Clatford Bottom and the sarsens

The widely-favoured theory of sarsen stone formation, by Goudie and others.  This is 

the "periglacial" theory involving the downslope migration of blocks of silcrete.  

The arrow with the words "the ice melts" should really have been labelled "the ground 

ice melts."  Or should it.....??

This is an interesting article from Peter Worsley of Reading University.  Peter and I were contemporaries and met frequently back in the 1960s and 1970s at geomorphology conferences and other events.  I have great respect for his careful work, and it's good to see an article about sarsen sones which is free of wild speculations and assumptions.......

Here is the link:
https://www.researchgate.net/profile/P_Worsley/publication/336003003_Geology_of_the_Clatford_Bottom_catchment_and_its_sarsen_stones_on_the_Marlborough_Downs

Geology of the Clatford Bottom catchment and its sarsen stones on the Marlborough Downs

Peter Worsley
MERCIAN GEOLOGIST 2019 19 (4).p 242-252

Abstract:
The now vanished Palaeogene geology of the Marlborough Downs area can be plausibly reconstructed by extrapolation from the surviving rock record lying immediately to the east. The Lambeth Group succession dominated by the Reading Formation, formerly extended westwards over the Downs. Anastomosing river channels draining from NW to SE created linear belts of sand extending across a clay-rich floodplain and coastal plain. During the Palaeogene/Eocene Thermal Maximum (PETM) global warming event (c.55.5 Ma), groundwater silicification within the sand bodies led to concretion formation (sarsens). Denudational processes led to the sarsens and their Reading Formation host being incorporated into a complex residual deposit, the Clay-with- flints, over Chalk Group bedrock. Following dissection, the Clay-with- flints now occupies the higher chalk interfluves and spurs. During progressive late Cenozoicerosion over at least the last 3 Ma, the present-day relief and its dry valley systems developed under a fluctuating temperate/cold climatic regime. The dominant processes were dissolution of the chalk (this continues today) and fluvial incision, mainly during phases of permafrost development. Following exhumation, the sarsens were lowered as the chalk landscape evolved and later redistributed by solifluction process during repeated cold climate stages.

Most of the article is taken up with a discussion on the origins of the sarsens and the Sarsen stone concentrations of stone streams on valley bottoms -- worth reading since it is clear and concise, with good illustrations.  On the geomorphological context, there are two theories, one involving the breakup of the silcrete crust or cap, the formation of chalk dry valleys and the downslope movement of silcrete slabs and fragments under Ice Age (periglacial) conditions; and the other involving the creation of different types of sarsens depending on environmental conditions at the time of formation, with later resultant slabs and blocks being "let down" onto the landscape without much lateral movement or "migration."  This latter theory seems sensible to me,  since there is no particular reason why sarsen blocks should originally have been more abundant on interfluves.  Indeed, as Worsley explains, the "drainage line" theory might be more attractive since it might account for thicker silcrete crusts on drainage lines or in river valleys, and thinner silcretes on the interfluves -- so although the silcrete blocks might have been let down onto the current landscape by 100m or more, the lateral distribution of silcrete remnants (ie sarsen stones) might not have changed much over 50 million years or so.  

Worsley seems to prefer a composite theory, involving some lateral downslope migration especially at times of cold climate, since he argues that there is no reason to assume that the drainage pattern 55 million years ago looked anything like that of today.  I would argue that it might well have done, since structural lineations, fault lines, etc will have exerted an influence then as now.

Worsley has not got much time for the standard conviction, in archaeological circles, that the Marlborough Downs provided the "quarrying" or extraction locations for all of the big sarsens used at Stonehenge.  He concludes:  " Geological evidence in confirmation of this haulage hypothesis is conspicuous by its absence (John, 2018), although in 2019 the Natural England web site for Fyfield Down has no doubts! A pioneering heavy-mineral study (Howard, 1982) concluded that the Stonehenge sarsen materials differed from sarsen samples from the Marlborough Downs, namely Clatford Bottom and Piggledene. This conclusion has been supported by recent laser scanning data from Stonehenge, which indicate differing chemistries that in turn are likely to reflect source variability. Currently, a project by Nash and Ciborowski at the University of Brighton, using ICP-MS/AES (inductively coupled plasma: mass or atomic emission spectroscopy) analyses seeks to establish geochemical fingerprints of sarsens both at Stonehenge and at potential sources in southern England (including the Marlborough Downs). The jury is still out."

Definitions:

Silcrete:  A terrestrial, geochemical sediment that is formed by low-temperature, near-surface, physico-chemicalprocesses operating within the zone of weathering, in which silica has accumulated in, and/or replaced, a pre-existing soil, sediment, rock or weathering material. Silcretes contain more than 85% silica by weight, with some pure examples consisting of more than 95% silica (Sommer eld, 1983). The term was first introduced by Lamplugh (1902) to describe (in an Irish context) “sporadic masses of ‘grey wether’ type, indurated by a siliceous cement”. (Fr. silcrete)

Clay-with- flints Formation: A residual deposit formed from the dissolution, decalicification and cryoturbation of bedrock strata of the Chalk Group and Palaeogene formations. It is unbedded and heterogeneous, dominated by orange- brown and red-brown sandy clay with abundant nodules and rounded pebbles of flint. Locally it includes bodies of fine to medium grained sand, clayey silt, sandy clay, with beds of well-rounded pebbles and sarsen stones (modified from the BGS online Lexicon). (Fr. Argiles à silex)

Head:  Head is a uniquely British term (some call it archaic as is Drift!), for describing non-sorted and poorly stratified debris mantling hillslopes and partially infilling valley floors. It is the result of solifluction, the slow downslope flow of saturated unfrozen sediments over either dissipating seasonal frost or where the substrate is permafrost, the flow occurs within a thickening active layer. A synonym is gelifluctate. (Fr. depots de couverture) (Comment BSJ: Nowadays this material is generally referred to as slope breccia, stratified, unstratified or pseudo-stratified.)

Coombe deposits:  Compact gravel containing flints and clasts of chalk in a matrix of weathered finely divided chalk-rich and silty material. Frequently found beneath the floors of southern English chalkland dry valleys and where such valleys cut open onto low ground, they form low angle alluvial fans. They reflect a combination of solifluction and fluvial transport, the latter being meltwater deposits derived from either ground ice or snow. Can be crudely mixed or roughly stratified. (Fr. glisements de coombe)

There are interesting discussions relating to the origins of clay-with-flints and head in the Clatford Bottom area, and what impresses me is the sheer lithological variety encountered in exposures.  It is clearly difficult in many locations to decide what label to give to a particular deposit -- and within the "clay-with-flints" category there are brickearths, clay deposits, sandy and gravelly layers and deposits that are really rather similar to tills.  Geoffrey Kellaway made the mistake in 1977 of trying to label ALL of the clay-with-flints as remnants of the Plateau Drift, referring to them as the decalcified remains of a cover of glacial till.  He later revised this opinion, in 1991, but I still wonder whether there has been a very complacent labelling of the "clay-with-flints" as a low-lying blanket of weathered residues derived from overlying beds that have been eroded away over many millions of years.   I would not be at all surprised to find that some of the deposits lumped together under this one label prove to be very old true tills, the last remains of a patchy deposit now largely destroyed.   Has anybody seriously looked for erratics in the deposit?  There would not be many of them, since in tills we find that more than 90% of the stone content can be locally derived -- but the shapes and surface characteristics of many of the Stonehenge bluestones suggest that they are erratics, and they must have been collected from somewhere not too far away.......

 

Exposure of clay-with-flints.  The appearance is not a million miles away from that of a true glacial till.......

Black dots:  locations of silcretes or sarsen stones across southern England.  Many of them are NOT located on the chalklands.... (from a talk by Katy Whitaker).