Figure 1: Aerial view of Aggersborg, looking north-west. The partly restored circular fortress is located next to the Limfjord; the manor house of Aggersborggård is to the front of the picture, Aggersborg church in the background, and Aggersborg village to the right. Image credit: L. Helles Olesen.
Figure 2: Location of Aggersborg Circular Fortress. The site is located in Aggersborg parish, Vester Han herred, Thisted Amt, and recorded in the Danish national records of sites and monuments as site no. 100701-5. The fortress is centred at 56.995457 latitude, 9.254838 longitude. (Map data: © Google Earth)
Figure 3: Reconstruction of Aggersborg fortress. Image credit: Edward Clarke.
Figure 4: Reconstruction of the structural elements of the rampart and outer defences. Image credit: Louise Hilmar.
Figure 5: Area around the west gateway (looking south). The rows of posts follow the curve of the rampart. In the middle of the excavation four large posts flank (in pairs) the axial street. About 8m from the gateway, in the front of the picture, is a double row of posts at right-angles to the rampart – presumably a strengthening element of the gateway area. Also to be seen are traces of drainage ditches, presumably leading from the fortress. Image credit: Nationalmuseet 1950.
Figure 6: The completed excavation of courtyard 5, looking south-east. All the courtyards in the fortress were formed of four similar large buildings, based on the same plan. Image credit: C.G. Schultz/Nationalmuseet 1952.
Figure 7: Schematic plan of the fortress showing the layout of defences and buildings and the system of reference used to identify the courtyards and their buildings. The courtyards are numbered anti-clockwise from 1 to 12, starting at the east gateway; the individual buildings are then designated according to their position within a courtyard. Thus the building on the northern side of courtyard number 5 is designated 5N(orth), that on the southern side of courtyard 4 is 4S, etc. Image credit: Louise Hilmar.
Figure 8: Map of Denmark and Southern Sweden showing the position of the 'Trelleborg' type fortresses and other sites discussed in the text.
Figure 9: Comparison of the plan of Aggersborg (a) with the closely related fortresses (b) Trelleborg (Sjælland) and (c) Fyrkat (Jutland). The so-called 'Trelleborg' group of fortresses also comprised the less well-investigated sites of Nonnebakken (Fyn) and (presumably) Borgeby (Skåne). Further sites may yet be discovered. After Olsen and Schmidt (1977).
Figure 10: Plan of the excavated area with principal features identified from the pre-fortress settlement. Post-built buildings are labelled alphabetically as building A, B, C, etc. SFBs (sunken-featured buildings) are identified by their number in the general list of finds and features in the excavation records. (Map data: © Google Earth)
Figure 11: Plan and reconstruction of house GS, one of the best-preserved buildings of the pre-fortress settlement. The preserved details include a chalk floor in the main room and the foundations of an oven and adjacent open hearth. Image credit: Edward Clarke.
Figure 12: Comparison of proportions of four houses of the pre-fortress settlement, A, C, F and GS. The middle room is in yellow. The correspondence between the lengths of the rooms must reflect a common system of measurement and a common system of arrangement.
Figure 13: Photo of house D, looking east, showing rounded gables and a stone paving in the near end, presumably relating to a byre or stable. At 40.8m in length, house D is substantially larger than other houses excavated in the pre-fortress settlement. It also remained in use for the longest time, and presented the richest finds, including a gold arm ring, found in a posthole in the eastern (far) end of the building. Image credit: The National Museum of Denmark.
Figure 14: SFBs in excavation area trench just east of the eastern gate of the fortress during excavations in 1948 or 1949, looking north-west. The photo gives an impression of the thick occupation layers and the dense clusters of SFBs that characterise parts of the site. Image credit: The National Museum of Denmark.
Figure 15: Two adjacent SFBs during excavation in 1949. The chalk-filled ring around the building to the right is the remains of a low wall built of material from the pit. The person in the centre of the picture is field supervisor Svend Søndergaard. Image credit: The National Museum of Denmark.
Figure 16: Section of fill layers in SFB nos 555 and 852 excavated east of the fortress, looking north. Image credit: The National Museum of Denmark.
Figure 17: Layers of fill in a sunken-featured building. Schematic sketch. The organic material is often compacted together, so that the overlying culture layer with its later material sank low into the pit. Image credit: Louise Hilmar.
Figure 18: (a) Hannah Brown and Frida Norstein conducting GPR survey SW of Aggersborg fortress. In the background the church and the reconstructed rampart are seen. View towards NE. Photo: S. Sindbæk (b) Helen Goodchild conducting gradiometer survey in the Aggersborg fortress area. In the background the church and the reconstructed rampart are seen. View towards N. Image credit: S. Sindbæk.
Figure 19: Location of gradiometer (dark red) and GPR (yellow) survey areas in relation to the current landscape, the reconstructed fortress plan and the excavation plan of pre-fortress settlement. (Map data: © Google Earth)
Figure 20: Gradiometry data after applying zero mean traverse and destep processes. Data are presented with a grey-scale range of -2 to +3nT in order to maximise the visibility of archaeological detail.
Figure 21: Gradiometry data related to the grid system used in earlier excavations.
Figure 22: Location of GPR survey Grids 1-6 (yellow) in relation to gradiometer survey data.
Figure 23: Gradiometry data and interpretation
Figure 24: Interpretation of gradiometry data.
Figure 25: Iron objects found using a metal detector on the spot of a particularly strong anomaly in the western area of the fortress. Image credit: S. Sindbæk.
Figure 26: GPR Grid 2: time-slices for two-way travel times 1-37ns, calibrated to depths of c. 0.05-1.8m below the surface (velocity=0.098nsm-1).
Figure 27: Simplified interpretation of gradiometry data: possible archaeological features.
Figure 28: XY plot demonstrating the difference between increased magnetic responses interpreted as probable SFBs or cut archaeological features, ferrous sources and geology. Each trace represents data collected along an east–west traverse (with coloured amplitude scale for clarity). The continuous 'background texture' of the geology running NW-SE dominates the sample area (several of these trends are identified by arrows); ferrous responses are present, both from individual discrete sources and the backfill of a previous excavation trench. The inset contains SFB signatures (two are ringed) for comparison.
Figure 29: GPR Grid 1: time-slices for two-way travel times 0-36.5ns, calibrated to depths of c. 0.00-1.79m below the surface (velocity=0.098nsm-1).
Figure 30: GPR Grid 1: survey profile collected east–west across the grid at y=8m. The possible SFB is visible at x=8m at a depth of around 0.7m.
Figure 31: GPR Grid 5: time-slices for two-way travel times 1.0-37.0ns, calibrated to depths of 0.05-1.81m below the surface (velocity=0.098nsm-1).
Figure 32: GPR Grid 5: consolidated time-slices for two selected depths demonstrating anomalies interpreted as sunken-featured buildings and associated features. Above: time-depth 10-12ns/0.49-0.59m. Below: 25-40ns/1.23-1.96m.
Figure 33: Possible archaeological features (outlined in red) identified from the full depth of the GPR dataset. Above: displayed against time-slice at 0.49-0.59m for clarity. Below: the same interpretation superimposed on the gradiometer data. Note the close correspondence between the major features identified in both surveys.
Figure 34: GPR Grid 4: time-slices for two-way travel times 1.0-37.0ns, calibrated to depths of 0.05-1.81m below the surface (velocity=0.098nsm-1).
Figure 35: GPR Grid 3: time-slices for two-way travel times 1.0-25.0ns, calibrated to depths of 0.05-1.23m below the surface (velocity=0.098nsm-1).
Figure 36: The area of the southern front of the fortress rampart, now the manor-house garden, seen towards NW. The supposed location of the south gate has been identified according to GPS measurements and is marked out by four ranging rods. Note the slope immediately in front of the supposed outer line of the rampart. Image credit: S. Sindbæk.
Figure 37: Single profile running north–south through Grid 6 at x=24m. The slope highlighted in Figure 35 runs between approximately x=9 (top) and x=13 (bottom) (marked). Note that the lack of topographic correction has the effect of seemingly detecting pronounced strata that run uphill towards the surface in this area, when in fact the stratigraphy is probably closer to horizontal, with the effect caused by the converse running of the antenna downhill and therefore closer (in terms of response time) to the sub-surface cause of the reflection. The position of the surface reflection visible at c. 9-13m corresponds to the gateway area off the fortress and may represent a flat area dug into the slope, similar to open areas in other gateways. The profile demonstrates the various areas of disturbance.
Figure 38: GPR Grid 6: time-slices for two-way travel times 1.0-37.0ns, calibrated to depths of 0.05-1.81m below the surface (velocity=0.098nsm-1).
Figure 39: 180° panorama of the Limfjord landscape looking south with Aggersborg ring fortress (front), manor (centre, back) and church (right) seen from a point near the north gate of the fortress. Image credit: S. Sindbæk.
Figure 40: Basic viewsheds from the supposed central watchtower at Aggersborg with view heights at: a) 1.7m, b) 6m and c) cumulative viewshed with viewer heights from 1.7-6m. (ASTER GDEM is a product of METI and NASA).
Figure 41: Direct line of sight between Aggersborg and Lindholm. Green represents the visible areas, and red is not visible. (ASTER GDEM is a product of METI and NASA).
Figure 42: Combined viewsheds from the supposed central watchtower at Aggersborg (height 6m) and Lindholm (1.7m). Light green represents areas that are visible from one site, while dark green indicates areas that are visible from both sites (ASTER GDEM is a product of METI and NASA).