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Irish lithic landscapes: the macroscopic, petrographic, and geochemical characterisation of chert from archaeological and geological contexts

Irish Lithic Landscapes Project

Introduction

The Irish Lithic Landscapes project (IRLL) is investigating the places where prehistoric communities obtained the raw materials for their flaked stone tools during the Irish Mesolithic, Neolithic, and Early Bronze Age, which dates to about 8,000–2,000 BC. This time period and region are critically important in global human history, as they include the complex transition from hunter-gatherers to farmers in what is often referred to as the 'Neolithic revolution', with related research questions regarding sedentism and social change.

Raw material sourcing studies are a vital part of archaeological considerations: they help us understand human behaviour and the relationships between people and the environment, allowing us to reconstruct the movement of peoples and objects in the landscape, including seasonal mobility, social boundaries and interactions, exchange systems, and trade routes. They also provide evidence for the territory, location, and size of resources that relate to the social and technical organisation of crafts and industries.

While Ireland has a very rich archaeological heritage, there is a significant gap in the country's raw material sourcing research. This project will begin to fill this gap, and therefore deepen our understanding of the prehistoric communities there. The first season's geoarchaeological prospection for the project is centred on the northwest of the country which includes a group of sites from the UNESCO World Heritage candidate site of the Céide Fields on the north coast of County Mayo, as well as sites located in and around the Knocknarea/Carrowmore and Carrowkeel prehistoric complexes in County Sligo.

Knocknarea

Knocknarea Mountain. Image courtesy of Stefan Bergh.

Fieldwork and Case Studies

Geological Prospection. The first season of the project's geological prospection, funded by the National Geographic Society / Waitt Grants Program will involve eight weeks fieldwork in the two sub-regions of northwest Ireland. The investigation of the geology includes fieldwork to collect geological samples in primary (i.e. outcrops), near primary (i.e. erosive deposits from, and in proximity to, outcrops), and secondary contexts (i.e. river gravel and glacial contexts). The fieldwork is recorded using a standardised reporting form, and mapped using Global Positioning System (GPS) for integration with GIS software. For the primary contexts the sampling is undertaken in such a manner as to sample outcrops both horizontally and vertically in order to identify the homogeneity or heterogeneity of the material over the course of an outcrop. An important part of the fieldwork concerns understanding the accessibility to the source - for primary, near primary, and secondary - both seasonally and diachronically, as well as matters such as ease of extraction and quality of the material.

Case Studies. The two sub-regions chosen for the first season of geological prospection are the Céide Fields complex from North Mayo and the Mullaghfarna/Carrowkeel and Knocknarea/Carrowmore complexes about 60 km to the east in County Sligo. The sites to be included from North Mayo are the Mesolithic to Bronze Age site at Belderrig; the Neolithic and Bronze Age settlement and enclosure site at Belderg Beg; the Behy court tomb, the Glenulra enclosure, the Céide Fields visitors centre excavations, and the Glenulra lithic scatter; the Rathlackan court tomb; and the Ballyglass court tombs and Neolithic house. From Sligo, the sites to be included are the chert outcrop and associated lithic scatters and enclosures from the Knocknarea megalithic tomb complex; the Early Neolithic causewayed enclosure from Magheraboy; the megalithic tombs from Carrowmore; and the megalithic tombs and hut sites from Mullaghfarna and Carrowkeel.

Céide Fields prehistoric field walls

Seamas Caulfield and the prehistoric field walls of the Céide Fields. Image courtesy of Seamas Caulfield.

The Céide Fields area represents a prehistoric landscape of field walls, enclosures, settlement sites, and megalithic tombs which have been buried undisturbed under bog for up to five millennia, and also includes a Mesolithic coastal site at Belderrig amongst the subsequent prehistoric field wall complex. There are no local sources of chert in the area, which stands in contrast to the second sub-region selected, the series of domestic and ritual sites in County Sligo which lie in a "chert-rich" landscape. Furthermore, unlike the North Mayo area, the archaeology of Sligo is not preserved under bog, therefore there is an apparent strong contrast between the prehistoric field wall system and its related settlement sites and megalithic tombs still visible in Mayo, and the Sligo area where no prehistoric field walls per se have been identified, but where a similar range of sites and material culture, including chert stone tools, appear. Stefan Bergh has identified the only known Neolithic chert quarry site in Ireland at the Knocknarea complex and Graeme Warren has been involved in the identification a probable Mesolithic chert quarry in the Irish midlands near Lough Derravaragh.

Research questions. Based on these two sub-regions a series of research questions have been proposed.
1. With no local chert outcrops in North Mayo, where did the prehistoric communities source their raw material?
2. For both the Mayo and Sligo sub-regions: are there intra-regional differences in sourcing amongst the North Mayo and Sligo sub-regions under consideration? This question pertains to possible differences between the various sites in terms ritual/domestic sites, chronological differences, and of the location of the sites (i.e. in North Mayo, from either the eastern or western side of the north coast).
3. How can the sourcing of stone tool raw materials shed light on the relationship between the prehistoric communities along the northern coasts of Mayo and Sligo? This broaches chronological and societal questions relating to the introduction of early farming, landscape use, and megalithic morphology amongst others.
4. The link between the large chert quarry on Knocknarea and the extensive production of lithics on the mountain above is unproven: can we positively link the "domestic lithics" in a ritual context on the mountain?
5. While the similarities within the ritual sphere are strong between the Carrowkeel and Knocknarea complexes (see Bergh 1995), a further research question is whether this is also reflected in the sourcing of stone tool raw materials, and to what extent can we identify further chert outcrops utilised in Sligo during prehistory.
6. Related to the last question, with the mountain of Knocknarea being a conspicuous focal point in the landscape, would the rich chert sources from this landmark have been of significance to people from outside the immediate region? This raises questions regarding natural places being charged with meaning and symbolism.

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Case study locations chert provenancing Ireland: case study area outline

Methods

Geoarchaeology has been succinctly defined as the use of geological concepts, methods, and knowledge base in the direct solution of archaeological problems (Rapp Jr. and Hill 1998). In the case of this project, the archaeological problem to be solved concerns the provenancing of chert used by the prehistoric communities in Ireland. Provenancing studies are an important part of archaeological considerations. They can help us understand human behaviour and the relationships between people and their environment, providing evidence for the reconstruction of systems of movement of peoples and objects in the landscape, including patterns of mobility, social boundaries, exchange systems, and trade routes. They can also provide evidence for the resource locations, scale of exploitation and the extent of distribution of products that may relate to social stratification and the social and technical organisation of crafts and industries.

Our project has at its core an interdisciplinary approach - combining social science research questions with natural science methods - to answering the archaeological questions of lithic raw material provenance. The methodology can be viewed as composed of two main parts - archaeological and geological - with a third part, the geographical information system (GIS) and database construction as an overarching component. The sampling strategy for the archaeological assemblages will be devised to allow a representative sample to be analysed, taking into account spatial and chronological aspects of the assemblage as well as technological aspects, i.e. not just the sampling of cores and 'tools' which is in effect sorting, but also sampling the suite of debitage products, sensu Inizan et al. (1999).

The artefacts and the geological samples will then be analysed using the three laboratory analytical steps - macroscopic, geochemical, and microscopic - which are described in the figure. As noted above, the geochemical and petrographic avenues for chert provenancing studies are often taken separately, or with researchers favouring one over the other. The objective of this proposal's methodology is to formulate and use an original methodological framework which will include both of these aspects. The analysis compares the archaeological and geological datasets in order to understand the similarities and differences between what chert sources were used in prehistory and those that are available in the present-day landscape, thus answering the various research questions posed.

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Laboratory analysis chert

The Team

Principal Investigator

Killian Driscoll, Département d'anthropologie, Université de Montréal

Investigators

Stefan Bergh, Department of Archaeology, National University of Ireland, Galway
Adrian Burke, Département d'anthropologie, Université de Montréal
Gilles Gauthier, Département de chimie, Université de Montréal
Graeme Warren, UCD School of Archaeology, University College Dublin

Acknowledgments

Killian Driscoll was awarded a two year Banting Postdoctoral Fellowship, administered by the Government of Canada to undertake this research project at the Université de Montréal, and the team was awarded a National Geographic Society / Waitt grant for fieldwork for the first season of geological prospection in 2014. This project has been continued for a further two years as 'Returning to the source: investigating flaked stone tool raw material procurement strategies in prehistoric Ireland.'

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2014 Fieldwork Update

During the summer of 2014 we undertook the geological prospection component of the project, centred primarily on the northwest of Ireland. Over the course of c.900 person/hrs we were able to collect over 350 geological hand samples, with the primary focus on in situ chert from outcrops. In total, we examined 170 outcrop groups from nine counties. The chert from the northwest is associated with 20 rock units, with 83% of the outcrop groups surveyed in the northwest from five principal rock units: the Dartry Limestone Formation, the Bricklieve Limestone Formation (upper and lower), the Ballyshannon Limestone Formation, the Ballina Limestone Formation (Upper), and the Ardnasillagh Formation. As well as the fieldwork in the northwest, we surveyed two rock units in the northern midlands – the Derravaragh Cherts from around Lough Derravaragh and the Lucan Formation, with the latter sampled at one outcrop c.15km north of Lough Derravaragh.

Whilst the maps show the underlying bedrock geology, the outcrops available for inspection are variable from extensive exposures along terraces and seaside cliffs, to very small outcrops exposing just a few centimetres of bedrock. Therefore some rock units are difficult to assess; the Croghan Formation, for example, centred on the town of Carrick-on-Shannon covers c.110km2 but is largely inaccessible as it lies within an area of a low-lying drumlin belt. On the other hand, the Dartry Limestone Formation, which includes the outcrops at Knocknarea, outcrops extensively in the west and is therefore, and was in prehistory, easily accessible.

The geological prospection was based on examining outcrops marked with strike and dip symbols on the geological maps that are based on the original 19th century geological maps, plus examining outcrops in the vicinity of the archaeological case studies such as at Knocknarea and Mullaghfarna / Carrowkeel. Overall, we were able to locate all but 24 of the marked outcrops, with the 24 not found due to land use changes and building construction. As well, while we were travelling across the countryside we surveyed outcrops we came across, including modern quarries and roadcut outcrops. The sampling strategy, therefore, was mostly planned in advance, and provides a good representation of the chert availability and variability in the areas surveyed.

As well as the geological survey of the in situ chert in outcrops, we undertook a number of river / lake surveys. These were undertaken to assess the distribution of surficial chert along the waterways in areas where the bedrock is non-chert-bearing, and also to assess the availability of chert in areas where the bedrock is chert-bearing but with few known or visible outcrops. For the former, this included a number of localities between Counties Sligo and Donegal where the rivers’ sources are within the chert-bearing Dartry Limestone Formation; these have shown a sharp drop of availability of chert as one moves downstream from the chert-bearing bedrock portion of the rivers. For the latter, the Ballina Limestone Formation (Upper) is poorly exposed in Co. Mayo, therefore the geological prospection included numerous surveys along rivers and Lough Conn.

The next phase of the project is the geochemical analysis. For this we have selected 140 samples for ED-XRF analysis from the 350+ geological samples collected, and we will also be using ED-XRF to analyse c.200 archaeological artefacts selected from the case study assemblages. In addition to this, we are designing and constructing the online database that will complement the physical reference collection – the Irish Lithics Landscapes Lithoteque – composed of the geological hand samples collected. These will be physically housed at the UCD School of Archaeology as a permanent reference collection of flaked stone tool raw materials. Along with the ED-XRF results, the online database will include a macroscopic description of the hand samples, as well as a photographic record of the survey points and the samples.

Seaside outcrop with chert, Co. Sligo Poorly exposed outcrop with chert, Co. Mayo Adrian Burke sampling chert from the Dartry Limestone Formation at Knocknarea, Co. Sligo

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IRLL14 Map

The map below provides an overview of the IRLL14 geological prospection, and contains two layers: a survey point layer that includes survey points with and without sampled material (i.e. negative evidence of survey points with no in situ chert / outcrops not found), and a sample layer with all the geological samples collected that also includes descriptions of the samples' survey points. The layers can be toggled using the layer box on the left, or the map can be opened in a new window and layers toggled from there. The project's images' locations can also be viewed using Picasa.

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IRLL14 Images

The image gallery contains the IRLL14 geological prospection fieldwork images and sample images. The images' locations can be viewed by clicking the image to open in Google Photos. All the images taken for this project are licensed under Creative Commons Attribution-ShareAlike 4.0 International Licenses. Creative Commons License

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2014 fieldwork surveypoints and chert-bearing bedrock Chert-bearing bedrock Google Earth view of the drumlin belt overlying the Croghan Limestone Formation, with suveypoints marked Chert hand sample
  • Lithics Ireland Consultancy
  • lithicsireland@gmail.com
  • Galway, Ireland