpment when thawing of permafrost released meltwater from the ground at the end of glacial phases (Quaternary again).
For our purposes it is safe to rule out both 1 and 2. The Manger and its adjacent coombe are cut back to near the top of the White Horse Hill, and the hilltop would never have produced enough surface water to have allowed the formation of two large river valleys.
This leaves us with spring sapping and headward erosion of the escarpment, either due to high water tables or the melting of permafrost. Both involve landslips in the formation of the valleys. If this is correct then landslips in both The Manger and Britchcombe must have happened in the past.
Return to rainfall
I’ve already said that a landslip in The Manger is unlikely under present conditions, but the section above suggests that they have happened here in the distant past. But could conditions have been different enough at the time of the formation of the White Horse, back in the late Bronze or early Iron Age?
Now the slope at this time wasn’t significantly steeper, the rocks were no different, and the land certainly wasn’t frozen due to glacial conditions. However, could the weather have been significantly wetter then – say, wet enough raise the water table to the top of the valley, or to temporarily saturate the chalk and cause it to fail? The answer is a possible, though not a definite, ‘yes’.
The Late Bronze Age and Early Iron Age climatic deterioration, more properly called the 2.8 kA BP event or the ‘Homeric Minimum’, occurred between around 850 and 550 BC when NW Europe experienced a ‘particularly wet and cold’ phase, unmatched in previous millennia, associated with very low Sun activity (the link is to do with protecting the Earth from cosmic rays, apparently, not the cooling of the Sun). River catchment data for the British lowlands also back this up, suggesting a major peak in flooding events between 850 and 750 BC. This may be a suitable event to have caused either the saturation of the ground or the raised water tables that are needed for a landslip.
The chances of such an event causing a landslip at Uffington are increased by human activity at the site. For much of the Holocene the slopes of chalk coombes were stabilised by the growth of scrub and trees. The removal of this tree cover by farmers from the Neolithic onward, coupled with the increasingly intensive use of the land for agriculture from the Bronze Age, is now thought to have caused considerable erosion of the chalk landscape. This can be seen in the accumulation of chalk-rich sediment (known as ‘colluvium’) on the slopes and in the bottom of chalk valleys from the Bronze Age until Roman times and beyond.
Whether this is enough to have caused a landslip in the Manger I don’t know, but the dates of the White Horse (1380 and 600 BC – 68% confidence) and the climatic deterioration overlap and are at least suggestive. Whatever, there is also a third piece of evidence to consider; the excavations of the White Horse and the Manger by the Oxford Archaeology Unit in 1990 and 1994. I’ll discuss these is in the third part of this post.
References
Ballantyne, C.K. & Harris, C. 1995 The Periglaciation of Great Britain, Cambridge, pp342 (p152-155).
Background to views on coombe formation.
Brown, A.G. 2008 The Bronze Age climate and environment of Britain, Bronze Age Review 1, 7-22.
Discussion of high rainfall in Britain during the early 1st millennium BC.
Cranfield University 2020 The Soils Guide.
This site details the kinds of soils found on the Berkshire Downs, including Upton 1 soils which are probably those underlying the White Horse.
Duperret, A. et al. 2004 Coastal chalk cliff instability in NW France: role of lithology, fracture pattern and rainfall, Geol. Soc. Lond. Engineering Geology Spec. Pub. 20, 33-55.
Laurenz, L., Ludecke, H.J. & Luning, S. 2019 Influence of solar activity on changes on European rainfall, J. Atmospheric & Solar-Terrestrial Phys. 185, 29-42.
Highlights a Grand Solar Minimum between approximately 810 BC and 610 BC.
Martin-Puertas, C. et al. 2012 Regional atmospheric circulation shifts induced by a grand solar minimum, Nature Geoscience 5, 397-401.
Small, R.J. 1964 The Escarpment Dry Valleys of the Wiltshire Chalk, Trans. Inst. Brit. Geog. 34, p33-52.
USGS Earth Explorer website.
Source for SRTM (Shuttle Radar Topography Mission) 1″ Digital Terrain Elevation Data of the Berkshire Downs.
Wilkinson, K. 2009 Regional Review of Geoarchaeology in the Southern Region: Colluvium, English Heritage Research Dept. Report Series 3/2009, pp48.
Discussion of chalk land use and soil erosion.
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