Salisbury Plain: serene landscape but lethal geochemistry

 

Almost all of the textbooks and archaeological papers that devote space to the Stonehenge bluestones make three claims:  (1) that there is no till or any other glacial deposit on Salisbury Plain; (2) that glacial erratics are entirely absent; and (3) that Salisbury Plain was beyond the reach of any known British glacier or ice stream.  

The latter claim was of course made by HH Thomas back in 1923, and has been taken as an article of faith ever since then, even by certain geomorphologists who should know better.  We now know that Thomas misrepresented the evidence by falsely claiming that the outermost ice edge in the Celtic Sea / Bristol Channel arena lay just to the south of the south Pembrokeshire coast, flying in the face of evidence from many of his peers that the ice had on occasions extended much further southwards and eastwards.  We now know that there are abundant traces of glaciation on the coasts of Somerset, Devon and Cornwall -- although evidence of the inland incursion of glacier ice is still hotly debated.  Nonetheless, we know from field research and from modelling studies that it was perfectly possible for glacier ice to have extended well to the east of the chalk escarpment.  Many of these studies have been cited on this blog.

As far as erratics are concerned, it is disingenuous to pretend that they do not exist on Salisbury Plain.  As pointed out by Kellaway, Olwen Williams-Thorpe and many others, they are abundant -- with at least 40 different rock types represented in the bluestone monolith / cobble / packing stone assemblage and in the debitage at Stonehenge and further afield.  Whether these erratic materials can be labelled as "glacial erratics" is another question entirely.  But as I have repeated many times, bluestone shapes and surface characteristics point unequivocally towards glacial entrainment and transport -- and if things look like glacial erratics, that is probably what they are. Why is that such a big deal?

Then we come to the claimed absence of glacial deposits on Salisbury Plain. Well, absence of evidence is not evidence of absence -- and the great majority of the sediments on Salisbury Plain are of course unexplored and unknown.  Even within the Stonenge stone setting, 50% of the surface sediments have never been investigated.  With the "impossibility of glaciation" accepted as an article of faith, we see confirmation bias in almost every study in the literature.  Over and again we see that sediments containing foreign erratic material are immediately dated as younger than the supposed known date of bluestone arrival; it does not seem to have occurred to the establishment faithful that "date of first use" is not at all the same as "date of arrival", and that erratics might well have been lying around in the Stonehenge area for hundreds of thousands of years before people arrived and started messing about with monumental ambitions.

So could there be glacial deposits lying around on Salisbury Plain, undiscovered?  Of course that is a possibility.  We see various clues in the strange and intriguing deposits lumped together under the "clay-with-flints" label.  But then we come to a point that is hardly ever made by archaeologists, geologists or anybody else: namely that the chalk downs of Salisbury Plain and adjacent areas are geochemically lethal, and are perfect places for the efficient degradation and destruction of any scattered patches of till and glaciofluvial deposits that might once have existed. 

If glacial ice did once extend across the chalk downs, what might the  traces of this glaciation have looked like?  Well, there will probably not have been any massive terminal moraines located at ice edges.  Prior to ice arrival near the peak of a glacial episode, we can assume that the ground surface would have been deeply frozen or locked in permafrost.  That points in turn towards cold-based ice, moving slowly and not affecting much basal erosion -- and in turn not depositing vast thicknesses of till and other materials such as we see in parts of Eastern England where chalk bedrock underlies Late Devensian and other till sheets.

So, around 450,000 years ago, if the chronology of ancient glaciations is correct, there may well have been a patchy spread of glacial and glacio-fluvial deposits on the chalklands of Salisbury Plain.  So should they not still be there, available to be mapped and examined?  The short answer to that is "No".  The long answer is that the geochemical environment on Salisbury Plain is far too aggressive -- and even lethal -- for ancient glacial deposits to have survived.  The deposits -- however extensive they might have been -- would have been quite calcareous, as demonstrated by the Devensian Irish Sea till of the west Wales coasts and by the Fremington Till on the coast of Devon.  It must have been composed for the most part of clay and silt deposits dredged from the floor of the Bristol Channel, with other calcareous additions coming (for example)  from the Mendips and from the Black Rock limestone beds around Frome.  Gilbertson and Hawkins, and other workers, have described the till deposits of Somerset and their associations with marine shell fragments, suggesting a shared chemistry.  

So the Salisbury Plain till was probably calcareous -- meaning that its chances of long-term survival were virtually zero.  Because the chalk is highly porous and lacks cementing minerals, rainwater mixed with soil acids percolates straight down, soaking the landscape. Any loose calcareous sediments dropped onto the plain (such as limestone erratics or older shelly sands) are subjected to continuous, downward leaching by acidic water. Without the specific point-source degassing or coastal evaporation seen in Wales to precipitate cements, the carbonate is entirely dissolved and flushed deep into the aquifer, leaving no physical trace.  This aggressive leaching is so intense that even relatively resilient detrital minerals (like apatite) are chemically dissolved and structurally stripped from Salisbury Plain's surficial layers over hundreds of thousands of years.

However, in this broadly very destructive scenario, the chemistry also shows that erratic boulder survivals are not just possible but probable.  Sarsen stones derived from an ancient duricrust layer were eminently survivable, being composed of almost 100% silica.  The "bluestone" lithologies from Pembrokeshire—primarily spotted dolerite (diabase) and various rhyolites/tuffs—are composed of highly resilient silicate minerals like quartz, plagioclase feldspar, and pyroxene. Unlike calcium carbonate, these minerals are resistant to weak carbonic acid.  Even if small carbonate grains dissolve, un-weatherable microscopic minerals do not disappear.  Igneous rocks like the Pembrokeshire igneous bluestones are packed with ultra-durable heavy trace minerals, such as zircon, rutile, tourmaline, and apatite.  Placed on Salisbury Plain, a large dolerite block would be expected to survive hundreds of thousands of years of chemical weathering, whereas a limestone or chalk pebble in a surrounding till matrix would dissolve completely.

In addition to the "chemical hostility" of the Salisbury Plain environment, we have to consider the physical weathering effects of climate change over almost half a million years -- with  multiple oscillations in wetting and drying, heating and cooling.  Chalk absorbs a lot of water.  During past permafrost freeze-thaw cycles, the surface layer of chalk must have shattered into a fine, paste-like slurry.  Any glacial till, erratic boulders, or paleosols caught in this zone would also have been violently churned up by cryoturbation and washed down valleys as mobile gravelly solifluction deposits (historically termed "coombe rock" or "head"). Because these redeposited sediments could not cement together, they remained loose, soft, and completely vulnerable to being wiped away by subsequent fluvial and colluvial processes during warmer episodes.

So the scenario is one of widespread sediment destruction with some survivals.  What of the residues and lag deposits that must remain when calcium carbonate rich rocks are dissolved away?  This is where we return to some of the enigmatic sediments on Salisbury Plain -- and in particular the "clay with flints" that we have discussed on many occasions.  The physical and mineralogical composition of this material strongly proves that it contains a large amount of detritus that did not come from the underlying chalk.  Pure chalk contains very little clay and sand. To produce a one-metre-thick layer of clay-with-flints purely from chalk dissolution, you would have to dissolve hundreds of metres of bedrock. Mineralogical analyses show that much of the matrix clay (like illite and kaolinite) and quartz sand actually comes from reworked, younger overlying formations (like the Paleogene Reading and Thanet Formations) and unknown exotic surficial inputs.  In many areas of the downlands, the clay-with-flints loses its classic red-clay appearance and grades into poorly sorted, unstratified, gravelly deposits packed with sub-angular flints and erratic quartzites. This structural signature is virtually indistinguishable from a highly weathered, decalcified, cryoturbed periglacial "head" or an ancient, deeply rotted till or fluvioglacial accumulation.

Despite the heavy dilution, geologists have occasionally confirmed a persistent presence of exotic "foreign" stones. Round, exotic quartzite pebbles (often historically lumped together as part of the "Northern Drift" or derived from old Triassic Pebble Beds) are well-documented across the downlands. These are classic indicators of far-travelled, non-local glacial, fluvioglacial or fluvial transport.  Over the decades, independent Quaternary geologists (most notably our old friend Geoffrey Kellaway) reported finding small, erratic pebbles of igneous and metamorphic material—including fragments resembling southwestern and Welsh lithologies—in deep solution features and surficial deposits across the wider Wessex region.

Ah -- solution features.  Now we are getting serious, and feel another comment about Parker Pearson's "periglacial stripes" coming on.  Watch this space.........