‘Indirect’ Evidence in Assessing the Lower and Middle Palaeolithic: two case studies from the West Sussex coastal plain

Executive Summary

West Sussex Coastal Plain: Quarry expansion in raised beach deposits

Indirect means of assessing archaeological significance are commonly used in Pleistocene contexts given the properties of Palaeolithic sites and the thickness of associated stratigraphic sequences.

Approaches comprise the excavation of test pits and drilling of boreholes, and less commonly, use of electrical conductivity tomography and other geophysical techniques. All have been employed on the West Sussex coastal plain, where fine-grained strata associated with Pleistocene raised beaches have been shown to be of considerable archaeological significance (most famously at Amey’s Eartham Pit, Boxgrove).

Two such test pit/borehole studies carried out prior to quarry expansions are discussed here: Lavant Quarry, which lies about 280m north of the supposed Aldingbourne raised beach, and Drayton Lane Oving, 1.2km south of the Brighton–Norton cliff line. The results from both investigations demonstrate the value of indirect evidence in improving stratigraphic/geomorphological models and hence future archaeological mitigation.

Despite field observations to the contrary, subsequent laboratory sedimentological analysis demonstrated that deposits of the Aldingbourne raised beach were not present in the proposed Lavant Quarry extension. On the other hand, raised beach deposits do outcrop on the Drayton Lane site, albeit that the sequence is entirely of sands, suggesting that archaeologically significant Norton Silts outcrop north of the location.

Keywords

Where: West Sussex (quarry sites)
Region: South-East
Palaeolithic period(s): Lower Palaeolithic (MIS 13–7)
Type of investigation: Fieldwork
Methods: Boreholes; Trenches; Deposit Modelling
Type(s) of deposit: Raised beach; Solifluction deposits

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Development Context

The West Sussex coastal plain is unusual in the British Isles in preserving a ‘staircase’ of Pleistocene raised beaches dating between >500,000 and 120,000 BP (Figures 1–2) (Bates et al. 1997, 2010; Roberts 1999; Roberts and Pope 2018). However, it was only in the 1980s that the plain was first recognised for its importance in preserving Lower Palaeolithic archaeological remains in in situ contexts (Woodcock 1981; Roberts 1986).

It is within this geological and archaeological paradigm that the impact of commercial (mostly aggregate exploitation), housing and infrastructure development on the cultural heritage resource of the West Sussex coastal plain is assessed.

This contribution discusses two so-far unpublished studies (Lavant Quarry, Chichester and Drayton Lane, Oving, Figure 1), where boreholes and test pits have been used to map and indirectly evaluate the archaeology of Aldingbourne and Brighton–Norton raised beach strata prior to quarry expansion.

The aim is to set out both the cultural resource management (CRM) and scientific value of the evidence that was obtained.

Archaeological Context

The investigation and subsequent discovery of hominin fossils in deposits associated with the Slindon raised beach at Amey’s Eartham Pit, Boxgrove, in the 1990s prompted a series of investigations to map, date and determine whether Palaeolithic evidence existed in other raised beaches (Roberts et al. 1994; Bates et al. 1998a; 2000; 2010; Roberts and Parfitt 1999; Roberts and Pope 2018).

As a result of these the Marine Isotope Stage (MIS) 13 Goodwood–Slindon raised beach is well mapped and is known to contain Palaeolithic remains at various localities.

The MIS 11, 9 or 7 Aldingbourne raised beach is associated with some Palaeolithic artefacts, but its extent is less certain, while archaeological materials have yet to be found in the MIS 7 Brighton–Norton and MIS 5e Pagham raised beaches (see Figure 2 for chronology) (Bates et al. 1998a; Pope et al. 2009; Roberts and Pope 2018).

The mapping of these latter three is also less well constrained than those of the earlier raised beaches.

Methodology & Research Questions

The Pleistocene deposits of the West Sussex coastal plain are a combination of marine gravels, sands and silts (i.e. the raised beaches) forming during interglacials, solifluction debris derived from the Chalk of the South Downs to the north during cold stages, and fluvial deposits from rivers crossing the coastal plain in both warm and cold stages (Bates et al. 2010).

Palaeolithic archaeological remains, or rather those that retain relative coherence as discrete in situ assemblages, are associated with fine-grained facies of the raised beaches (Roberts and Parfitt 1999; Pope et al. 2009). The long-term uplift of southern Britain means that the oldest raised beaches are at the highest elevation and furthest from the present coast, while beaches from later interglacials are progressively lower and closer to the coast (Figure 1) (Westaway et al. 2006).

The Pleistocene stratigraphy of most parts of the plain is thick (ranging up to 12m below ground level [BGL]), as a result of the dynamic nature of the depositional environments outlined above, in turn meaning that conventional archaeological trenching cannot often be employed as a means of prospection. Rather investigation of the Pleistocene archaeology of the West Sussex coastal plain has been by indirect means, i.e. through a combination of geotechnical-style test pits and boreholes (as reported here), and occasionally electrical resistance tomography (ERT) (e.g. Bates et al. 2000; Roberts and Pope 2018).

Such geoarchaeological approaches are intended to locate and map in three dimensions those strata most likely to contain near in situ archaeological remains. These strata, if found, are then sampled in archaeological trenches in order to determine whether artefacts are in fact present. In the case of the Lavant Quarry site, test pits had been dug during geotechnical works preceding the archaeological investigation, although unfortunately not in that part of the site with the greatest archaeological interest.

The first stage of the geoarchaeological investigation at Lavant Quarry therefore comprised the transcription of test-pit locational and lithostratigraphic data into the geological utilities program, RockWorks 15 (RockWare 2012).

Preliminary cross-sections were then produced in RockWorks and used to plan the locations for four geoarchaeological boreholes which were then drilled using a sonic drilling device (Figure 3) (Boart Longyear 2017).

At Drayton Lane, Oving, following a test-pit study, six boreholes were drilled using a cable-percussive rig equipped with a baler auger to penetrate solifluction deposits and a U4/100 sampler to collect continuous cores from the base of the solifluction to the top of the London Clay bedrock.

Borehole lithology was recorded in the field (deposits recovered in the baler auger at Oving and the sonic cores from Lavant Quarry) and laboratory (U4/100 samples from Drayton Lane, Oving) using standard descriptive systems (Jones et al. 1999; Munsell Color 2000; Tucker 2011), and the resultant descriptive data transferred to RockWorks.

Results

Lavant Quarry: stratigraphy

The Lavant Quarry extension lay within 200m of the mapped position of the Aldingbourne raised beach (Figure 1B) (Shephard-Thorn et al. 1982) and was investigated both to test the accuracy of the mapping and determine whether potentially important fine-grained intertidal deposits outcropped within the proposed quarry footprint.

The White Chalk Subgroup bedrock was found to slope from +28.5m OD in the northern part of the site to +22.0m OD in the south, albeit that depressions within the Chalk (e.g. to +16.5m OD in LQ 2008 BH2) suggest that channels are present (Figure 4).

While there is no obvious cliff line indicated by the Chalk outcrop, 0.85m of strata in the south of the site at LQ 2008 BH4 were categorised in the field as possible marine and intertidal deposits (at +22.50 to +21.65m OD) (Figure 4). These comprised 0.25m of laminated yellow brown clays overlain by 0.50m of bedded (in 0.1m-thick sets) medium sands and in turn succeeded by 0.10m of red-brown clays containing moderate quantities of rounded flint pebbles.

However, given the uncertainty of the field attribution, laboratory mineralogical and granulometric studies were carried out and demonstrated that the strata were in fact alluvial facies of the Lambeth Group (Eocene). The Lambeth Group in LQ 2008 BH4, and the Chalk in all other boreholes and test pits was overlain by poorly sorted sets of predominantly cobble and pebble-sized flint gravels in a fine sand and silt matrix.

These latter are thought to have formed in periglacial environments and are attributed to the Chichester and/or Nyton Members (Table 1 and Figure 2). Brickearths cap the periglacial sequence.

Drayton Lane, Oving: stratigraphy

The proposed quarry at Drayton Lane, Oving, lies 1.2km south of the mapped Brighton–Norton cliff line (Figure 1A and Figure 1C). Geoarchaeological work was therefore to determine whether fine-grained deposits of the Norton Formation outcropped on the site and if so, to assess their archaeological and palaeoenvironmental significance. Pleistocene deposits were found to outcrop unconformably above London Clay bedrock at between +4.5 and 6.0m OD and comprised:

i. Between 0.8 and 0.2m of poorly sorted, angular matrix-supported flint gravels infilling a depression represented by DLO BH4 and BH5. Probable solifluction deposits.
ii. 0.5–1.2 of rounded flint gravels in a coarse sand matrix and containing fragments of Cerastoderma sp. in DLO BH2 and BH3. Beach gravels.
iii. 1.5–2.6m well-sorted medium sands with Cerastoderma sp. fragments overlying i., ii and in DLO BH6, the London Clay (Figure 5). Beach sands.
iv. 1.1–1.2m of fining upwards chalk gravel in a chalk mud matrix in BH1 and BH2. Solifluction.
v. 1.5–6.0m of fining upwards clast- and matrix-supported angular and sub-angular flint gravel overlying iv in BH2 and BH3, and iii in all other boreholes. Solifluction/fan gravels.
vi. 1.2–1.8m of yellow brown silts with occasional sub-angular and sub-rounded flint pebbles. Brickearth.

Significance

Lavant Quarry

Despite initial indications to the contrary, deposits of the Aldingbourne raised beach were not found in the proposed Lavant Quarry extension and thus the Palaeolithic archaeological significance of the site was assessed as being negligible. However, the geoarchaeological investigation did provide relevant CRM information in demonstrating that the northernmost outcrop of the Aldingbourne raised beach lies south of the Lavant Quarry.

Drayton Lane, Oving

The beach deposits at Drayton Lane can be attributed to the Norton Formation, a unit that has possible archaeological significance. At the Norton Farm stratotype, the Norton Sand Member outcrops between +6.0 and +11.8m OD, while the Norton Silt Member occurs between +9.0m OD and +12.2m OD (Bates et al. 2000).

The Norton Sand outcrop at Drayton Lane has a maximum elevation of +8.78m OD (in DLO BH5) and it is therefore probable that the site was too far south of the contemporary cliff line for intertidal deposits, i.e. the Norton Silts, to form. Given that the latter represent the most likely position for near in situ archaeological preservation, the raised beach deposits at Drayton Lane were not assessed as being of archaeological significance.

Key Insights

As has been pointed out by Bates et al. (2010), periglacial activity and the formation of fan gravels has hidden topographic features on the West Sussex coastal plain that might otherwise have been used to map former cliff lines and raised beaches. Except in the case of the Goodwood–Slindon raised beach, where a great deal of intrusive work has been undertaken (Roberts and Pope 2018), the positions of cliff lines are approximations. Therefore, the results reported above — and indeed those of other CRM investigations of Pleistocene strata on the West Sussex coastal plain — demonstrate the value of an indirect geoarchaeological approach to archaeological prospection.

The absence of the Aldingbourne raised beach deposits at Lavant Quarry indicates that deposits of this feature do not extend to the site and must therefore only outcrop to the south. In a similar manner the absence of the Norton Silt Member at Drayton Lane, Oving, further restricts the area of archaeological potential south of the mapped Brighton–Norton cliff line.

To generalise, therefore, as more geoarchaeological investigation is carried out on the West Sussex coastal plain, the resolution and precision of maps showing the distribution of strata of high archaeological potential will be refined. Thus, resources can be focussed on those areas of greatest importance.

Other conclusions resulting from the two projects are methodological in nature:

1. Lithostratigraphic recovery was markedly superior in the U4/100 cores obtained with the cable percussive rig when compared to the sonic drilling device. However, a problem with use of a cable percussive rig is knowing — particularly for a previously uninvestigated site — at what point to switch from a baler/gouge auger (from which intact samples cannot be retrieved) to a U4/100 sampler (to recover cores).
   
   Since the 2008 investigations at Lavant Quarry, ARCA have employed a dynamic probe rig for all investigations of the West Sussex coastal plain and elsewhere (Geotechnical Engineering 2017). The latter device retrieves continuous 1.5m long cores of a comparable quality to those achieved with a U4/100 sampler, but continuously through a sequence.
2. A difficulty remains with what mitigation approach is taken once strata with archaeological potential are revealed by geoarchaeological investigation. Had the Lambeth Group deposits in the Lavant Quarry at 6.7m BGL instead proved to be strata of the Aldingbourne raised beach an archaeological evaluation trench with a surface expression of 10x10m and removing 326.9m³ of overlying sediment would have been required to expose an area measuring just 2x2m.
   
   Had Norton Silts been found at Drayton Lane, Oving, such a trench would have been even more challenging to excavate given that it would have had to be dug several metres below the water table. In other words, the only realistic option for archaeological mitigation in such circumstances is an augmented watching brief undertaken during aggregate extraction.

Acknowledgements

John Mills, West Sussex County Council, is thanked for his help and enthusiasm on matters Palaeolithic during both the projects reported here and others conducted by ARCA on the West Sussex coastal plain. Funding was from Tarmac PLC in respect of the Lavant Quarry site and Hall Aggregates (South Coast) Ltd for Drayton Lane, Oving. ARCA's work on both projects was on behalf of Cotswold Archaeology, for whom the project managers were Cliff Bateman (Lavant) and Neil Holbrook (Oving).

References

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