The Work

Specialist Report 2004 - Charcoal

Posted by steven on 11/04/2005 at 02:17 PM

High Pasture Cave, Isle of Skye: Preliminary Charcoal Assessment
Report No. 1059

Michael Cressey - CFA Archaeology Ltd.

Introduction

In March 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 interior of High Pasture Cave, Isle of Skye.  Charcoal samples included flot samples ranging from 4-6mm (Box 1 and 2) and sample bags of replicated ‘wet’ samples.  Only the dry flot samples have been examined (Appendix 1). 

Objectives

The main objectives were to determine the species composition of the assemblage and to help target contexts for future radiocarbon dating.  An assessment of the preservation quality of the assemblage was also made.

Methods

Identifications were made using a binocular microscope at magnifications ranging between x10 and x200.  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 were 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 relatively good considering the nature of deposition within a highly active cave system.  Repeated saturation has not affected the assemblage, nor has any of the charcoal become coated with any calcareous concretions likely to compromise future radiocarbon dating submissions.  Some vitrified material was present within the assemblage and its presence may be attributed to secondary burning with high temperatures reached sufficient to convert the charcoal to a glass-like consistency.  No fragments of charcoal were attributed to timber; all observed material is attributed to small branch wood.

Identification results

A total of 492 identifications were made providing a combined charcoal weight of 3.45kg (Table 1).  Eight species of wood are represented within the assemblage; these include Alnus glutinosa (alder); Pinus sylvestris (pine); Betula sp. (birch); Quercus sp. (oak); (Prunus type (cherry or blackthorn); Salix (willow); Corylus avellana (hazel); and the shrub Calluna vulgaris (heather).

Hazel was the most abundant material identified, followed by birch, oak, heather and pine.  Willow, alder and cherry/hawthorn are represented by only trace amounts.  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. 40mm.  Two pieces of charcoal had evidence of oblique trimming (hazel/001/S9 and willow/003/60).

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 only been exploited from streams and wetland areas.  The presence of pine (Context 002-004) and heather (Context 001/005) could be attributed to tinder.  When dry, both species burn vigorously and make excellent starter fuels.  There is insufficient material to make any further conclusions on the palaeoecological implications of the charcoal assemblage. 

The quality of the Box 1 charcoal is very good.  However the assemblage from Box 2 contained samples that were slightly abraded, a process that has occurred during on-site wet sieving (S. Birch, pers comm).  Given the high potential for the re-working of sediments within the active cave environment, in general the charcoal examined was in good condition and it is likely that any further charcoal recovered from the cave will be suitable for single entity AMS radiocarbon dating.

References

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

Tables
Species No of ID's Sample Weight (g)
Alder 1 0.4
Birch 91 40.7
Hazel 360 286.9
Pine 5 3.7
Cherry/blackthorn type 1 0.3
Oak 5 7.4
Willow 4 1.30
Heather 25 5
Total 492 345.7
Table 1 - List of species, number of identifications and weight from the sampled assemblage.

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