Introduction
We were invited to evaluate the geoarchaeology of a site at Trowbridge Mawr, Cardiff, to help the excavating archaeologists, both in understanding the origins of the deposits and in assessing the archaeological potential of the site.
Background
The bedrock of the site was known to be Mercia mudstone overlain by alluvial clays and silts containing layers of peat and we found the soils on the site to be a pelo-calcareous alluvial gley which commonly forms on marine alluvium. The site lies on a low-lying alluvial area between the Rivers Rumney and Usk on the northern side of the Severn Estuary. The site is currently undeveloped and covered with short-cropped pasture, and groundwater levels are controlled by ditches and pumps.
Due to the size of the site and the wet nature of the deposits, the site was examined using a combination of coring and geophysical survey to create a detailed 3-dimensional deposit model.
Findings
Our 3D deposit model showed that in the past an island or promontory was present on the eastern edge of the site, the bedrock beneath it rises up, showing that this feature would have existed as an area less subject to flooding over much of the Holocene. Palaeoenvironmental evidence told us that the vegetation on site varied between saltmarsh and the deposition of clay and wet terrestrial and the deposition of peats.
Conclusions
From our investigations and 3-D deposit model, we were able to inform the client that we had identified a palaeoshoreline and an associated peninsula or palaeoisland, and microfossil and sedimentary evidence was used to help understand the palaeoenvironmental history of the site. Excavation failed to find any archaeological material, but possible evidence of land clearance and the deliberate infilling of minor drainage ditches was found, and the excavation evidence was compatible with and supports the geoarchaeological model.