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Report on the Sediment Stratigraphies of Three Shafts

Deborah. J. Long & Richard Tipping, Department of Environmental Science, University of Stirling. 1998.

summary | introduction | methods | sediment stratigraphies | discussion | radiocarbon dating | concluding remarks | references

Summary of Findings

Detailed semi-quantitative sediment-stratigraphic data are presented for three deep (2.5-2.8m) shafts closely grouped at Nosterfield sand & gravel quarry.

The morphological and sedimentological data support the archaeological interpretation of these shafts as anthropogenic in origin.

The sediments and infilling processes are complex and different between each of the shafts, allowing a relative sequence for the formation and infilling of the shafts to be suggested.

Two of the shafts (F44 & F45) clearly contained standing water, probably during use. The shafts are assumed to have served as water-sources. The third shaft (F46) has no evidence for standing water, but is thought to have ceased to function very soon after its excavation.

Sediments at critical depths for defining (i) the ages of formation and (ii) close to the end of sediment infilling are organic-rich and should be able to define the time-span covered by the deposits.

Introduction

Three features (F44 - F46 inclusive) in the Nosterfield (NON 98) sand-& gravel quarry were sampled on the 25th September 1998. This report describes the subsequent laboratory analyses, which focused on detailed semi-quantitative sediment-stratigraphic description in order to define the possible modes of formation of these features and the processes of sediment infilling. Features F44, F45 and F46 were identified during excavation as probably of anthropogenic origin, and sediment-stratigraphic description was designed to explore this possibility. An anthropogenic origin is confirmed as most likely, and the complex processes of sediment infilling are seen as resulting from a combination of recutting, probably whilst the shafts were in use, and post-use sediment accumulation.

We present a suggested relative chrono-sequence for the formation and infilling of these features derived from these descriptions. The report also defines the depths of critical sediment samples recommended for AMS 14 C dating, and details the significance of these assays in characterising the dates of sediment infilling.

In addition a sediment description of a 25.0cm monolith taken from the marl/peat boundary within the lacustrine deposit adjacent to F44-F46 is presented. Radiocarbon dating of the peat immediately above this sedimentological boundary is recommended, which will allow us to establish the age of terrestrialisation of the former lake and its relation to F44-F46.

Methods

The three features discussed in this report had been partially excavated prior to the sampling described here on September 25th 1998. Attempts to section the sediment fills, prevented by a high groundwater-table, made sampling with corers a little difficult. Nevertheless, all cores taken can be related to site and Ordnance Datum, levelled by Electronic Distance Measurer.

The sediment fills at and below the groundwater table were sampled using a 1.Om long, 6.Ocm internal diameter Russian peat-corer, samples placed in clean plastic guttering, labelled and wrapped in plastic. Monolith tins of 50.Ocm length had already been sampled in the overlying sediment fills; these were returned with the cores to Stirling University for sediment description.

Cores and monolith tins are stored in a refrigerator at 40C to prevent fungal growth. Sediments were cleaned in the laboratory, and sediment-stratigraphic descriptions carried out according to the conventions of Troels Smith (1955). This is a semi-quantitative objective descriptive method designed to produce a full, consistent and universally applicable appraisal of discrete sediment units (deposit elements).

Troels-Smith descriptions are presented in Tables 1-5. In the text simpler but less precise descriptions of the sediments are used.

Colours of sediments were described using Munsell Color Charts on moist sediment under artificial light.

Sediment Stratigraphies

The three features are described and interpreted in numerical order. Observations made during archaeological excavation on feature morphology prior to sediment sampling are incorporated. Sediment stratigraphies are described from base up, e.g. in the order of deposition.

F44 figure 1

F44 is the easternmost of the group of three features, closest to the former lake margin. Cut into the underlying gravel, F44 had a near-circular planform c. 3m in diameter, the uppermost fill lying at 39.50m). A shallow basin-like feature rapidly changes below c. 50cm to a steep- or vertical-sided shaft a little less than 3m wide.

The uppermost loamy inorganic fills (8055 and 8054) contain considerable quantities of modern pottery and are interpreted by the excavator as fills to level the depressions. Below these recent loamy fills is context 8056, described by the archaeologist as a pure peat. This and the underlying context 8057 were sampled by 50.Ocm monolith tin, and two overlapping Russian core samples in lower sediments, at the centre of the shaft. The lowermost Russian sample probably reached gravel but a 15.Ocm nose-cone prevents sampling of these basal sediments. The total depth of fill recorded in samples is 241.Ocm (Table 1); the base of the fill is probably 15.Ocm lower or at an altitude of 36.53m OD.

The basal fill (241-229cm) is an unstratified organic-rich clay with sand grains and common small rounded stones. Coarse components probably derive from the sides of the shaft, but the intimate mix of clay and amorphous organic matter suggests deposition and mixing in water. An abrupt boundary to overlying clay occurs at 229cm. Nearly a metre of completely minerogenic, structureless clays (although varying in particle-size from clay to silty clay) were deposited to 138cm. A well-humified peat at 191-195cm (Table 1) is clearly a discrete lens with a sharp boundary within the clay. Small rounded stones are rare. The clay is almost certainly a product of settling-out in standing water. During this period the stability imposed by a high groundwater table and equal pore-water pressure within the gravel and shaft may have prevented slumping of the shaft sides. Despite the abundance of minerogenic sediment in the shaft it is likely that the adjacent contemporary ground surface was vegetated because the sediment fill is largely limited to the clay-sized fraction of 'soil'. Sediment was probably sorted before deposition, possibly by a complete vegetation cover.

An organic-rich clay deposited between 138-134cm is oxidised, and is overlain by a similarly iron-rich sandy gravel (134-127cm). The gravel is very probably a collapse feature from the shaft sides, oxidised above the contemporaneous water level, and the underlying peaty clay may also be derived by shaft-edge collapse. Overlying these slumped sediments are a series of well-humified peats and well-defined clay lenses with sharp boundaries between 127 and 11Ocm, suggestive of intermittent though probably rapid infilling or of fluctuating water levels within the shaft: the peats probably formed on the wet surface of former sediments and are in situ; the clays may have sedimented out in ponds on the peat surfaces or be derived from slope-wash, representing single small-scale erosion events.

Above this is a thick well-humified structureless peat (110-72cm), probably that identified in excavation as context 8057, and a wood-rich peat between 72 and 60cm, it is not clear if these wood fragments are in situ. Finally, extending to context 8056 is an in situ peat of varying humification in which sedge remains (Cyperaceae), including Phragmites, are seen in poorly humified parts. These less decayed peats are felted, retaining the original sub-horizontal structure. This peat accumulated with no slumping or recognisable slope-wash activity. However, the uppermost 10 or so cm are severely disturbed by recent earthworm activity.

F45 figure 2

F45 is cut into gravel, the surface at around 39.50m OD. The feature is more neatly circular than F44, 2.2m wide and with little evidence for the surface horizons to be wider than the shaft. Archaeological excavation to around 60cm showed a sequence of shallow loamy and stony fills at the surface in the centre of the shaft (contexts 8066 and 8065) to 20cm depth, containing modern pottery. These are interpreted as fills to level the surface as at F44. Beneath these recent inorganic fills are a series of peat deposits (contexts 8067, 8068) sampled in the centre of the shaft by 50.Ocm monolith tin and two overlapping Russian cores. The total depth of fill sampled is 281.Ocm, as at F44 the Russian core probably grounded on underlying gravel but could not sample it, and the base of the shaft is estimated to reach 36.99m OD.

The basal sediment sequence (281-189cm) is an organic-rich clay with discrete thin pure clay bands. It is comparable to the basal sequence at F44 and probably represents mixing of mineral material with in situ amorphous organic matter in standing water. However, well-humified peats are also present and Phragmites stems are preserved, and if in situ, which is most likely, these peats formed in short periods when the water level was lowered. Wood fragments occur at 257-260cm, and may be inwashed. Small rounded stones are present, but in general there is little evidence for shaft-collapse or major erosional events. Between 227-221 cm a structureless silty clay was deposited in standing water, probably quite rapidly and suppressing organic matter production, but coarse particles were not inwashed. The silty clay is oxidised, probably indicating its derivation from a soil surface.

Above 189cm is 23cm of structureless and highly humified in situ peat formed under stable conditions, but above 166cm peat formation is interrupted by the deposition of discrete clay lenses. These clay bands probably represent the sorting of eroded soils by a complete vegetation cover and the washing into the shaft of only the finest particles, they may represent individual storm events and the setting-out of sediment following temporary flooding of the shaft. Recorded in excavation are a series of sandy silt slumps in the surface horizons (context 8069), predating peat infilling the top of the shaft, these may relate to this period of soil inwashing.

These intermittent but probably rapidly deposited sediment fills continue to 119cm, when minerogenic sediment inwashing ceases and structureless well-humified in situ peats accumulate. As at F44 the humification state of this final pure organic fill varies, and less decayed parts contain Phragmites (Cyperaceae) stems, preserved in growth position (i.e. vertically) above 50cm.

F46 figure 3

The surface of this feature lies at 39.93m OD, furthest from the lake margin. This is a distinctive funnel-shaped feature, over 4m wide at the surface, with a steep edge to the shaft at the east, a more bowl-shaped slope at the west. The shaft commences about 70cm below the present surface, and is assumed to be steep or vertical. The top of the funnel was excavated and seen to be infilled with context 8059, a crumbly dark brown loam rich in modern debitage, and context 8060, a discrete layer of pebbles and gravel extending across the depression. These contexts are interpreted as deliberate fills above the shaft, although the absence of artefacts in context 8060 is problematic in this regard.

Beneath these in section is an organic-rich loam (context 8061) that grades into the underlying black peat of context 8062. These contexts were sampled by 50.Ocm monolith tin in the centre of the shaft, and two overlapping Russian cores obtained deeper sediments. With the assistance of a JCB bucket the otherwise intractably stiff sediments were sampled and the Russian corer was able to demonstrate coarse gravels beneath the shaft fill. Samples were obtained to a depth of 250.Ocm but the contact between gravel and basal fill is at 223.Ocm or 36.92m OD.

The fill is very uniform, and comprises 215.Ocm of highly organic, well-humified in situ peat with rare herbaceous and woody remains visible. Accumulation appears to have been continuous. There is no evidence for mineral components and the diffuse contacts between peat units suggest undisturbed accumulation without wall collapse or minerogenic infilling. Minerogenic sediment is virtually absent until the admixed loam of context 8061, above 8.Ocm depth. Seen to be lining the sides of the shaft in excavation are sandy silts (context 8064), but these inwashed or slumped deposits predate all the peat infill of this shaft.

Marl/Peat Boundary (Find No. 14)

The altitude of this sequence is at 39.23m OD, not significantly different to the ground surfaces at F44-F46. The monolith tin was sampled through in situ lacustrine and fen deposits where undisturbed sediments are closest to F44, around 75m away to the north-east. Marls are recorded from very close (within 15m) to F46, however. The proximity of open-water sediments to the shafts leads to questions concerning their contemporaneity, to be resolved through 14C dating (below).

The horizontally colour-laminated marl is clearly an in situ lacustrine deposit rich in calcium carbonate. A well-defined transitional unit to the overlying peat is indicated by thin laminae and bands of alternately deposited mari and organic matter, before a well-humified structureless terrestrial peat replaces lacustrine sediment.

Discussion

(1) Probable Mode of Formation of F44-F46

The steep sides to these features are indicative of deliberate cuts into the gravel substrate. The abrupt boundary between gravel and well-humified peat at F46 also indicates that this is not a natural feature: these shafts are clearly anthropogenic in origin. It is not known whether the shafts were dug to their maximum depth at one time or in a more piecemeal fashion because the shafts could not be half-sectioned. It can be suggested that re-cutting or emptying of the sediment fill occurred during use in one shaft, F44 (below).

The sediments demonstrate that two shafts, F44 and F45, at one time contained standing water. This cannot be demonstrated for F46 although it is argued that this is because this shaft, dug last, became redundant through a drop in groundwater level before water-lain sediments could form. It is likely that all three shafts were intended to contain water. However, whether these were wells intended for human consumption of water is uncertain: the amount of clay sedimented out in F44 in particular indicates that the water in the shafts was at least periodically very muddy; further analyses will be required to define water quality from, for example, particle-size and diatom analyses.

The three shafts are cut to approximately the same altitude: F44 = 36.53m OD; F45 = 36.99m OD; F46 = 36.92m OD. These differences in altitude are not considered significant. At their fullest depths these shafts were probably dug to reach a groundwater table at roughly the same depth. However, current data cannot establish the original groundwater level: pollen and aquatic macro-plant analyses will help here. Sedimentological data indicate that during infilling the groundwater table varied, with some clay-rich fills probably indicating standing water, and well-humified peats suggesting drier conditions, although many of these sediment fills may not relate to use of the shafts (below).

The observation that all the shafts were cut to broadly the same depth does not necessarily imply that the shafts were dug at the same time because groundwater levels may have remained stable for long periods. However, the close grouping of these shafts suggests that one was visible or known when others were sunk. Equally, this close grouping might imply that not all shafts functioned at the same time, and that some shafts went into disuse before others.

(2) The Relative Chronology of Sediment Inkling

Non-contemporaneous use of the shafts is suggested by the very different stratigraphies of the sediment fills at each shaft. These major contrasts are interpreted here as representing different modes of sediment formation occurring at different times. This relative chronology is speculative and makes assumptions that are currently untestable.

F44 and F45 have comparable sediment infills at the base of the two shafts. Depositional conditions were of standing water where in situ organic matter, amorphous but perhaps derived from micro-faunal and floral components in addition to plant growth, was intimately mixed with clays being washed into the shafts by storm events and soil erosion around the shafts; small-scale slumping of shaft edges also occurred. Initial depositional conditions were comparable between these features which might suggest contemporaneous use of the shafts. In F45 this sequence of intermixed clays, organic matter and peat continues until replacement by in situ peat, and there is no indication that deposition was interrupted. The simplest interpretation is that F45 simply filled with water-lain sediments before these were replaced by peats.

At F44 this basal sequence is truncated. The abrupt boundary between the basal units and overlying clays is interpreted as the result of recutting, or an attempt to empty out the sediments accumulating in the shaft. The purely minerogenic clays and silty clays within F44 are not seen at F45. F45 was seemingly not re-cut; no attempt at rejuvenating the function of the shaft is indicated. So although F44 and F45 may initially have been dug at the same time, F45 was not cleaned out, and may have been abandoned before F44.

The clays of F44 are water-lain sediments, deposited when minerogenic input was much greater than at any time during the infilling of F45. This is a second reason for suspecting that F45 was infilled before F44. The source of these large thicknesses of clays in F45 is not presently clear. The sediments are assumed to derive by slopewash from gravelly soils, but sediment-sorting through the trapping of coarse particles by a complete vegetation cover, even of grasses, would seem necessary to explain the very fine particles being deposited in this shaft. The abundance of clays suggests that soil erosion, by wind, perhaps, as much as by water, was much more prevalent than during the infilling of F45. Reasons for this could relate to an intensification of settlement or land-uses around the shafts, closer proximity of these activities to the shafts, or to climate change to increasing storminess or lower groundwater tables. Evidence is recorded for only one episode of re-cutting at F44, and eventually peat accumulated in the shaft.

The peat sequences in the upper parts of F44 and F45 are interpreted as representing post-use phases of sediment infilling; it is difficult to see these shafts functioning as sources of water when the sediment indicates only moist terrestrial conditions.

F46 is astonishingly different in its absence of water-lain sediments. The basal fill of this shaft is a well-humified peat which at F44 and F45 is interpreted as an indicator of abandonment. It is difficult to see this peat as anything other than a post-use indicator here also. The absence of water-lain sediments may be interpreted in several ways:

1. the function of this shaft was different to F44 and F45, but morphologically these three shafts are strikingly similar;
2. water-lain sediments formed during use were removed by a re-cutting episode, after which the groundwater table fell rendering the shaft non-functional;
3. similar but simpler, groundwater fell after initial excavation but before the shaft could contain water.

Given that both F44 and F45 clearly contained water at some time, either of the latter two explanations for the sequence at F46 would most reasonably imply that F46 was dug later than the other two shafts, close in time to abandonment of the features.

(3) Relation of F44-F46 to the Lake Sediments

Discussion of the role of groundwater fluctuations in the use and abandonment of these shafts necessitates evaluation of the lacustrine sediments only tens of metres to their east and north. A marl lake certainly existed at some time in the Holocene period, and this was replaced by terrestrial peat. It is assumed here that if the shafts were dug to seek water, the lake had by then ceased to exist. This is not demonstrable from existing stratigraphic relations but can be established by 14C dating.

Radiocarbon Dating

Radiocarbon dating is necessary to establish

1. the ages of the three shafts F44-F46,
2. the duration of their infilling,
3. possibly differences in the periods of use of the three shafts and
4. the temporal relation between the lacustrine sediment sequence and those in the shafts.

Objectives (a), (b) and (d) are readily attainable. Objective (c) is less clear-cut. the sequence of use and sediment infilling within F44-F46 is a relative one, and all of these changes may have occurred over time periods too short to be resolved by 14C dating.

The sequences in these shaft sediments have a very high potential for resolution by 14C dating. The sequence is complex, and in time it may prove necessary (and will be easily achievable) to define for each of the shafts the following changes:

1. a terminus post quem age for initial excavation of each shaft;
2. a terminus ante quem age for re-cutting in F45;
3. the ages of homogeneous peat-formation, possibly indicating the disuse of each shaft-,
4. palaeo-hydrological and palaeo-ecological changes;
5. the ages of cessation of peat-growth at the tops of the shafts.

Such a dating programme is ambitious, and here it is recognised that the fundamental concern at present is to bracket the time periods represented by the shaft deposits (Points (1) and (5)).

Terminus post quem ages for initial excavation of each shaft

In situ organic sediments are recorded from the basal sediments of each of the shafts. In F44 and F45 these are pond-sediments with organic contents estimated to between 10-20% (F44) and >75% (F45). In these deposits there is a possibility of some organic matter being reworked from surface soils by shaft-edge collapse, but this is thought to be negligible. In F46 the basal fill is a highly organic (c. 80-90%) in situ peat.

Age/s of cessation of peat-growth at the tops of each shaft

The topmost peats within the monolith tins have evidence of drying, probably through 19th century drainage and bioturbation. However, below c. 20cm from the top of each sequence the peat is moist, with good plant preservation and no evidence of bioturbation. Assays from these localities will provide very secure age-estimates close to the age of cessation of peat infilling.

The age of the marl/peat boundary

A 14C assay is also requested on the terrestrial peat immediately above the marl described in Find no. 14, providing a terminus post quem date for the terrestrialisation of the former lake. Identification of the precise depth within the peat would be defined after laboratory analyses for loss-on-ignition, CaCo3 content and pH to avoid the possibility of hard-water contamination.

Dating

AMS 14C techniques will need to be employed because of the paucity of available sediment. 'Beta Analytic Inc.’, Miami, Florida can process the results in between 14 and 50 days of receipt of samples. Currently seven assays would be required.

Concluding Remarks

It should be stressed that the interpretations derived from these sediment-stratigraphic data are not conclusive and that the reconstruction attempted here is only one possible sequence of events. However, this reconstruction is supported by the limited evidence available to date and provides a testable model for future work. AMS 14C radiocarbon dating of six assays from tops and bottoms of the shafts is feasible and would place these sediment infills into a broad temporal context.

The potential of the deposits described in this report is very great in enabling a detailed temporal reconstruction of landscape change. Depending on the broad age-ranges established from this recommended dating programme and their relevance to existing archaeological finds in the region, further analyses might be encouraged. These would include further 14C dating to refine the sequence further (above), together with sedimentological analyses to define more precisely the depositional mechanisms, and palaeoecological analyses (pollen; diatoms; plant macro-remains) to define the local vegetation and land-use history, water depths and water quality at times during sediment infilling.

References

Troels-Smith, J. (1955) Karakterisering af lose jordater. Characterisation of unconsolidated sediments. Danmarks Geolosika Undersogelse iv, 3, 39-73.

Tables & Figures

Table 1. Detailed Sediment Stratigraphy of F44.

Table 2. Detailed Sediment Stratigraphy of F45.

Table 3. Detailed Sediment Stratigraphy of F46.

Table 4. Additional stratigraphic evidence at F44, F45 and F46.

Table 5. Sediment Stratigraphy of marl 1 peat interface (Find no. 14).

Figure 1. Troels-Smith Depiction of the Sediment Stratigraphy of F44.

Figure 2. Troels-Smith Depiction of the Sediment Stratigraphy of F45.

Figure 3. Troels-Smith Depiction of the Sediment Stratigraphy of F46.

© Archaeological Planning Consultancy Ltd 2003 - 2006
URL: http://www.archaeologicalplanningconsultancy.co.uk/mga/projects/noster/speciali/long98.html
HTML Markup: Guy Hopkinson • email: . Updated February 6, 2004

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