The Work

Specialist Report 2005 - Charcoal Analysis

Posted by steven on 21/03/2006 at 09:15 PM

CHARCOAL IDENTIFICATION AND ASSESSMENT

Mike Cressey - CFA Archaeology Ltd

Introduction

In December 2005, West Coast Archaeological Services commissioned CFA Archaeology Ltd to carry out a preliminary assessment on an assemblage of charcoal recovered from various contexts excavated from the High Pasture Cave site (HPC), Isle of Skye during the 2005 season.  Charcoal samples included flot and hand- picked samples ranging between 4-6mm and these have been identified to species level to allow an assessment of the wood exploited.

This work follows on from a previous examination of a charcoal assemblage from the same site collected in 2004 (Cressey 2005).  This work was carried out to assess the range of wood species exploited and its general condition for radiocarbon dating.  It was concluded that the charcoal was in good condition and it was likely that any further charcoal recovered from the cave would be suitable for single entity AMS radiocarbon dating.  No fragments of charcoal could be attributed to timber (branch wood squared-off from the round) and all observed material is attributed to small to medium sized branch wood.

Objectives

The main objectives of this study were as follows:

• to determine the species composition of the assemblage in relation to previous work;

• to identify any worked wood and assess its preservation quality;

• to provide a report on the results of the work suitable for incorporation into the High Pasture Cave 2005 Data Structure Report.

Methods

Identifications were made using a binocular microscope at magnifications ranging between x10 and x200.  Only identifiable charcoal was quantified by count and weight.  Generally identifications were carried out on transverse cross-sections on charcoal pieces not smaller than 4-6mm.  Anatomical keys listed in Schweingruber (1992) and in-house reference charcoal was used to aid identification.  Asymmetry and morphological characteristics were recorded.  In this report, branch wood is used as a term for wood that has not been modified. 

Preservation

The level of preservation within the charcoal assemblage is good to excellent and very few fragments could not be identified.  Such material was confined to either root wood or extremely small fragments (<4mm).  Taphonomic alteration, such as repeated saturation, has not affected the quality of the charcoal nor has any of the charcoal become coated with any calcareous concretions that would likely compromise future radiocarbon dating submissions. 

One sample of highly vitrified non-vascular material was present within the assemblage (Sample 2225) probably either pitch or resin material that has amalgamated at the base of a hearth.

Identification results

A total of 824 identifications were made providing a combined charcoal weight of 820.6 kg.  Seven species of wood are represented within the assemblage.  These include: Corylus avellana (hazel); Betula sp. (birch); Pinus sylvestris (pine); Prunus type (cherry or blackthorn); Quercus sp. (oak); Salix (willow); and the shrub Ilex aquifolium (holly).

Hazel was the most abundant material identified, followed by birch, pine and oak.  Cherry/blackthorn type and willow are also present and holly is represented by a single fragment.  In terms of the overall assemblage weights, hazel and birch provided the largest volume of charcoal.  Some of the hazel fragments were large with a diameter of c. 40-50mm showing that mature branch wood had been exploited.  Modified material was present but not in abundance.  The main modification was trimming, forming an oblique facet caused by a sharp blade.  A number of heels are also present.  These are round wood with a distinct basal curve, and are typical of stems that have been torn from a hazel stool.  Straight stems or rods of young growth were also present and reflect the growth pattern of stems commonly found in coppiced hazel, although this species will self-coppice as part of its natural regeneration cycle.  It is interesting to note that Context 205 contains the highest assemblage of worked or modified material and includes hazel, birch and pine.

Conclusion

All the above-mentioned species are native to Western Scotland and all would have been present within the locality of the High Pasture Cave during the prehistoric period.  Hazel and birch are well represented within Scottish charcoal assemblages and both types thrive on open acid soils.  Alder and Willow would have been exploited from streams and wetland areas.  The earlier pilot study mentioned that pine could have been exploited for tinder.  When dry this species will burn vigorously and makes an excellent starter fuel. 

The presence of holly in Scottish charcoal assemblages is rare.  This species has only ever been recorded once by the author, at Kaimes Hill, Edinburgh (Simpson et al 2004).  Holly is insect-pollinated and is therefore not represented in pollen diagrams.  It requires damp frost free environments and would be well suited to sheltered valleys on Skye in the prehistoric period.  It is also a species which has often been associated with settings near holy wells (Jones 1992 and Shepherd 1994).

Hazel appears to be the most abundant exploitable fuel source.  The local hazel woodland was certainly mature enough to provide not only large diameter branch wood for fuel but also hazelnuts as attested by the plant macrofossil evidence from the site. 

The charcoal represented in the High Pasture Cave assemblage must represent only a fraction of the local wild wood exploited for fuel.  It is likely that future work will provide additional species to add to the inventory thus far assembled.

References

Cressey, M 2005 High Pasture Cave, Isle of Skye. Preliminary Charcoal Assessment.
CFA Report No1059.

Jones, F 1992 The Holy Wells of Wales. University of Wales Press, Cardiff.

Schweingruber, F H 1992 Microscopic Wood Anatomy. Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf.

Shepherd, V 1994 Historic Wells In and around Bradford. Heart of Albion Press .

Simpson, D D A, Gregory, R A and Murphy, E M 2004 ‘Excavations at Kaimes Hill, Ratho, City of Edinburgh, 1964-72’. Proc. Soc. Antiq. Scot 134, 65-118.


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