Soil thin section descriptions - Stephen Carter

1. Contexts 129-131 Deep sediments on south side of settlement

1(a). Context 129.

Basic mineral components

c/f limit: 20 m

c/f ratio: 10:90

Coarse material

Quartz. Very few angular and subangular grains up to medium sand size with undulose extinction.

Mica. Few angular acicular grains up to medium sand size but mostly only silt size.

Rock fragments. Dominant poorly sorted angular to subangular metamorphic rock fragments up to 23 mm in diameter. Of the 54 fragments larger than 1 mm, there are 40 % pelite, 20 % psammite, 20 % marble and 15 % metabasite.

Biological residues. Frequent bone fragments up to 20 mm with 18 greater than 1 mm. Few marine shell fragments with 8 greater than 1 mm.

Fine material

Organo-mineral nature with a dotted appearance. Yellow brown to brown in PPL, isotropic in XPL and reddish yellow to brownish yellow in OIL.

Basic organic components

c/f limit: 20 m

c/f ratio: Variable but 50:50 ?

Coarse material

Organ residues: Very few carbonised root fragments up to 1.4 mm in diameter. Very few multicellular spores up to 300 m in diameter.

Tissue residues: Few fragments of carbonised vascular tissue up to 4 mm long, but mostly less than 500 m.

Peat: Very dominant fragments of carbonised peat up to 4 mm but mostly less than 500 m; very few fragments of probable unburnt peat consisting of cell residues, diatoms and phytoliths in a yellow brown groundmass. Fragments up to 200 m.

Fine material

Dominant fragments of black amorphous organics and punctations, probably burnt peat. ?Frequent fragments of yellow brown amorphous organics. Variable concentration of pigment.

Groundmass

Fabric of coarse material: not random, tend to be clustered.Fabric of fine fraction: undifferentiated bfabric.Related distribution of c/f: porphyric.

Microstructure

Channel structure breaking up into a crumb/granular structure in areas of abundant fine excrement. Channels are 100-400 m diameter in the larger aggregates; see excrement pedofeatures for description of aggregates.

Pedofeatures

1. Crystalline

Very few rounded fragments of impregnative vivianite nodules; up to 200 m in diameter.

2. Excrement

The whole soil structure is created by excrement. Areas of channel structure, up to c.1 cm in diameter, are coalesced mammillated earthworm excrement which in part forms infillings to burrows a few mm wide. Indicated by the variable groundmass (abundance of coarse material, colour in OIL and PPL).Areas of crumb/granular structure consist of weakly to strongly coalesced clusters of frequent 600-800 m ellipsoids/bacillo- cylinders and rare less than 200 m ellipsoids.

1(b). Context 130.

Basic mineral components

c/f limit: 20 m

c/f ratio: 20:80

Coarse material

Quartz. Few angular grains up to coarse sand size with undulose extinction.

Mica. Few angular acicular grains up to medium sand size but mostly less than very fine sand size.

Rock fragments. Very dominant angular and subangular metamorphic rock fragments up to 8 mm long. Of the 30 fragments larger than 1 mm, 30 % are pelite, 30 % psammite and 30 % metabasite. NB, in slide 130/131, the base of 130 does contain marble.

Biological residues. Few fragments of bone and marine shell, mostly less than 1 mm.

Artefacts. Very few fragments of fused ash, up to 1mm in diameter.

Fine material

Organo-mineral nature with a dotted appearance. Yellow brown to brown in PPL, isotropic in XPL and yellow red to dark yellow red in OIL where burnt organics are more abundant.

Basic organic components

c/f limit: 20 m

c/f ratio: variable but reaches 70:30

Coarse material

Organ residues: Very few carbonised roots up to 2 mm in diameter. Very few multicellular spores up to 200 m in diameter.

Tissue residues: 1 sub-rounded fragment of wood charcoal (1.2 mm). Very few fragments of uncarbonised lignified vascular tissue up to 2 mm long (?from roots)

Peat: Very dominant burnt peat with fragments up to 13 mm diameter. Contains fungal spores and tissue residues but very little mineral matter. Very few fragments of mineral rich burnt peat, as above but with up to 10 % mineral grains (mostly less than 100 m but up to 1 mm). All less than 5 mm in diameter unless one 23 mm fragment in slide 130/131 is this rather than a pottery fragment.

In slide 130, the burnt peat is concentrated in two bands at 0- 2.5 cm and 4.5-7.5 cm. The basal 5 cm of 130 in slide 130/131 is also rich in burnt peat.

Few fragments of probable unburnt peat, many elongate up to 200 m long with humified tissue residues, phytoliths and diatoms.

Fine material

Very dominant black isotropic fragments and punctations (burnt peat) and ?common yellow brown amorphous organic matter. Variable organic pigment.

Groundmass

Fabric of coarse material: not random, clustered with bow shaped areas.

Fabric of fine material: undifferentiated bfabric.Related distribution of c/f: porphyric.

Microstructure

Channel structure breaking down into a crumb/granular structure in more porous areas. Channels in the size range 200-500 m with no preferred orientation. Larger chamberlike voids may be invertebrate channels up to 4 mm in diameter. Crumbs and granules are excrement.

Pedofeatures

1. Crystalline

Very few fragmented impregnative subrounded vivianite nodules; up to 200 m in diameter.Very few impregnative hypocoatings of yellow amorphous and crystalline vivianite phosphatic material. c.1 mm thick on some burnt peat fragments.

2. Excrement

As in 129, the whole slide is excrement. Again, three size classes can be identified:

i) Large mammillated masses, 110 mm across.

ii) 600-800 m ellipsoids

iii) Less than 100 m ellipsoids.

The large mammillated areas show as fabric pedofeatures and are heavily reworked by smaller organisms and plant roots. The larger ellipsoids are common and tend to be clustered in voids as porous microaggregates. The smaller ellipsoids appear to be rare, or perhaps not easily identified.

1(c). Context 131.

Basic mineral components

c/f limit: 20 m

c/f ratio: 30:70

Coarse material

Quartz. Few subangular to angular grains up to medium sand size with undulose extinction.

Mica. Frequent angular, acicular grains up to medium sand size.

Rock fragments. Very dominant angular and subangular metamorphic rock fragments up to 11 mm long. Of the 88 fragments larger than 1 mm (176 for a whole slide), 40 % are pelite, 10 % psammite, 35 % marble and 10 % metabasite.

Biological residues. Very few bone and marine shell fragments. There is one large smashed shell and only less than 1 mm bone fragments.

Artefacts. One fragment of pottery, 2 mm long with a distinctive b-fabric.

Fine material

Organo-mineral nature with a dotted appearance. Colour: variable yellow brown, brown to dark brown in PPL, isotropic in XPL and yellow brown with some areas of yellow red in OIL.

Basic organic components

c/f limit: 20 m

c/f ratio: uncertain but coarse material is less abundant than above.

Coarse material

Peat: Frequent fragments of burnt peat up to 500 m with very few fragments up to 2 mm. Tend to have a clustered distribution. Few elongate fragments of probable unburnt peat.

Fine material

Common amorphous black fragments (burnt peat) and organic pigment. Both have a very variable distribution.

Groundmass

Fabric of coarse material: not random, patchy

Fabric of fine material:undifferentiated b-fabric Related distribution of c/f: porphyric.

Microstructure

Complex crumb/granular structure with a channel structure in larger (10 mm) aggregates. Channels are 200-500 m (plant roots) with some larger infilled invertebrate channels (25 mm) identifiable.

Pedofeatures

1. Crystalline

Fragmented impregnative vivianite nodules, as in 129 and 130 but up to 1 mm in diameter.

2. Excrement

As in 129 and 130 there are three scales of excrement, mammillated areas and larger and smaller ellipsoids.

2. Context 680 Deep burnt layer within broch.

One 8 x 5 cm thin section was prepared from this layer. The soil block was damaged before it was impregnated and as a result only the lower 5 cm of the thin section has intact structure. This area consists of numerous thin layers of plant ash partially disrupted by areas of dense mineral fabric and coarse rock fragments. For the purposes of this description, 4 types of fabric have been identified:

Type 1. Areas of dense heat reddened mineral material.

Type 2. Bands of amorphous and partially recrystallised colourless ash.

Type 3. Bands of pale yellow amorphous ash with some inclusions

Type 4. Bands of microlaminated ash with abundant often highly reflective inclusions.

Context 680, type 1.

Basic mineral components

c/f limit:20 m

c/f ratio: 20:80

Coarse material

Dominant rock fragments up to medium sand size (400 m) with few, generally smaller single mineral grains of quartz and mica. Few fragments of burnt bone.

Fine material

Mineral nature with a dotted appearance. Colour is very variable, dark brown, brown or grey brown in PPL, very dark red in XPL and bright red in OIL.

Basic organic components

c/f limit: 20 m

c/f ratio: very little fine material?

Coarse material

Very few VAM spores, 140-200 m in diameter, not birefringent. Few amorphous black (in all light) fragments, up to 400 m but mostly c.50 m diameter with some internal cracking. May be organic, or burnt organic.

Fine material

Some material similar to the coarse material above but in smaller fragments.

Groundmass

Fabric of coarse material: not random, bow shaped distributions. Fabric of fine material: undifferentiated bfabric. Related distribution of c/f: prohyric.

Microstructure

Almost massive with very few voids. Some channels c.400 m in diameter.

Pedofeatures

Excrement

The whole of this type of fabric appears to consist of mammillated excrement with coalesced rounded masses of groundmass up to 10 mm in diameter identified by variable colour in PPL and the arrangement of the coarse material.

Context 680, type 2.

This ash type consists of a groundmass of amorphous, slightly granular mineral material, colourless to very pale yellow in PPL, isotropic in XPL and nonreflective in OIL. This is organised into coalesced equidimensional areas 100-200 m in diameter with intervening voids.

In some ash bands, the amorphous groundmass has partially crystallised to form cryptocrystalline radiating aggregates up to 400 in diameter. These may be separated by bands of silt sized acicular crystals, birefringent in XPL. Very rare coarse mineral grains (up to 600 m) may be present.

In most bands of this ash type, there are complex concentric nodules clustered in voids or forming void hypocoatings. These are up to 200 m in diameter (mostly 515 m), red brown or black in PPL, isotropic and black in OIL. They may be organic or ferruginous in nature.

Context 680, type 3.

This ash type consists of an amorphous groundmass that is yellow brown, pale yellow or grey in PPL, isotropic in XPL and has low reflectance in OIL (Colourless). It contains mineral inclusions that are concentrated in particular areas giving a generally banded appearance to the ash; individual bands are c.100-500 m wide. Mineral grains up to 3 mm are present but most are less than 200 m in diameter. Individual phytoliths or fragments of ashed plant tissue are also present. Phytoliths are only common in very limited bands where they include frequent dumbell and trapezoid types and some rods. Plant tissue fragments consist of only a few straight walled cells commonly 10 m wide and up to 100 m long arranged in parallel groups. Tissue fragments are parallel to the ash banding and they are very rare. More frequently, very fine inclusions are arranged as approximate pseudomorphs of plant tissue. These include the concentric nodules noted in the type 2 ash and also very fine irregular shaped highly reflective particles (110 m) which are more abundant in type 4 ash (see below)

Context 680, type 4.

This ash type is charactarised by the presence of abundant inclusions. It is dark reddish brown in PPL, dark red in XPL and bright red in OIL (highly reflective). These colours are created by common very fine inclusions in the size range 110 m which are dispersed throughout the groundmass. These are highly reflective and their small size creates the overall groundmass colours at low magnification. Similar material is concentrated in parallel bands, 10-100 m wide, often as aggregates up to 40 m long or patches 100 m in diameter. These are associated with indistinct plant tissue pseudomorphs; actual phytoliths are very rare or rarely visible and there are no well defined tissue fragments. Concentric nodules, also seen in types 2 and 3, are associated with the other banded inclusions. Coarse mineral grains are present in a similar abundance to that in type 3.