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IDENTIFYING CULTURAL STRUCTURES IN CLITTER

In this section of the paper we attempt to set out a number of geological criteria for identifying the presence of cultural structures in the clitter. In the following section we then relate these to the specific case of the clitter on Leskernick hill.

From a geological point of view we need to think about how natural unmodified landscapes might appear.

Solifluction

The most common periglacial process affecting frost susceptible sediments is solifluction. This is a mass wasting process by which stoney material is moved downslope at velocities of the order of 1 - 7 cm per year. Hillslopes affected by solifluction tend to be concave in form and soil depth may thicken considerably towards the base of the slope. Flow of the top metre or so of material during solifluction is initiated by the freezing of fine-grained wet soils followed by thaw-induced instability leading to mass-wasting. Freezing of the ground draws water to the freezing front allowing ice lenses to develop. This increase in ground volume results in frost heave of the ground. During thawing of the ground the excess water cannot be expelled efficiently from the soil and the consequent stresses are transferred from inter-granular contacts to porewater pressure (Ballantyne and Harris 1994) with an associated decrease in frictional strength. This means that the water pressure increases until particles are no longer in contact with each other. The process is most effective when the soil is composed of fine sand or silt-sized particles. Finer or coarser materials inhibit the development of ice lenses during freezing and solifluction will not occur in these sediments. The implications of solifluction for the archaeological identification of cultural structures are threefold:

1. The process creates a characteristic macrofabric (spatial arrangement) to the deposit in which elongate stones are aligned with their long-axes (a-axes) in a downslope direction parallel to the direction of the flow vector. Such flows also result in clast imbrication where their long axes dip into the slope whilst maintaining their downslope a-axis macrofabric (see Benedict 1970; 1976 and Fig. 1
[pdf]).

2. Solifluction can only occur in frost-susceptible soils (Harris 1981). Where the soil matrix is either too fine or too coarse for solifluction to occur, another process must be invoked to explain clast orientations and/or movement. The sandy silty nature of the soils on Leskernick hill, exposed during excavation, are highly likely to be frost-susceptible.

3. Such mass-wasting processes are very old and have not occurred to any significant extent in upland areas of southwest England since the Younger Dryas. By implication, a solifluction layer will always be found below a cultural horizon.

Frost heave and clast displacement

In areas where large clasts are abundant (e.g. in clitter fields below tors) frost-heaving, frost sorting and clast displacement may have occurred by a variety of processes. The a-axes of elongate clasts may display high dip angles (up to perhaps 80 degrees) as a result of macrogelifraction (large-scale frost shattering) and frost heave and may thus mimic standing or placed stones of a cultural origin. However, for this to occur naturally, several conditions must be satisfied:

1. 'Standing boulders' are unlikely to form in isolation since intense cryostatic pressures are required to elevate the clast and this requires the presence of other large clasts. The depth of burial in soil determines the pressures required to elevate the clast. It follows that shallow burial requires large cryostatic pressures.

2. Although standing boulders may occur on flat and gently sloping ground, they are unlikely to survive rapid mass-movement on very steep slopes (greater than 30-40 degrees).

3. Where they have formed in response to mass-movement pressures they will form a distinct pattern whereby groups of standing boulders will be separated by groups of boulders exhibiting low dip angles. The spatial scale implied here may be of the order of 1-10 m. This pattern is caused by the separation of extensive and compressive flow regimes on slopes and the differing response of elongate clasts to these mechanisms. Compressive flow in a viscous medium tends to occur when flow is retarded (see Hooke 1998) and is therefore most likely to appear in the downslope edge of topographic hollows. Conversely, extension flow occurs during accelerated flow and therefore may be seen on convex hillslope segments. Such flow must have occurred within a viscous or plastic medium such as fine soil or within an ice matrix. It is not possible for clasts to move on slopes below about 30 degrees angle without such a matrix.

Perched boulders and rockfalls

In many archaeological sites boulders are found perched on top of one another or on the top of bedrock outcrops. In a geomorphological context such arrangements imply three things:

1. The perched blocks have been deposited as a result of rockfalls from bedrock masses upslope

2. They have been deposited by avalanches

3. they are in situ and the result of weathering out of corestones from a larger bedrock mass.

However, rockfalls require the presence of a steep source area for the rock debris (usually a cliff), the slope must be steep and long enough for the debris to gain sufficient kinetic energy to travel horizontally. Although determination of whether a perched block has been emplaced by rockfall is site specific, and therefore can only be achieved in the field, it is clear that perched boulders when they are found on low-angled slopes and at considerable distances from cliff faces or on hillsides with no backing cliff must be cultural relicts.

Stone circles and polygons

There are periglacial processes which may form stone circles and polygons. These include frost sorting (without the presence of permafrost) and large-scale frost wedging which implies permafrost development. The former process creates small (less than 1 m in diameter) stone circles in the uplands of Britain in the present day (e.g. Ball and Goodier 1968, 1970; Ballantyne 1986) whilst large-scale permafrost development may create large circles (perhaps up to 5 m in diameter). Hence, large, naturally formed stone circles in the landscape can only be attributed to intense ground freezing, probably during periglacial periods. These have not been found so far in the British Isles. All natural stone circles degrade to form stone stripes on ground sloping more than five degrees (see Goudie and Piggott 1981; Tufnell 1985). The implications of this are that stone circles are likely to be cultural if (a) they are larger than 1 m or so in diameter and/or (b) they are on sloping ground. Straight lines of clasts may occur on steep ground due to frost sorting, but these lines are generally composed of small debris.

Cultural landscapes (Fig. 2 [pdf])

Elements of a landscape must be considered to be of cultural origin if they include:

1. Perched boulders on gently sloping ground or near hill tops

2. Isolated standing stones or standing stones whose buried portion is only a small proportion of the total length

3. Elongate clasts whose a-axes are at right angles to the dominant flow vector during mass wasting

4. Large-scale circular patterns in stone streams or stone circles on steeply sloping ground

5. Grooves on rocks found on bedrock or other rocks which are not parallel to the bedding planes or where the joint surfaces of the perched block are not aligned with joint surfaces on the resting block.

6. Straight lines of large clasts, especially when these are oblique to the slope angle.



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