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Pattern database home
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Interior divisions of space
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Midden patterning
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Burial patterning
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Entryway patterning
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Pathway patterning
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Boundary patterning

Structure Patterning

The identification of structures not visible on the ground surface is possible in phosphate analysis through identification of the specific interior patterns described above.  Areas that exhibit a variety of phosphate levels in a relatively small locale are identifiable as interior areas.  The location of relatively high and low phosphate concentrations clustered near each other suggests the presence of divided interior space (Craddock et al.1985: 368; Matthews et al. 1997: 293).  The presence of increased phosphate content within a roughly circular area of low phosphate content can also indicate a structure that was kept clear of organic debris within the interior (Terry et al. 2004: 1245).  The increased areas of phosphate content relates to interior walls where debris collected or to exterior walls where debris was deposited as a result of clearing the interior.  Matthews et al. (1997: 293) identified this patterning as a result of sweeping to clear interior areas.  It is also possible that increased phosphate content against the exterior of the walls results from natural accumulation in a protected area.  Both patterns result from the presence of structures, which were used in two different ways.

The association between these specific phosphate patterns with the presence of structures not visible on the ground surface was examined by Crowther (1998) at the site of a small upland hillfort in Wales and by Craddock et al. (1985) on several prehistoric sites in England.  In Wales, several circular hut platforms and two rectangular structures were recognisable in the phosphate patterning through the identification of well-defined increases in phosphate content (Crowther 1998:118).  In England, dwelling structures were also easily identifiable in the phosphate patterning through the presence of clusters of relatively high and low phosphate content in close proximity to each other, as well as roof drip line patterns (Craddock et al. 1985: 368).  In both instances these results were tested and proved through excavation.


Pattern Maps and Interpretations

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Structure patterning, Dun Bunnafahy

Structure Patterning indicating possible hut structures at Dun Bunnafahy

Two areas in the undefended land on the exterior of the Dun Bunnafahy promontory fort display phosphate patterns generally associated with the presence of circular structures (Figure 4).  In this location, areas of increased phosphate content (Level 4) are located within mid-level phosphate concentrations (Level 3), indicating the presence of structures not visible on the ground surface. Areas between the higher increases (Level 4) remain lower (Level 3).  The two central areas of decreased phosphate content are both between 6 and 9 meters in diameter.  The patterning suggests the two possible structures were used in different ways.  Phosphate patterning in the area of the western feature indicates a structure that was swept clear, with the build-up of some organic debris against the walls.  Phosphate patterning in the area of the eastern feature is likely associated with the presence of an enclosed hearth, as has been identified and ground-truthed at a number of sites (Kerr 1993: 138; Weston 1995: 22; Smith et al. 2001: 258; Crowther 1998:118; Craddock et al. 1985: 368).  

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Structure patterning indicating the presence of an outbuilding at French Azilum

The patterning in this area consists of two larger areas of slightly lower phosphate content, completely surrounded by higher levels, and two smaller areas of variation in phosphate content (Level 2 and Level 4).  This area has a base-level of 4. The smaller concentrations of Level 2 and Level 4 are located in the boundary area between base-level 3 and base-level 4.  This patterning suggests the presence of a structure, or specific activity area, which dictated the deposition of organic debris in specific ways.   Features on the ground surface were likely present in the areas of slightly depressed phosphate content, which would have inhibited the deposition of organic material with a higher phosphate content generated through use of the identified structure.  A line of four post-holes and a single post-hole on the southern perpendicular, forming a corner, were found in this area through excavation.  The post-holes are located in the boundary area between the larger Level 3 concentrations and the Level 4 base-level concentrations.  This fits well with the abovementioned scenario where material was stacked near, or even against, the feature.  Excavation of this area has disturbed the area directly beside the post-holes.  As a result, it is impossible to determine if organic debris collected along either side of the identified feature. 


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Floorboarded structure patterning at French Azilum

The area to the south of the exposed slave quarters shows a great, and widely dispersed, increase in phosphate content.  There is evidence in the written records of a foundationless structure appended onto the exposed slave quarters.  The phosphate patterning in this area supports this; organic material would fall through the wooden floorboards and decompose on the soil surface, causing the increase in phosphate content.

Johanna Ullrich, Ph.D.