An Introduction to Peatland Archaeology and Palaeoenvironments

By Benjamin R. Gearey and Henry P. Chapman

Peatlands are regarded as having exceptional archaeological value, due to the fact the waterlogged conditions of these wetlands can preserve organic remains that are almost entirely lost from the majority of dryland contexts. This is certainly true, although the remarkable preservation of sites and artifacts is just one aspect of their archaeological importance. This book provides an accessible introduction to the ecology and formation processes of peatlands, and to the different archaeological and palaeoenvironmental techniques that have been developed and adapted for the study of these environments. It provides an outline of the major themes and methods and as a guide to other more detailed and technical literature concerning peatland archaeology. The book is aimed at undergraduate and postgraduate students in archaeology, earth sciences and cognate disciplines, but will be useful to professional archaeologists who are looking to develop their expertise in this field. Whilst the assumption is that the reader has little knowledge of peatlands, a general archaeological background is necessary, including some knowledge of techniques and approaches.

• Language: English
• Publisher: Oxbow Books
• Release date: Dec 2, 2022
• ISBN: 9781789257564

Book preview

Introduction

Peatlands are regarded as having exceptional archaeological value, due to the fact that the waterlogged conditions of these environments, like other wetlands, can preserve organic remains that are almost entirely lost from the majority of dryland contexts (e.g. Coles and Coles 1986). This is true, although the remarkable preservation of sites and artefacts is just one aspect of their archaeological importance. This book is an introduction to these issues, written to provide an outline of the major themes and methods and as a guide to other more detailed and technical literature concerning peatland archaeology. It is aimed at undergraduate and postgraduate students in archaeology, earth sciences and cognate disciplines, but we hope it will also be useful to professional archaeologists who are looking to develop their knowledge and skills in this field. Whilst the assumption is that the reader has little knowledge of peatlands, a general archaeological background is necessary, including a foundation in methods, techniques and approaches.

Figure 0.1 : The Ballachulish ‘goddess’: a re-crafting of the Iron Age wooden figure discovered in the Scottish peatland of the same name (see Chapters 3 and 7). Carved by Mark Griffiths. Photograph: Brian MacDomhnaill/Pallasboy Project

There are a number of introductory textbooks, including Mennotti and O’Sullivan (2012) and Mennotti (2012), which provide excellent and accessible coverage of wetland archaeology in its broadest sense. Both these books cover the full range of wetland environments, including lakes, peatlands, intertidal and submerged sites and landscapes. Whilst a broad definition of ‘wetland archaeology’ allows for the inclusion of a range of remarkable sites, landscapes, associated methods and techniques, it does mean that the scope of the subject can appear slightly bewildering to the student or even professional seeking to learn underlying principles, methods and approaches. Other texts are also available as introductions to related studies such as alluvial geoarchaeology (e.g. Brown 1997).

To date, there has been no attempt to outline the practical developments and applied techniques that have arisen from the archaeological investigation of peatlands. This book aims to fill that gap, in particular to provide an introduction to the ecology and formation processes of peatlands, and to the different archaeological and palaeoenvironmental techniques that have been developed and adapted for the study of these environments.

Many of the themes and issues involved, of course, overlap with other archaeological approaches to wetland and often also dryland landscapes. In particular, peat and organic sediments are found on other wetland sites, including intertidal, alluvial and lacustrine environments. However, taking our starting point from a broad ecological definition of peatland allows us to be clear in terms of the landscapes and thus sites that will be included in this book.

This permits a practical and, we hope, accessible introduction to the ecological, palaeoecological and archaeological themes that are the foundations of the method and theory of peatland archaeology. Once a basic understanding is attained, the acolyte can proceed with knowledge that should assist in closer readings of the original case study publications and other sources. In addition, this should provide access to the study of wetland archaeology in its broadest definition.

A basic understanding of physical geography, palaeoecology or environmental archaeology would also be an advantage but is not a prerequisite. Also, this book is not intended as an exhaustive geographical or disciplinary review or technical discussion of the subject. To this end, our case studies in part reflect the research focus of the authors and contributors (the United Kingdom, Ireland and the Netherlands).

Layout of this book

Chapter 1 outlines key peatland concepts; understanding the formation processes and ecology of peatland systems. An in-depth knowledge of the rather complex environmental factors and controls on peat and peatland ecosystems is not required, but a basic understanding is an essential prerequisite to archaeological interpretation. This will become clear in the case study chapters, when we consider the relationship between pattern and character of peatland growth and the archaeological record.

Chapter 2 provides an introduction and brief historical overview of the development of peatland archaeology.

Chapter 3 considers the fundamentals and practice of peatland archaeology in the context of the previous chapter, discussing approaches to the identification, investigation and analyses of peatland sites and deposits.

Chapter 4 outlines palaeoenvironmental methods and approaches. As will become clear, both the palaeoenvironmental record and robust chronologies are a crucial component for reconstructing the development of peatland landscapes and for contextualising archaeological sites and finds. Again, this does not necessitate an in-depth scientific knowledge of palaeoecological methods, rather a general appreciation of the different forms of information they can provide.

Chapter 5 presents a series of case studies, describing the work of selected peatland archaeological projects and studies in terms of the background to these investigations, approaches and techniques adopted, knowledge generated, innovations as well as lessons learnt.

Chapter 6 summarises interpretative themes and methodological issues, arising from the case studies in chapter five.

Chapter 7 considers archaeological study in the context of issues surrounding the management and conservation of peatland landscapes in the present day. This chapter also discusses the particular challenges associated with the heritage management and promotion of peatland archaeological sites and reflects on the future for peatland archaeology.

A note on language and dating conventions

As mentioned above, any discussion of peat and peatlands inevitably includes technical language; whilst this will be kept to a minimum, it is not possible to avoid utilising certain terms and words. Chronology and time are a key element of archaeology and palaeoenvironmental research and again different conventions can be used (BC, AD, BP etc.). In this book, unless otherwise stated, all dates are given as calibrated years BCE and CE (BC and AD). The chronological data and associated interpretations have been drawn directly from the case study publications. Unless otherwise stated, no attempt has been made to assess the chronological robustness or precision of sequences of dates, such as those associated with palaeoenvironmental records.

1. An introduction to formation processes and ecology of peatlands

Introduction

Peatland archaeology is part of the broader field of ‘wetland archaeology’, which can be obviously, if perhaps unhelpfully, defined as the archaeological investigation of wetland environments. However, this is where the obvious ends! One of the problems of wetland archaeology, at least as far as initial introductions to this subject, is that definitions of ‘wetland’ within archaeology, biology, ecology and related disciplines can be very broad (see Table 1.1). For example, the definition of ‘wetland’ by the Ramsar Convention (an international framework for the conservation and wise use of wetlands and their resources) is:

areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water, the depth of which at low tide does not exceed six metres. The Ramsar Convention on Wetlands of International Importance Especially as Waterfowl Habitats 1971. (Named after the Iranian city where this convention was signed).

Although this convention was not formulated with archaeology especially in mind, there has been a tendency (see Foreword) for ‘Wetland Archaeology’ to include a similarly wide range of environments. In this book we take a more focused approach: the archaeology of peatlands, ecologically speaking, an important and relatively clearly definable subset of wetlands. There are various definitions and characterisation of peatlands, some of which are rather vague and imprecise (see Wheeler and Proctor 2000). We follow a broad categorisation into fen, raised mires/bogs and blanket bogs/mires (Table 1.1).

Peatlands occupy an estimated 4 million km² (c. 3%) of the Earth’s land surface (Minasny et al. 2019) and are found on every continent except Antarctica. Europe contains around 1 million km² of peatland, around 10% of the total land area, most of which can be defined as fen (c. 600,000 km²) and the remainder as mire/bog (Tanneberger et al. 2021). As we will see below, peatlands have been reduced considerably in extent by human activity and much of the surviving resource has been damaged to varying extents. Despite the extensive global distribution of these environments, archaeological research has focussed on areas where drainage, agriculture and peat extraction has led to chance discoveries of sites and artefacts: predominantly in northern Europe, and in particular those countries where disturbance has been most intensive: Ireland, the UK and the Netherlands.

Table 1.1: Summary of wetland and peatland terminology. The final two terms will not be used in this book. Based in part on Charman (2001)

There are vast areas of peatland elsewhere in the world where our knowledge of the archaeological resource is very sparse in comparison to the total area (e.g. Canada, Russia, although what has been found in the latter country is fairly spectacular, see Chapter 2). Peatland archaeology is for the main part, distinctly European. As we will see, there are clear ‘hot spots’ within different countries, as well as areas of extensive bog for which we lack archaeological information almost entirely.

Although intertidal peatlands are important for archaeological and palaeoecological studies (e.g. Bell 2013), our focus will be on freshwater peatlands and we will not consider intertidal environments, for which see Wickham-Jones (2018). This requires that we adopt a slightly artificial definition in terms of formation processes, as many current intertidal and coastal peatlands, originally formed as freshwater systems during periods of lower relative sea level (see below).

The archaeological sites selected as case studies for Chapter 5 of this book are located in the so-called ‘Atlantic Bog’ region of western Europe. They have previously been published in full and discussed, sometimes at length, in various other outputs. However, the excavation reports are often very detailed, frequently technical and sometimes difficult not only for the undergraduate archaeology student, but also the professional archaeologist to digest! We attempt to summarise the key approaches and results of these studies. Similarly, there are a wealth of excellent and very detailed publications concerning peatland ecology, biology and geography (e.g. Charman 2002). These are generally not written with the budding peatland archaeologist in mind. To this end we outline the key aspects of peatland ecology, growth and development to provide a foundation for informed understanding and further investigation.

We will consider how peatlands connect the wider discipline of archaeology to contemporary concerns and action around anthropogenic destruction of ecosystems, and the current climate crisis. Drainage, agriculture, peat-cutting, afforestation have impacted on peatlands to varying extents, and the survival of archaeological sites and deposits in situ is of course linked directly to these processes. However, recent years have seen a shift in the perception and management of these environments, amid the recognition that healthy peatlands are important for biodiversity, hydrology and as net ‘carbon sinks’ with the potential to mitigate global heating. Peatlands are directly threatened by global warming, in which increased global temperatures reduce their capability to maintain CO2 sequestration, enhanced exposure to drying out and hence ecosystem function collapse (Swindles et al. 2019).

We discuss these issues in terms of the loss and damage of peatland archaeological sites and deposits, and the future protection and communication of the resource (see Chapter 7). The drainage and reclamation of peatlands is also of historical and archaeological interest but we will touch on this only briefly. Recent studies have included comprehensive discussion of late Holocene (medieval period onwards) ‘improvement’ and exploitation of peatlands (see Rotherham 2020).

Overview: peatland archaeology

Large and spectacular, small and mundane. The exquisite and the everyday. Vessels and vehicles, cauldrons, weapons, agricultural tools, decorative fittings, animals and instruments and craft objects, food and drink. (Giles 2020, 154)

The above quote summarises in three short sentences, the astonishing range and character of archaeological finds from European peatlands. As Earwood (1993) also observed, the list of such artefacts is bewildering: spades, hoes, forks, ox and horse yokes, deer traps, bowls, tubs, boxes and wooden containers; mundane objects such as shoes, basketry, through to rare and valuable items, for example the remarkable late 6th century CE wooden Crozier from Lemanaghan Bog, Co. Offaly, Ireland (O’Carroll 2003) and the Iron Age Pallasboy ‘vessel’ from Toar Bog, Co. Meath, Ireland (Gearey et al. 2019). Fontijn (2020, 120) describes such objects as ‘exquisite’; ‘conjuring the notion of exceptional beauty or intensity’. Whilst some ‘mundane’ finds (shoes, spades…) might well reflect accidental loss or casual disposal, these ‘everyday treasures’ (Giles 2020, 120) and the other ‘exquisite’ items were most certainly deliberately deposited in peatlands for reasons that remain the subject of considerable debate and discussion.

Archaeological finds and sites date from the Mesolithic through to the early Modern period, although certain periods are better represented than others. The later Bronze Age, Iron Age and early Roman periods in northwest Europe saw activity in peatlands and ‘bog deposition’ of valuable and sometimes unusual artefacts (e.g. Becker 2013). Many discoveries were made during agricultural improvement, drainage and cutting (milling) of peatlands, often during the early modern period (Chapter 2), and represent chance finds, generally shorn of archaeological context or investigated in any detail at the time. Some artefacts were lost or kept by the finders: the Late Bronze Age Dowris Hoard, discovered by farm labourers in the early 19th century in Birr, Co. Offaly, Ireland, was depleted in this way. In fact, although often described as a ‘bog find’, the Dowris Hoard was originally deposited within a shallow pool or lake, subsequently overgrown by peat via the process known as ‘infilling’ or ‘hydroseral succession’ (see below). Understanding context is a key challenge and peatland archaeology is no different.

In the case of organic artefacts discovered during the late 19th and early 20th centuries, once removed from their preservation environment, many dried out (for example, The Iron Age Ballachulish ‘goddess’ from Scotland; Chapter 2) and partially or fully disintegrated. Museums such as the National Museum of Ireland also contain effectively unprovenanced ‘bog discoveries’, whilst others exist only as ‘paper finds’, recorded but subsequently lost or destroyed. Most organic archaeological material is extremely susceptible to degradation once removed from anoxic waterlogged conditions.

This presents a significant problem for storage and long term conservation of material recovered during archaeological excavations, which we consider towards the end of the book in Chapter 7. Whilst the preservation of wood is a defining feature, stone built monuments are known: for example the White Horse cist, Dartmoor, southwest England (see Chapter 2 – this site also included some astonishing organic preservation) and the Copney Stone Circles, Northern Ireland, two Bronze Age sites found within and beneath (pre-peat archaeology) peatlands respectively.

Case studies in peatland archaeology

In this book we focus predominantly on archaeological sites rather than individual finds and artefacts, summarised very briefly above. The main reason for this is most recent archaeological excavations have been of sites rather than individual finds or artefacts. The most ubiquitous intra-peat peatland monuments are trackways of varying length, form and date found across Europe (Fig. 1.1). For example, the Neolithic Sweet Track, the Somerset Levels and the Iron Age structures of Corlea 1, Mountdillon Bog Complex in Co. Longford, Ireland, and the Valtherbrug (Dutch for ‘bridge of Valthe’) trackway, an estimated 12 km in length. These trackways are often regarded as solutions to the problem of traversing treacherous and unproductive ‘bogland’ with little relevance to other questions of past society and culture, but this is an archaeological misconception as we will see with our case study of the Lisheen Archaeological Project, Ireland. In the Hatfield Moors, UK case study we consider how we can investigate and model processes of landscape change associated with the growth of peat across previously dryland surfaces: modelling peat inception and spread. This study will also discuss how pattern and process of environmental changes related to wetland development can be used to contextualise the peatland archaeological record. This case study also reflects on how we can assess the loss of peat through industrial scale milling and the implications of this for understanding and management of the surviving resource.

Figure 1.1: A later prehistoric timber hurdle trackway under excavation at Lisheen, Co. Tipperary, Ireland (second phase of excavations 2013)

Perhaps best known of peatland archaeological finds are ‘bog bodies’, for example the famous Iron Age bodies of Tollund Man, Denmark (Fig. 1.2), the Yde Girl, Netherlands, and the Lindow Man, UK, amongst the most evocative of intra-peat archaeological discoveries but actually comparatively rare finds. Again, a huge amount of scholarship has focussed on how human beings ended up in peatlands, from accidents to unusual burials, to human sacrifice and offerings to the gods (see Giles 2020). However, in our case study we describe how we might develop ‘best practice’ approaches to excavating bog bodies, focussing on the work carried out at Lemanaghan Bog, Ireland. This project is one of very few excavations that were explicitly designed to investigate the deposition of a bog body within its landscape context. This kind of information is critical to informed reflection on the circumstances and events that might have surrounded the final moments of a human being.

Figure 1.2: The Tollund Man ‘bog body’ (on display in Museum Silkeborg, Denmark)

Our penultimate case study concerns the later prehistoric site of Flag Fen, Peterborough, UK, situated at the western edge of the lowland fenlands of East Anglia. The fenlands have long been a key location for archaeological and palaeoenvironmental discoveries and study (see Chapter 2) and Flag Fen has featured in many publications. However, the case study seeks to provide an accessible summary of the work that has been carried out over the last forty years and also to situate this in terms of threats to the future survival in situ of the extensive organic archaeological remains.

The final case study takes a different approach. The Netherlands and northwest Germany, have been important locations in the development of wetland archaeology. The peatlands of the Bourtanger have produced numerous archaeological finds, and have been the focus of numerous studies. However, this work has never been summarised: the account in Chapter 5 provides the first such synthesis of the research and discoveries over the last decades.

Peatlands are also ‘archives’ of past environmental changes (Godwin 1981): the palaeoenvironmental or palaeoecological record. The waterlogged conditions preserve ‘proxies’ including pollen, plant remains, beetles and other material, crucial to understanding the development of peat and peatlands, and the role of processes such as climate in their growth. Palaeoenvironmental records from bogs are also important for the ‘reconstruction’ of vegetation change and human impact on dryland areas beyond the wetland. The potential to integrate and combine records of cultural and environmental change is one distinguishing feature of peatland (and wetland) archaeology, what we might describe collectively as the ‘archaeo-environmental record’ (see Chapters 3 and 4; Chapman and Gearey 2013). All our case studies illustrate palaeoenvironmental approaches to peatland sites and landscapes and how we cannot hope to understand past human activity without such datasets.

We begin with an overview of peat, peatland inception, growth and ecology. There is an extensive literature on these themes which can appear daunting and inaccessible to those without (and sometimes even those with…) education or training in the natural sciences. However, a basic appreciation of the fundamental terms and concepts of peatland development and ecology is an important foundation. This chapter is an introduction to the key concepts and terms essential for archaeological and palaeoecological understanding of peat and peatlands.

To add to the potential sources of confusion, there are a number of different concepts and expressions used in the description of peatlands, a profusion of terms that can be confusing, especially since some of these words have specific ecological and scientific meanings, whilst others have vague or imprecise definitions. Therefore, we will first define the key peatland terms that we will employ throughout this book (see Table 1.1) and address the following interconnected questions:

•What is peat?

•Where are peatlands found?

•Why and how does peat form?

•‘Drivers’ of peat formation

•Peatland stratigraphy

•Peatland flora and stratigraphy

•Non-humans and peatlands.

Basic ecological concepts

What is peat?

one quality of peat that remains oddly consistent regardless of how one views it—peat is a poorly defined material. (Rich 2015)

Very simply stated, peat consists of the undecayed or partially decayed remains of plants that have died and remain in place (in situ) where processes of decay have been slowed down or effectively stopped due to the waterlogged, low temperature conditions. The International Peat Society provides the following definition: ‘sedentarily accumulated material consisting of at least 30% (dry mass) of dead organic material’ (Joosten and Clark 2002). An important distinction here is ‘sedentarily’, in other words, the organic remains formed in situ. In their natural undrained state, most peats consist of 85–95% water.

Peat forming plants are generally those adapted (see below, Flora) to live in wet conditions, although in theory almost any plant can become incorporated into peat. Chemical processes in waterlogged conditions produce substances such as humic acids (see Chapter 4) that also contribute to inhibiting decay processes. A range of environmental variables are significant in terms of associated plant communities: in particular hydrology, nutrient and base (pH) status, which leads to important distinctions in terms of ecological categorisations and definitions of peatlands. The ecology of peatlands, in particular, the fauna and flora of these ecosystems, have further significance in terms of past human activity and the archaeological record (see below).

Whilst peat can be defined first and foremost as undecayed or partially decayed plant material, different types can be recognised depending on the vegetation community involved. Hence peat may be wood-rich, moss-rich, herbaceous or a mixture of different components. Entirely unsurprisingly, the term ‘peatland’ refers to peat covered terrain (Plate 1), although the depth required to define a landscape as such varies according to different classification schemes. A minimum of 30–40 cm is conventional although, as we will see, maximum peat depths can exceed this figure substantially where peat has been growing for several millennia. Peat accumulation rates will vary depending on various factors, but a very approximate rule of thumb would be c. 1 mm/year. However, accumulation of peat can lead to the compaction of underlying deposits, whilst drainage can significantly reduce the water content also leading to compression of deposits. In these circumstances, one centimetre of accumulation may effectively equate to c. 20 years or more. It is important to note that peat accumulation will proceed vertically but also horizontally, with the latter rate of spread controlled in part by local topography. Both lateral and horizontal accumulation rates can only be reliably calculated using radiocarbon and other scientific dating techniques (see Chapter 3).

Where are peatlands found?

As stated above, the global area of peatlands is estimated at c. 4 million km². Whilst these ecosystems are found in areas where annual rainfall is high and the loss of water through evaporation is low, the conditions peat for formation are met across the world in different climatic regimes. Hence, whilst the anecdotal conception of peatlands associates them with cold and damp countries such as Scotland and Ireland, they are also found in humid locations: for example, enormous areas of peatland are present in the Amazon Basin (over 62,000 km² in the lowland Peruvian Amazon; Hastie et al. 2022). There is an estimated 500,000 km² of peatland in Europe (just over 5% of the total land area; Table 1.2, Fig. 1.3) but nearly half of this total can be defined as ‘degraded’, in the sense that active peat accumulation is no longer taking place due to drainage and other impacts.

Almost all peatland archaeology, and all of the case studies discussed in this book, are in countries which have long histories of drainage and peat extraction (Fig. 1.4). It is invariably these interventions, and large infrastructural (e.g. road building) developments, which result in the exposure and discovery of archaeological sites and finds that would otherwise remain hidden and preserved. The archaeological potential of peatlands in countries where these environments survive in pristine or near pristine condition, remains entirely unknown.

Table 1.2: Summary of peatland extent in selected countries and indicative archaeology. Based in part on data from Tanneberger et al. (2017)

Why and how does peat form?

Hydrology, the role of water in terms of source and flow, is essential in terms of the formation, growth and development of peat. The basic conditions for peat formation are relatively easily stated: there must be an imbalance between precipitation and evaporation (abbreviated to the ‘P:E’ ratio) whereby the effective supply of water through precipitation (and potentially other sources such as snowmelt) exceeds the loss of water via evaporation. This results in a high watertable and under these conditions the productivity (growth) of plants exceeds the rate of decay of organic material after the vegetation dies and this material forms peat at the surface of the bog.

Peat accumulation and other biological processes are active in this ‘living’ surface layer, referred to as the acrotelm for raised mire systems. Beneath this is the catotelm: the ‘dead’ plant material, the main body of peat, in which biological processes are slowed down or effectively halted. There is an important link between climate and peat formation, which we will return to below. Other factors including geochemistry, pH (acidity) status and temperature are associated with inhibited organic decay and peat formation, but the principal factor is the exclusion of oxygen via saturation: anoxia, or anoxic conditions, which restricts or curtails the respiratory activity of microbes that usually degrade organic remains. Microbial activity does take place in anoxic environments, albeit at very low rates in undisturbed systems.

Figure 1.3: Extent of peatland in northwestern Europe, comprising both lowland mire and upland blanket bog

This leads us to another important point: the initial processes through which peat formation begins, sometimes referred to as peat inception. As outlined above, the basic conditions for inception require a positive (or at least null) hydrological balance rather than a negative balance. Two main ‘pathways’ can be defined which lead to these conditions and via which peat inception may commence:

Figure 1.4: Industrial peat extraction has ceased on many, but not all, lowland peatlands (upturned peat container, Hatfield Moors)

1. Paludification: this refers to peat formation directly over a previously dryland surface. Soils that were relatively dry and may have supported woodland can become waterlogged due to a number of processes. Many blanket peat landscapes apparently developed via paludification (see below) leading to the very large tracts of peatland in parts of the uplands of the UK, Ireland and a few other countries (see below). Sometimes, primary formation might also be distinguished, which is similar to paludification but where peat forms directly over a surface without any previous stage of terrestrial vegetation;

2. Terrestrialisation: perhaps somewhat counter intuitively, this refers to the process by which a peatland develops via an open body of water and is sometimes referred to as ‘infilling’ (Rydin and Jeglum 2013). Aquatic plants colonise open water such as lakes, and as these plants die their remains gradually accumulate in the basin in the form of an organic mud. The lake infills from the margins, with floating leaved plants, such as Potamogeton (pond weeds) and Nuphar and Nymphea (water lilies), followed by Phragmites (reeds) and Carex (sedges) colonising the shallower water. This process continues, until the lake ‘infills’ with sediment, allowing the growth of wetland trees and shrubs such as Salix (willow), Betula (birch) and Alnus (alder) on the waterlogged (semi-terrestrial) surface, and sometimes Quercus (oak) forming fen. Note, the source of water is important in terms of the