Castlegate, Sheffield: Geoarchaeological Monitoring of Ground Investigation (GI) works

The GI works undertaken by GEL at the Site comprised: - 12 no. Dynamic Sampling boreholes (DS101-DS204B), undertaken using a rotary drilling rig equipped with a dynamic sampling head; - 9 no. Rotary Core boreholes (VC201-VC207); - 9 no. Vertical Concrete Core boreholes (VC101-VC109), undertaken to determine the location of either the culvert or rockhead; - 8 no. Horizontal Concrete Core boreholes (HC101-HC203); - 9 no. Observation Pits (OP101-OP111), undertaken to inform the location of concrete footings associated with the remaining structure of Castle Market; - 2 no. Inclined Concrete Core boreholes (IC101 and IC 102), and - 8 no. Trial Pits (TP101-TP204). Of these interventions, Dynamic Sampling boreholes DS101 and DS201-DS204, Rotary Core boreholes VC201-VC207 and Horizontal Concrete Core boreholes HC104 and HC105 were subject to monitoring, along with four trenches excavated specifically for Unexploded Ordnance (UXO) clearance (Trenches UXO11, UXO16, UXO17 and UXO35), Observation Pits OP101 to OP111, trial pits for Horizontal Concrete Core boreholes HC201-HC204 and Trial Pits TP101 to TP104. Deposit modelling The deposit model presented in Wessex Archaeology (2023) was updated to include the data arising from the geoarchaeological monitoring, and a review of the geotechnical logs arising from the remaining interventions that were not monitored. All available data points were entered into industry standard geological utilities software (Rockworks" 23). An updated model for the surface of the bedrock at the Site was generated using an inverse-distance weighted (IDW) algorithm. Two-dimensional stratigraphic profiles ('transects') of selected interventions across the site were also generated using RockWorks 23". These include west-east orientated Transects 1 to 3, and north-south orientated Transects 4 and 5 showing the main stratigraphic units and their lateral and vertical variability across these areas of the Site. A significant proportion of the sediments overlying the bedrock encountered during this phase of GI works have been assigned a post-medieval or modern date, and comprise the various elements of the stratigraphic unit referred to here as 'Made Ground'. In addition, only OP101 and TP101 were undertaken within the anticipated area of the castle motte, and both of these were excavated to levels that recorded only post-medieval/modern made ground/brick structures. No additional information can therefore be provided for the research questions associated with the castle motte. However, additional stratigraphic data has been provided associated with the castle moat. Suspected moat deposits were discovered in the south-west of the site underlying modern made ground and concrete, with these deposits being identified in borehole DS101, close to Trench 9 and a series of boreholes undertaken during the 2018 archaeological investigation (Wessex Archaeology 2020). Given that the scope of the GI works was engineering led, it was not possible to obtain samples from any of the deposits associated with moat fill. Previous palaeoenvironmental assessment and radiocarbon dating of organic moat fills at the site (Wessex Archaeology 2019d) demonstrated the complex taphonomy of moat fills, with significant age inversions noted in the results of the radiocarbon dating, and a high degree of uncertainty was placed on the reliability of the palaeoenvironmental data as an indicator of environmental conditions contemporary with the castle. Similar to other records of the earlier moat fills, the sediments in borehole DS101 are exclusively inorganic clays and silts, which appear mixed in places and may be disturbed or redeposited with no visible signs of waterlogging. These moat fills have low potential for palaeoenvironmental assessment and radiocarbon dating, and no further work is recommended.