Resistivity Characterisation of Shallow Stratigraphy in Delineating Shell Midden at Guar Kepah, Penang, Malaysia

Abstract

Good stratigraphy mapping of a site is essential in archaeological explorations but Guar Kepah, Penang, Malaysia had no detailed subsurface mapping done previously up till now. As many archaeological remnants in the vicinity have an affiliation with shell middens, better understanding on Guar Kepah’s strata will increase the efficiency of uncovering the shell middens while also saving a huge amount of energy in excavating. This research aims to distinguish Guar Kepah’s buried shell midden from the surrounding strata using electrical resistivity method. The study area is composed of three shallow strata of unconsolidated soil with low resistivity values. The sandy-clay layer has the lowest resistivity (< 40 Ωm), followed by densely packed shell layer (40–70 Ωm) and sandy layer (> 70 Ωm). The presence of shells indicates that the uppermost strata is of marine origin, which automatically makes Gula Formation the youngest formation in Guar Kepah due to the absence of terrestrial deposits (Beruas Formation) on top of the layer. The shell layer from the resistivity profiles suggests that the layer has relatively uniform thickness throughout the site. The layer also becomes undulating and shallow towards the south (dipping), making the top sandy-clay soil become thinner or absent in the south. The dipping feature of the shell layer acts as an indicator that Guar Kepah used to be a beach ridge during sea transgression, and further proves that the area was once a marine environment. However, the shell layer at the southern zone poses an anomaly where it suddenly dips toward the centre. As a result, two zones were marked as potential shell middens at the south-western zone (only exhibits one side of the midden that is dipping towards centre) and south-eastern zone (shell midden elongated to the east). Subsequent excavation based on this study had successfully exposed the south-eastern midden and Shell Midden at Guar Kepah 100 proved that resistivity method could be used not only in stratigraphy identification, but also in characterising shell layer and detecting shell midden.