Fig 1.
a) Location of the study area at the North Sea coast of Germany. Grey areas show the tidal flats and yellow areas show the recent coastline, redrawn after [4]. The dashed line indicates an approximate model of the former coastline of the island Strand before the Marcellus flood 1362 AD. b) Aerial photo of the tidal island Hallig Südfall and surrounding tidal flats [5]. The white transparent box indicates the investigated site, named Niedam area, dots mark known archaeological findings recorded in the catalogue of the archaeological state department of Schleswig-Holstein. Aerial photos are a combination of photos 32480_6034, 32480_6036, 32482_6034, and 32482_6036, republished from Regional government authority for coastal preservation, national park and ocean protection (LKN) under a CC BY license, with permission from LKN, original copyright 2013.
Fig 2.
a) map summarizing all significant archaeological structures that were visible in the tidal flats and recorded by a local resident during the early 20th century after [8, 17]. Drawn maps of these sources were georeferenced, combined and redrawn for this figure. Observations were usually located by bearing a fix object on the mainland or the island and triangulation [17]. b) sketch illustrating the main features of the coastal environment connected to the medieval dyke. Typical rectangular dwelling mounds were either attached to the dyke or lay isolated in drained cultivated marshlands and protected the settlements from being flooded. The report by [8] and [9] further shows two tidal gates embedded in the dyke, that opened during low tide to drain the marshes on the landward side but closed with the incoming tide.
Fig 3.
a) Magnetic gradiometer survey cart built for prospection in tidal flat areas. Large wheels prevent the system to get stuck in muddy areas of the tidal flats. b) The used marine seismic acquisition system mounted on an inflatable catamaran in front of a small rubber dinghi. The equipment is made for lightweight transport to the island and based on the system used in [37]. c) Percussion coring in the tidal flat area. Equipment needs to be carried by special carts (seen in the front), d) Map showing the part of the study area that was accessible to magnetics and percussion coring, the position of the corings, and the seismic lines recorded during the two campaigns. Aerial photos are a combination of photos 32480_6034, 32480_6036, 32482_6034, and 32482_6036, republished from Regional government authority for coastal preservation, national park and ocean protection (LKN) under a CC BY license, with permission from LKN, original copyright 2013.
Fig 4.
Results of the magnetic gradiometry.
a) Magnetic gradient map of the tidal gate and western dyke covering the part of the investigation area that was accessible by walking. b) Redrawing of the magnetic map showing the observed main features. Colored areas and lines correspond to roman feature numbers placed in boxes with equal color. I, area of reduced magnetic signal amplitude compared to the areas II and III; IV, elongated structure, related to the dyke; V, signatures that are connected to the inner side of the dyke; VII, anomalies connected to the tidal gate position that is probably related to I; VI elongated anomalies with no corresponding structures in the recorded observations.
Fig 5.
a) Percussion core locations and stratigraphies in relation to magnetic prospection results. b) Transect A shows the increasing thickness of recent tidal flat deposits (facies type H) in northern direction. c) Transect B shows the limited occurrence and different depth levels of facies type D in the eastern part of the study site.
Fig 6.
a)-e) example seismic profiles and a map (f), showing their locations, the magnetic gradient map and the main features highlighted in Fig 4. Colored rectangles (profiles) and lines (map) indicate the position of several features in the magnetic map and their corresponding features in the seismic profiles.
Fig 7.
a)-c) selected seismic profiles and their preliminary interpretation based on coring results, which are also indicated by coloured bars (facies colors are similar to Fig 5). The positions of the profiles and the cores are shown in the d).
Fig 8.
a) Final interpretation based on the presented dataset. b) Comparison of the results in a) with the old recordings (lines outlining the features already shown in Fig 2 after [8, 17]).