Investigation of the Wishaw Burn next to Trench 6 by coring

The coring programme was undertaken in the summer of 2014, by Dr Richard Tipping of Stirling University, and concentrated on the investigation of the area of the Wishaw Burn to the south-west of Trench 6. The intent was to investigate the extent of the timber platform, identified within the trench, and to gain an understanding of the sediment sequence within the narrow channel that today contains the burn. A series of 31 boreholes were sunk with a Eijelkamp 2.5cm diameter peat gouge. The location of the sequence is depicted on the figure below, with each borehole numbered in the sequence in which it was recorded.

The sediment layers of the Wishaw Burn are dominated by two types of peat, an amorphous peat with no recognisable plant matter, and a herb peat, which retains much recognisable plant material.

In addition a substantial wood content has also been noted, in the case of the present samples, this component varies between fragments and round-wood. In the case of the round-wood component, the original form is maintained, often including bark.

The borehole location plan, showing transect A as blue dots and Transect B as red dots.
The borehole location plan, showing Transect A as blue dots and Transect B as red dots.

Interpretation

The report attempts to describe the sequence of events, in chronological order, as far as they can be constructed from the observed data. Some 13 phases of activity has so far been identified and are described below:

(1) The shape of the channel

The cross-section of the Wishaw Burn has been defined by Transect A (marked in blue on the figure), and has been found to be asymmetric, with a steep side on the north-east side and a more gradual slope on the south-west side (though the bank itself has so far not been reached on this side of the burn).

(2) The base of the channel

The base at the north-east side of the channel was identified as boulder clay, but over much of the transect the basal layer was an impenetrable stone, which may represent bedrock, but is more likely to represent a gravel base.

(3) Contrasts in basal sediment along Transect A

A major contrast in the basal sediments between boreholes north-east of B15 and those SW was noted. To the north-east of B15 mineral sediments with little or no organic matter were identified. To the south-west stone is overlain by herb peat entirely free of mineral sediment. The two sediments cannot have been laid down at the same time, and the peat layer is proposed as the later deposit.

(4) Erosion of the clay-dominated pond fill

The best explanation for the isolation of the clays on the north-east side of the burn is that an erosion event occurred. It is possible that this was severe enough to erode the south-west bank itself.

(5) Armouring the wider channel with course gravel

It is thought that the reason that the boreholes were stopped from penetrating deeper, south-west of B12 on Transect A, and all boreholes south-west of the channel edge on Transect B (marked in red on the figure), by stone, is because of the presence of coarse fluvial gravel that lined the channel floor, in a process called armouring. The movement of these gravels would require stream flow rates higher than any that have been seen since.

(6) The earliest sediment of the widened channel

Following the erosion and armouring events a structureless silt, seen in boreholes B1 an B14 on Transect A and B23 in Transect B, was laid down. The silt represents flowing water, but with no significant stream power, and the absence of the silt in other boreholes could mean that the stream at this time was quite confined. The increasingly clay rich quality of this sediment in B23 may hint at ponding. The basal sediments in B1 contain amorphous organic matter (AOM), which is significant of change in the local environment and it is assumed that this is in the form of increased plant and algal biomass in the channel. This can be an indication of warming of the water.

(7) Establishment of herb peat in the channel

The herb peat represents a valley mire or marsh environment. With sufficient water, such an environment will accumulate live material building up on dead material, which has not decayed. Such an accumulation could have the affect of raising the water table as the channel became choked with marsh vegetation. With almost no gradient to the burn the water would have drifted through the channel rather than flowed. Wood is a rare component and the few examples may represent trees that were growing close to, or within the channel, before the spread of herb peat forced them away. The presence of two hazel nuts and a fragment of charred wood hints at the presence of humans in the area, but this is not the only explanation for the presence of such material.

(8) Shell fragments and the development of an open water phase

In this phase shell fragments are found, common to abundant, in boreholes B1, B12, B13 and B14. The shell fragments are associated with the in-washing of mineral matter. To the south-west herb peat continued to grow unaffected by mineral sediment. As a result it is thought that perhaps a pool formed only around boreholes B12 to B1.

(9) The persistence of open water at B12

Above the peat with abundant shell fragments standing water persisted, as thin layers of pale grey to fawn clay was seen stratified with herb peat. The colour changes in the clay are thought to signify different organic contents, and the lamination could be seasonal markers.

(10) Soil surfaces and their disturbance on the north-east bank

Above the deposits of shell fragments there is another major difference in the sediments forming north-east and south-west of B12 on Transect A. To the south-west the herb peat continued to form, with interruptions, B11, B10 and B2 have sediments that are highly disturbed, perhaps from trampling, separated by sediment units where trampling cannot be identified. Trampling does not necessarily need to represent the presence of people, but in this case, given the general context, it is highly likely. Given this, it is worth considering what we know of the area at this time. There was no significant depth of water and the water surface is thought to have lain at 29.9m OD. It would have lapped the feet of people if they stood at the site of B11. Open water would have lain some 3-4m out from B11, but barely flowed most of the time. The depth of the marshy peat, in the middle of the channel, some 4m out, was less than 1m.

(11) Soil erosion on the north-east bank and re-deposition in the channel

Though the evidence for soil disturbance by trampling is episodic, many near-shore boreholes suggest sustained mobilisation and re-deposition of mineral sediment. Soil erosion occurred within and between mineral sediments that contain common-abundant wood, which is interpreted as human in origin. It seems clear from its distribution that the in-washed sediment originated from the location of the timber structure that has been identified by excavation within the extensions of Trench 6.

(12) The addition of wood to sediment on the north-east bank

Wood was very rare to rare in earlier deposits, but during the most intense phase of soil erosion and re-deposition on the north-east bank, wood becomes abundant in the ‘nearshore’ pond deposits and clays. The overwhelming bulk of the added wood is of tiny-small smashed fragments. Large round-wood was added only at one time. Large-very large fragments were added in the middle of the sequence and appear to have been oriented vertically. This accumulation of wood, first in mud, and then, above 31.2m OD, in herb peat, continues to 32.3m OD, some 1.6m. It is assumed that this wood was added to sediment at or under the contemporary water surface as it continued to rise. As the water rose the north-east bank would have been pushed north-eastwards.

Boreholes B21, B19, B25 and B26 contain wood in abundance and suggest that structures similar to that in Trench 6 were constructed along a 10m length of shoreline.

(13) Addition of wood to herb peat in the channel

At B10 and in the area to its north-east, on the sloping river bank, the addition of wood on soils and in mud was probably a comparatively easy task. Further out, towards the deeper part of the channel, the task would have been more difficult as wood was being added to wet and unconsolidated marsh vegetation and peat. This is important as the horizontal bedding shows that there has been no significant deformation of the timber structure, indicating that additions were in response to a rising water table and not the sinking of the existing platform. It is assumed that the addition of wood in the channel would be constrained by the water table, unless the timber was pegged down. The vertical or near vertical timbers identified both during the excavation and within the boreholes are believed to have performed this role. Even so, it could be expected that laid timber would at times float away, complicating the interpretation of wood within the boreholes. As a result it has been decided that intentional addition is assumed when pieces of wood are abundant and associated with the in-washing of eroded soil. Using this we are tentatively able to describe the spatial and temporal development of the wooden structure.

(a) The earliest additions of wood: 30.9m OD to 31.1m OD, at B14, B10 and B11.

(b) 31.1m OD to 31.5m OD: wood may have been added at B2 linking the structure to the shore as the water rose. Wood was also added at B15 and B25.

(c) 31.5m OD to 31.8m OD: wood was dropped into the peat at B8-B2 and also intensionally added to B12 at this time. On Transect B abundant wood was added for the first time at B21, B19 and B26. This appears to represent the greatest extent of the timber structure.

(d) 31.8m OD to 32.3m OD: the marsh that had formed between B8 and B2 had little or no wood added to it. Above 32.3m OD wood becomes less than abundant in boreholes where it had previously been abundant. It is more likely that aerobic decay is affecting preservation above these levels.

(e) cessation of the peat stratigraphy: sediment formed after the addition of wood is absent except at B11. A dry gleyed soil overlies peat everywhere and the boundary between peat and soil is gradual, suggesting that the soil is a product of peat mineralisation.

Conclusions

The coring has been very successful, and has been able to characterise the sediments in the channel and has been able to define in great detail the environmental changes within the Wishaw Burn. In addition the coring programme has been able to map the timber structure, not just in extent, but also to identify phases within its construction.

If you would like to read Dr Tipping’s interim report then click on the title below:

Wishaw Burn and Trench 6 Interpretations