Tales from prehistory


Microscopes and Monsters has been quiet for a couple of weeks, as I have been on holiday, part of which was spent “off grid” at the Green Man Festival in Wales.  From there, we headed to London for a Proms concert (two music festivals in a week!) via the Cotswolds and the ancient landscapes of Salisbury Plain.

My first visit to Stonehenge was 50 years ago, at which time you could pull off the A303 and wander amongst the columns unconstrained by fences and barriers.   Now the visitors are guided to a visitor centre two kilometres from the site, and offered a shuttle bus after being relieved of £20.  Or, if you prefer, you can walk across Salisbury Plain to the monument.  On a sunny afternoon in August this becomes part of the experience as there are ancient burial mounds (some pre-dating Stonehenge) both alongside the path and dotted around the horizon.   Stonehenge itself gradually rose up ahead of us, and we experienced a little of what the ancients must have felt as they approached Stonehenge along the processional way.

The last time that I was here was a stop off between field work on the nearby River Wylye and a meeting in Reading.   At the time I was engaged with two separate projects concerned with the health of chalk streams, which are characteristic of this part of southern England.   The approach we used elsewhere in the country was to compare what we found in samples we collected with what we expected to find if that site was in a pristine condition.

There was, at the time, a vigorous debate about how this “reference condition” should be defined.   This debate had a theoretical component (epitomised by Brian Moss’ paper in the reference list) but also a more pragmatic element (encapsulated by the other paper).  This was necessary because an ultra-strict, but theoretically sound, approach might not yield enough data from which a robust prediction of the “expected” ecology could be derived.   In essence, we searched out remote regions of the UK where population density was low and agriculture was not intensive and used these to derive our understanding of what to expect in the more densely-populated regions of the country.


Part of Avebury stone circle, Wiltshire, August 2019.  The photograph at the top of the post shows Stonehenge.

This worked quite well (although Brian Moss, predictably, had his own pithy thoughts on the approach).  However,  we simply could not find any sites that fulfilled our criteria of low population density and a low intensity of farming in those parts of lowland Britain where the underlying geology was Jurassic limestone, Cretaceous chalk or another formation that resulted in very hard water.  Our estimates of ecological health in such regions depend, as a result, on extrapolation and judgement rather than evidence.   That is all well and good for an academic journal but, in the case of the River Wylye, Wessex Water were being asked to spend hundreds of thousands of pounds to upgrade sewage works and, rightly, felt that they needed something more in order to explain the consequent price rises to their customers.

The OS maps of the region around my sampling points on the Wylye were dotted with symbols marking ancient monuments (long and round barrows, in particular) and the huge, mysterious religious sites of Stonehenge and Avebury lie just to the east.  Together, they point to continuous occupation of the area for over four thousand years, which means that it is hardly a surprise that we found no sites that met our criteria for a “pristine” stream.   The chalk streams of southern England are famous and rightly regarded as a threatened habitat, but they are not natural.  It is better to think of them as aquatic equivalents to hay meadows or hedgerows: ecologically-rich habitats that have been created by human activity, rather than as a result of “natural” ecological processes.

That means that it we need to diverge from a strict definition of “reference conditions” in order to set a baseline for ecological expectations in such circumstances.  For macrophytes – the larger aquatic plants – there is an expectation that the flora in this baseline state will be rich; however,  this assumption does not work for the microscopic algae in chalk streams.  We also found that river stretches where the macrophytes are thriving and, apparently, healthy, often have diatoms that suggest nutrient enrichment.  That is a puzzle for which we think we may have a solution, and which I will write about in a future post.


Silbury Hill, part of the Avebury World Heritage Site.  It is a Neolithic site whose original purpose is unknown though, to a visitor from north-east England, it looks remarkably like a slag heap.

We use low population density and absence of intensive agriculture as a proxy for “natural” in the uplands but need to treat this assumption with care too.  There might be fewer grand Neolithic monuments in the north of England or Scotland but signs of ancient habitation are there if you care to look (see “More reflections from the dawn of time”).   The moorland where these streams rise is, itself, an artificial habitat, created when early agriculturalists removed the natural tree cover.  Modern streams in these areas are, therefore, exposed to more light than in their primeval states and that will have important consequences for the plant life that lives within them.  They may be the best we have, but are hardly “natural”.

Two factors, both highly pragmatic, brought this debate to a close.   The first was realisation that, whatever the rights and wrongs of purist versus practical standpoints, most of our rivers are very degraded and alterations in the approach used to define the “expected” condition would be unlikely to change this broad scale picture.   About sixty-five per cent of our rivers fail to achieve good ecological status despite the flaws in the reference concept.  The second factor was simply that, since the financial crisis in the 2008-2010, the UK environment agencies have had too few resources to improve the reference concept.   As any such “improvement” will almost certainly make the true state of UK rivers look even worse than it does at the moment, a more cynical argument is that few of the bureaucrats involved in the process have any great enthusiasm for the task anyway.


Moss, B. (2008).  The Water Framework Directive: total environment or political compromise.  Science of the Total Environment400: 32-41.

Pardo, I., Gómez-Rodríguez, C., Wasson, J.G., Owen, R., van de Bund, W., Kelly, M., Bennett, C., Birk, S., Buffagni, A., Erba, S., Mengin, N., Murray-Bligh, J. & Ofenböeck, G. (2012).  The European reference condition concept: A scientific and technical approach to identify minimally-impacted river ecosystems.  Science of the Total Environment420: 33-42.