Preliminary Investigation and Interpretation
A LiDAR image of the same feature, collected during a bathymetric survey conducted by National Oceanography Centre provides a more details1. Figure 2. The LiDAR has also defined the trend of the underlying bedrock, which is estimated to be approximately 300 to the alignment of the structure.
In August 2023, during a low spring tide, it was possible to wade out to the most inshore of the stone barriers. A bearing to the structure was established along the wooden groyne extending from the Bandstand. A hand bearing compass was used until a line of round boulders were encountered.
Using snorkelling gear an attempt was made to visualise the boulders, but visibility was virtually zero. It was possible to gauge the size and shape by feel. The boulders extended either side of the initial contact point along a line bearing 0300 forming a low-lying barrier approximately 2m wide
The alignment of the boulders was established by wading along their contact with the seabed, using landmarks to judge bearing. Alignment conformed very much to that arial and LiDAR images, confirming the feature had been located. The boulders are likely to be Upper Greensand and between 0.5 and 1.0 m in size and at time of low water at a depth estimated to be between 1 and 1.2 m, at initial contact point.
The size of the boulders and trend of underlying geology demonstrates that the structure is discordant with the local geology and geomorphology. Therefore, likely of anthropogenic origin.
Initial Estimate of Age
To function a fish weir must be covered at HW and drained at LW. The depth of the far wall is estimated to be 3 m at spring LW. Assuming tidal ranges were the same during time of operation another 2 m is added to allow for difference in heights between neaps and spring. At a current estimated depth of 5m the fish weir would have been functional 5000 years ago using a eustatic sea level rise rate of 1 mm per annum.
Other Considerations
The apparent opening to the fish weir faces north westwards, towards Carlise Road. This connects to Devonshire Park Tennis courts, the former site of a Roman Harbour. It is possible that a river existed between the tennis courts, and the fish weir used to trap fish swimming down stream. However, the size and configuration in relation to the shore alignment are more in keeping with the round ecluse of Isle de Rei. These are designed to trap sea fish swimming inside during high water, which are recovered at low water.
Extant stone fish weirs are still being used at Minehead Bay, Somerset Levels. A photograph within an Historic England publication illustrates a similar stone structure2. The dimensions of the boulders, width and height are similar to the estimated dimensions of the structure lying off the Bandstand.
Further Investigation
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Under water survey: film, still camera, measurements, and positioning survey using tethered buoys
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Depth of supporting bedrock below sand veneer. This can be done with graduated rods.
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Establish accurate water depths of structure with respect to top of boulders and supporting bedrock, and use this to refine estimate of age.
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Compare the form and build characteristics with other examples.
References
1A. Colenutt & J. Evans 2014
Seabed Mapping: Dungeness to Selsey
Channel Coastal Observatory National Oceanography Centre European Way Southampton SO14 3ZH.
2Historic England 2018 River Fisheries and Coastal Fish Weirs: Introductions to Heritage Assets. Swindon. Historic England.