Geoarchaeological Priorities in the West Midlands
David JordanTerra Nova Ltd
Introduction – Geoarchaeology and Archaeology. What does the West Midlands need?
This paper is based on that given at the third West Midlands Regional Research Framework meeting on 30 October 2002. It considers the geoarchaeology of the whole of the West Midlands from the Palaeolithic to the present day. With such a wide focus it is bound to consider more general than specific issues but I hope it will stimulate debate.
Geoarchaeology is not yet a mature discipline and we are not sure what it should include and exclude. Those of us who are called Geoarchaeologists tend to be skilled in soil science, geomorphology and recent, shallow geology and our attention has often focussed on a few techniques – in the past Phosphate analysis, now perhaps soil micromorphology. Yet geoarchaeology could include all the earth sciences applied to archaeology and there is a particularly good reason to take this broader view.
Commercial practice, from which we now get most of our field data, tends to split archaeology into separate disciplines covered by different specialists. This makes sense since we can’t all be good at everything. But specialisation can go too far. Consider, for example, the case of geophysical survey, an approach which has become such a standardised “black box” that briefs do not require (and training does not equip) surveyors to know anything at all about soils and rocks. It is presumed that it is enough to have the right technical kit, and know how to use it, to produce pictures of buried remains. Yet very many geophysical surveys generate information about the soil and geological environment of the site as well as the location of remains - valuable information which can only be interpreted by those with broader earth science skills and which, for the lack of such skills, is now largely discarded unused. For this reason I would argue that archaeological geophysicists should be expected to understand a lot about soils and rocks before they are let loose on sites. We look to the County Archaeological officers and the IFA to set higher standards for practitioners.
But the problem goes deeper. I often observe, on site or during the geoarchaeology courses I run, that excavators are confused about fundamental issues of how sites form. There are the common confusions, reflected in so many site reports, between a soil and a sediment, a horizon and a stratum – terms with precise meanings which can help us to describe our observations and present reasoned interpretations of our excavations. Stratigraphy – the layered result of human activity which created a site – is often confused with properties inherited from the soil which preceded the site (the parent material) and the processes which have changed the colours and textures of the strata since the site was abandoned. This matters. If we don’t get these things right then our records and interpretations are unreliable and, since many excavators are aware that they don’t fully understand what they see, they compensate by being more tentative in interpretation than they might be and producing less solid archaeological evidence as a result.
Thus the first geoarchaeological priority for the West Midlands is training. Let’s make sure our excavators know enough about soils, sediments, rocks and natural landscapes to get the best from their excavations. It is striking how little such training is included in most archaeology degrees – yet, until we expect excavators to know about soil before they start digging through the stuff, the situation will not change. My own company, Terra Nova Ltd, helps to improve the situation by running training courses in archaeological sediments and soils for excavation staff.
There have been very few geoarchaeological studies in the West Midlands except, taking my broader definition, for geophysical surveys – although even these are sparse when compared with some other parts of the country. This lack of studies is despite the influential presence of Susan Limbrey and the late Frank Shotton at Birmingham University. Key studies, such as Shotton’s on the Severn and Richard Macphail’s at Deansway, Worcester, did not stimulate much further work at the time, although more recent studies have raised the profile of geoarchaeology so that it is now routinely written into briefs for excavations in Hereford and Worcester and, more rarely, elsewhere.
What is geoarchaeology and what can it do for us?
What is geoarchaeology? Broadly it comes down to:
1Interpreting the physical history of landscapes – how they have influenced and been influenced by past societies. Is this alluvium here in this valley bottom because people chopped the trees down and ploughed that hillside there? Could people have cultivated this soil with Bronze-age technology and what evidence might survive to tell us if they did so? Do these buried sediments and soils tell us that this buried ground surface was regularly flooded by the sea or dry enough to live on?
2Connecting sites to their landscapes – is this layer hillwash? What does it tell me about what was going on around my site? Might there be archaeology beneath it? Should I keep digging or can I go home?
3Interpreting strata – why does this stratum look and feel as it does? What does that tell me about how it formed? Where does the material within it come from? How was it laid down? What has happened to change it since? What does the soil buried beneath my site tell me about the landscape before the site was used?
Geoarchaeology is, evidently, relevant to site excavation and prospection – geoarchaeology within excavations is, in a sense, an extension of what the excavator already does. But geoarchaeology is also a palaeoenvironmental science and closely related to other such sciences. It can provide direct evidence of past environments (for example, the nature of an alluvial sediment can tell us the rate at which an ancient river flowed) but it has particular value in telling us about how other kinds of evidence were deposited – and if we can trust them. A vertical sequence of samples from a deposit may produce a lovely pollen diagram but if the geoarchaeological study of that same deposit shows it to represent, for example, a series of floods and dry periods, then we have to interpret the sequence as representing not so much changes in environment as changes in source and deposition – and to be more cautious. In many cases a study of pollen and other such fossils is incomplete without a study of the material in which they are found.
Geoarchaeology can tell us useful things about other kinds of archaeological evidence. Has my survey found nothing because there was nothing to find or because any sites are buried or eroded away? Where will I find soils which produce cropmarks every year and where will they do so only once a decade? Where are soils magnetically responsive – and produce good geophysical results – and where can we predict that they will be poor? How does this bias our SMRs and change the way we design commercial excavation briefs? Geoarchaeology can help us to find answers.
What is special about the geoarchaeology of the West Midlands?
The effects of geology and soil
The solid geology of the region has determined much of the way in which soils have formed. That, in turn, determines how archaeological features are preserved and appear in excavation. A quick look at the geological and soil maps shows that much of the area is underlain by clayey and calcareous rocks which have produced poorly drained soils called surface-water gleys (or stagnogleys – a gley is a soil which is seasonally waterlogged and ‘surface-water’ or ‘stagno-’ indicates that the waterlogging is due to poor drainage and not to rising groundwater). The main exception is an area to the south-west, west of the Severn and south of Telford, where a wedge of slightly coarser Silurian and Devonian rocks have produced better drained brown earth soils. Smaller areas of higher land, such as the Long Mynd, are covered by more acid soils such as brown podzols, while lowland sandy rocks have produced brown sand soils. Finally, the active river floodplains contain alluvial gley soils, which are waterlogged in winter by rising groundwater.
The differences between the surface-water gley and brown earth soils are mainly due to differences in the rock and boulder-clay on which they have formed – their parent materials. The acid soils of the highest hills and the alluvial gleys of the floodplains, however, have formed largely because of their location and the colours and textures of archaeological strata within them will have been partly determined by the same effects. These differences in the ways sites form are significant but are rarely made explicit by excavators. The sides of a ditch in clay erode quite differently to those of a ditch in a sandy soil and so ditch profiles and erosion histories must be interpreted differently. Despite the important efforts made at Butzer Farm, Overton Down and elsewhere, we do not have enough experimental models to tell us how to interpret such effects. Thus, we should consider whether the West Midlands should have some simple experimental earthworks of its own and gain whatever information we can from fortuitous observations of ditches cut or banks built up for other purposes.
Some decay processes which are common in West Midland soils have a significant effect on archaeological sites. The gradual downward movement of fine matter through the soil (a process called lessivage) is particularly destructive of West Midland sites in sandy and silty soils since the colour differences which define the uppermost 50cm or more of cut features disappear and only the lower cuts remain. Whole sites are lost – or almost lost - in this way although excavators are often unaware of the process and mystified by its results.
Another process, called redoximporphism (or redox), is active in clayey or wet sites, such as those in the larger river valleys. Here the fluctuating soil water levels cause strongly coloured mottles to form which can obscure much of the stratigraphy. Changes in drainage and agricultural regime, due to the coincidence of climate change and reforms of agriculture, may well coincide, over the coming decades, to alter the nature and rate of such destructive soil forming processes. These effects may be slow and insidious but they are often no less destructive than quarrying or road building, and they should be explicitly considered in surveys of archaeological risks. A baseline survey of the geoarchaeological state of sites in the West Midlands, a survey of sites at particular risk and a long term programme to monitor soil-formation change in the area may prove a good investment.
A problem specific to West Midland urban sites
Many urban sites in the West Midlands are formed from both sandy, acid, well-drained soils and clayey, calcareous, wet soils since many towns in the West Midlands are underlain by both these parent materials very close together. This leads to complex variations in the way in which sites have formed. This close proximity occurs because many towns – Hereford, Worcester, Stafford, Droitwich and others - occupy fluvioglacial sand and gravel terraces on strategic heights overlooking major rivers. The fluvioglacial deposits tend to be acid sands and gravels while the bedrock close by tends to be relatively calcareous mudstone or marl. The result of this proximity of contrasting parent materials is that strata vary greatly across and between sites, not only because of differences between the human processes which formed them but also because of the effects of parent materials and post-depositional change. In these cases in particular, therefore, excavators need to be aware of the ways in which local differences in parent materials and post-depositional effects will have superimposed colour and texture changes which are largely unrelated to the stratigraphy of a site – and must be interpreted quite differently.
Periglacial structures and archaeology in the West Midlands
Most of the West Midlands lay outside the last, Devensian, ice sheet. As a result many of the soils contain an abundance of periglacial features formed by long periods of freezing and thawing cycles. Some, such as ice-wedge casts, are usually fairly obvious to the experienced excavator (although the less experienced need to be given some training in their recognition if they are not to confuse them with ditches). Others, such as periglacial involutions, can be harder to spot – but it is important for excavators to recognise them and their effects since they have the potential to greatly affect the preservation of sites and can cause some sites to be misinterpreted. Involutions, in particular, cause problems since they bring about post-depositional changes in the stratigraphy above them which can greatly complicate the interpretation of sites.
Redox effects in the archaeological strata of the West Midlands
Many West Midland valley fills consist of gravel overlain by Holocene alluvium. The gravel units act as aquifers which alternately drain and flood the soils above. This constant wetting and drying changes the colours and textures of the soils – and of any archaeological deposits contained within them. It is, as a result, extremely difficult to identify the cuts of features in many of the West Midland valley alluvia. It is important to recognise these effects in planning the recording of archaeological deposits. To do this explicitly we have introduced recording forms for the Lugg Valley and elsewhere that make the effects of post-depositional change clearer. Such forms, which explicitly recognise that archaeological strata are the result of parent material, stratigraphy and post-depositional change, may be usefully applied to many sites and it would be helpful if the recording systems applied to all excavations are designed specifically with the problems brought about by the local soils in mind. Terra Nova Ltd can help with the specific design of such forms.
To summarise: we know that there are certain types of geology and soil in the West Midlands which cause feature and post-depositional change to archaeological deposits. Excavators working in the region would therefore do well to study these processes, and we should all attempt to research then in greater depth.
The importance of landscape context
To understand a site properly we need to understand its landscape context – a need for which the implementation of PPG16 often does not allow.
Fortunately, many sites lie close to others, previously excavated, and as a result, in my own practice, we have been able to build up a clearer picture of the way sites form in cities like Hereford and Worcester, and areas of quarrying and development, because we have examined the deposits found in many adjacent sites.
By far the best way to understand sites in the context of their landscape, however, is to study the pipe-trenches excavated by bodies such as Transco. A Terra Nova Ltd project to study the geoarchaeology revealed by pipe trenches through the Valley of Evesham taught us more about that landscape in two months than could have been gained through decades of excavation studies. We learnt about sites along the pipeline through normal excavation but we also had the chance to learn – at the same time - about the colluvium which covered them, the lynchets that trapped that colluvium, the pollen from the plants which grew on those lynchets, the soil eroded from the Cotswold headwaters and many other related kinds of evidence. We could thus learn about the history of land-use over hundreds of square miles and thousands of years in a single, unified study, giving the evidence of the pipe-trench a significance far beyond that of any individual excavation. Moreover, we learnt about how the soil and rock underlying the archaeological sites had influenced the survival of the stratigraphy above – and, because of the pipe-trench sections, we were able to understood why such changes had or had not occurred.
In fact we learnt so much that we found it hard to assess the potential of all the information we recovered. Moreover there was more than just potential for research. Pipe-trenches also form the perfect teaching tool since they expose many different kinds of sites and soils across a variety of landscapes and can thus help us to show the next generation of archaeologists how sites have formed and are exposed in their landscape context.
We therefore recommend that every opportunity is taken to promote geoarchaeological and palaeoenvironmental studies of pipe-trenches wherever possible since they seem to us to be an exceptionally important resource. We observe that far too little has yet been made of this potential.
Responding to regional priorities
So, how can geoarchaeology help to address these regional priorities? There are many ways in which it can help, including: