The Svanemølle Harbour site, Copenhagen. Groundtruthing of a submerged Mesolithic site detected by acoustic remote-sensing

Lately we have published positive results obtained with the HALD method (Human Altered Lithics Detection), which is a method for acoustic detection of submerged Stone Age sites based on that lithic pieces knapped by humans respond to acoustic signals within a certain frequency interval (3-23 kHz) whereas naturally cracked lithic pieces do not. This works even for pieces buried several meters into the lake- or seafloor sediments no matter what these sediments consist of, sand, clay, silt, etc. (Grøn et al. 2021). In our last paper we discussed some rather convincing acoustic indications of the existence of a quite extensive Mesolithic site off the Svanemølle Harbour, Copenhagen, which in the actual area is around 7.5-9.0 meters deep (Grøn et al. 2021). The dating of this supposed site was based on indications that it was a coastal site in combination with its depth. As the acoustic method (HALD) facilitates detection of knapped lithics buried under several meters of sea floor sediment (Grøn et al. 2021), fast and efficient groundtruthing of such buried cultural features is often demanding. As no Mesolithic material was exposed on the sea floor in relation to the Svanemølle Harbour site indications, it was decided to inspect cores from relevant positions of the seafloor for settlement material.

A series of vibrocores of 7.4 cm in inner diameter that were taken by GEUS (the Geological Survey of Denmark and Greenland) were able to penetrate only close to 1 m into the very hard seafloor in the central part of the area, which consists of compact clayey till with layers of sandy material. In each of the two central cores taken approximately 20 m from each other (Fig. 2) was found two small pieces of knapped flint. Two of these – from the one core - did not have their depth below the seafloor registered. The two others were found between 80 and 90 cm below the surface of the seafloor. Statistically this points to a density of knapped lithics around 230 pieces per square meter in the central part of the site. Smaller lithic densities in the more peripheral parts of the detected area could well explain the lack of knapped lithic pieces in the cores from these areas.