Event/Activity EPM292 - Geotechnical Survey and Watching Brief Southsea Coastal Defence Scheme, Sub Frontage 2

Wessex Archaeology carried out geoarchaeological and archaeological monitoring of ground investigation (GI) works undertaken as part of the Southsea Coastal Defence Scheme between the 12th and 22nd September 2023. The GI works consisted of ten cable percussion boreholes and six cone penetration tests which were undertaken following the excavation of ten initial Inspection Pits. The works were situated north-west of Pier Road to the rear of Sub Frontages 1 and 2 of the Southsea Coastal Defence Scheme, and fell partially within the extent of the former post-medieval town defences. They did not fall within the Long Curtain, King's Bastion and Spur Redoubt Scheduled Monument. A watching brief was conducted during the hand excavation of the ten inspection pits which measured 1.2m deep by approximately 0.3m in diameter. Two of the pits contained archaeology in the form of masonry, including a section of wall, thought to relate to the 17th century defences designed by Sir Bernard de Gomme. Historic plans suggest the pits were probably situated within a section of the inner glacis. Geoarchaeological assessment was carried out on the boreholes and a programme of deposit modelling undertaken to assess the geoarchaeological potential of superficial deposits at the site and their potential to preserve palaeoenvironment and dating evidence. The assessment was combined with the results of previous geoarchaeological work carried out by Wessex Archaeology in Subfrontages 1 and 2 in 2018-19 in order to update existing deposit models. Eight main stratigraphic units were identified. Data coverage across the site was good and the sediments modelled fit well with the previous model for the site and the British Geological Survey records. The updated deposit model for Frontage 1 and 2 confirmed the presence of a deeply incised, broadly north-east/south-west paleochannel filled with alluvium interbedded with organic alluvium and possible peat. The alluvial and peat deposits encountered within the evaluated boreholes likely reflects a former wetland environment across the south of Portsea Island. The alluvium represents mudflats and saltmarsh accumulating under the influence of sea-level rise, with the peat deposits representing periods of stable and/or falling sea-levels. The peat and alluvial deposit remaining undated but the elevations for peat around -0.40mOD suggests that they may be contemporary with peats dated to the Neolithic and Bronze Age period in the area. Although the sequence is undated its likely Holocene in date and have a high potential for preservation of paleoenvironmental and archaeological material, therefore further work (purposive geoarchaeological boreholes) is recommended to date the sediments in order to set the sequence in its archaeological and paleoenvironmental context. Portsmouth Museum Service Accession No: 2020/51/2