MARINE ARCHAEOLOGICAL SURVEY BY HIGH-RESOLUTION SUB-BOTTOM PROFILERS

BY OLE GRØN¹, ANNE NØRGÅRD JØRGENSEN², AND GERD HOFFMANN³

1 Langelands Museum. 1994-1996: The National Museum of . The Research Centre for Maritime Archaeology. 2 Kulturarvs Styrelsen, Copenhagen. 1994-1996: The National Museum of Denmark. The Research Centre for Maritime Archaeology. 3 GEOMAR (Research Centre for Marine Geosciences), Kiel.

1. Introduction

Today’s focus on marine resources and environmental factors has supported a rapid development of technology for monitoring the coastal zones. Together with an increasing awareness of the historical and archaeological sites located under water as well as a growing understanding of the enormous research potential of the well-preserved submerged settlements and landscapes as an indispensable supplement to the investigations on land (Grøn & Skaarup 2004, 2006; Skaarup & Grøn 2004), it has been a logical step to attempt application of the technology to practical marine archaeological survey in shallow water (0.5-10m). Where side-scanners have become widely used in marine archaeological survey because of their ability to quickly create a high-resolution acoustic picture of the surface and texture of the sea-bottom, high-resolution sub- bottom profilers have not gained the same popularity in practical survey. It takes longer to cover an area with profiles with a reasonable resolution/ distance between them. Furthermore the sub-bottom profiles are significantly more difficult to interpret than the side-scan data. However, this should be weighed against the fact that a large part of the cultural heritage under water, and that is often the better preserved one, can be located only with high-resolution sub-bottom profilers because it is covered by re-deposited sediments. Furthermore the sub-bottom profilers can provide valuable detailed insights into the bottom’s small-scale geomorphology, which in relation to settlement investigations is added value (van Andel & Lianos 1984, Grøn 1990).

115 Further factors stimulating the archaeological interest in the submerged cultural heritage resources are the industrial peat exploitation and agricultural drainage of the land-based peat-bogs which through the 20th century led to a significant degradation of organic artefacts in these basins. Large numbers of Stone Age sites with well-preserved dwelling remains such as the famous ones excavated at Ulkestrup Lyng and Muldbjerg in Denmark (Andersen 1978; Andersen et al. 1982; Troels-Smith 1960) have lost their important organic component and appear today as mere concentrations of lithic artefacts. We may here have witnessed one of the largest cultural heritage disasters ever. This sad development underlines the special importance of the submerged areas, where intact and well preserved peat basins still can be found, as a last research resort with regard to detailed studies of the Stone Age and of the important process of Neolithisation (Grøn 1990, 1995; Grøn & Skaarup 2004). The presence of entire submerged cultural landscapes from the Stone Age with impressive preservation provide in some areas a unique context for these sites (fig. 2). Where these landscapes have not been damaged by erosion or covered by significant layers of soft sediment, that make them practically inaccessible, such a cultural resource should be managed with extreme care and thoughtfulness. Settlement surfaces from later periods than the Stone Age must, due to the more stable sea-level in this phase, in general be expected to play a less prominent role even though landslides in some cases such as Port Royal are known with disastrous speed to have transferred parts of settlements to submerged positions with good preservation (Marx 2003). However, constructions of poles and stones from these periods, such as harbours, piers, ferry landings, bridges, barrages and other defences against naval attacks are features that have been largely overseen because they are often not preserved above the bottom level and because their surprising preservation can easily be mistaken for the Fig. 1: Locations where important marine-archaeological results were obtained remains of modern pound-nets. with the Chirp system in the period 1994-1996.

116 The main aim of this paper is to demonstrate the practical potential of acoustic high-resolution sub- bottom profilers for marine archaeological large-scale survey in shallow water (0.5- 10.0 m). The focus will be on the so-called ‘chirp’ systems, because they according to our tests produce better results with regard to archaeological sites than single and dual frequency penetration- systems. For general survey the chirp systems also at the moment seem to represent a cost-effective and manageable methodology compared to the experimental 3D-acoustic systems that exist today. Few of the features distinguished with chirp in the Fig. 2: Side-scan recording cases presented in this paper would have been distinguishable with non- showing tree stumps and penetration systems such as side-scanners and multi-beam scanners. trunks on the bottom Furthermore it will provide examples of the use of chirp systems to along the banks of a small demonstrate the significant improvement of the interpretation of submerged watercourse flooded archaeological features they facilitate. The initial development of the approximately 5000 BC cal. method was carried through as a 3-year project period from 1994-1996 by The area shown is only about the authors. The work has since been continued by Grøn. Apart from the one kilometre from the Late authors’ institutions, the Archäologisches Landesmuseum der Christian- Mesolithic dwelling and Albrechts-Universität, Schleswig, the Institut für Ur- und Frühgeschichte nearby boat-grave excavated der Christian-Albrechts-Universität, Kiel, and the Institute of Geology, at the same depth (Skaarup Aarhus, have participated. The results achieved demonstrate that acoustic & Grøn 2004). technology has reached a level where a more general application of it to marine archaeological survey will be profitable.

2. Background and Method

Sub-bottom profilers are echo-sounders that penetrate the bottom sediments with acoustic signals and from their reflections generate images (today a digital one) that reflect the features of the sea bottom’s sediments to a certain depth. Low frequencies give a good penetration but poor vertical resolution,

117 and so are not able to distinguish finer details. Higher frequencies yield bad penetration but good vertical resolution: that is a more detailed picture of the features below the bottom surface. Where the older systems employ only one or two frequencies, the chirp systems sweep a frequency interval. In this case a window of 2-23 kHz was used that combines a band of different frequencies with different penetration and resolution and has proved to be well suited for distinguishing archaeological features in shallow waters. Traditional systems with one hydrophone could not be operated successfully at depths less than a couple of metres. The application of more hydrophones to the systems has made it possible to obtain reasonable results at depths as small as 0.5 metres. While small depths are one problem, large depths are another. The longer the acoustic signal has to travel, the more it will be spread and accordingly lose its precision. We found that 0.5-10.0 m depth is an ideal operation area. At larger depths the ‘fish’ (transducer unit) with the hydrophones can be ‘deep-towed’ to obtain a reasonable recording distance to the bottom. An important improvement is the development of real-time processing of the data that facilitate a morphologically correct presentation of features embedded in the bottom. With earlier systems, anomalies could be distinguished only as diffraction hyperbolas that were mostly very difficult to interpret. The efficiency of the method is demonstrated by our findings in Denmark in the project period: 7 barrages (underwater defences against ship attacks), 2 ferry landings and 1 possible breakwater that may have protected an old harbour. One of the barrages found is of a type hitherto unknown in this area. So far, the seven structures from Denmark that have received radiocarbon and dendrochronological datings belong to the period 80-1635 AD (1). Furthermore, 25 acoustic anomalies were observed and Fig. 3: Left: Recording with low degree of processing. Object in water recorded as verified in the harbour area of diffraction hyperbola. Illustration modified from Stümpel & Borth Hoffmann the Viking Age trading centre (1983). Right: Immediately processed chirp-recording (CAP 6600 from Datasonics). Stake, 8 cm in diameter, standing up in water and visible in the soft bottom at Hedeby/Haithabu, Schleswig, sediments as seen in the 2-7 kHz interval (upper profile) and 8-23 kHz interval Northern Germany (Grøn et al. (lower profile). Recording: Ole Grøn and Lars Erik Larsen, 1996. 1998).

118 For the survey a Chirp II system from Datasonics/Benthos (CAP-6600) was used. This system sweeps the interval from 2-23 kHz on two channels and shows good penetration even in sandy sediments with some organic content and in shallow water. It is important that the transducer unit (the ‘fish’) which can be operated in depths as small as 0.3 m is of such a size and weight that it can be transported by a single person. For the dynamic positioning, an Ashtech DGPS-navigation system with its own basis station was used. Every second a position with an accuracy of ± 0.3 m was stored with the seismic data in the seg-y format. The precise dynamic positioning allowed us to deviate from sailing straight parallel lines when this was preferable. Data recorded by sailing straight parallel lines tend to be difficult to interpret. Potentially interesting anomalies registered during such a recording would demand follow-up recording with several cross-profiles to establish their character as much as possible by seismics alone. On the basis of our experiences we trained in instant recognition of anomalies and immediate seismic check of their character instead of accepting a later recognition through processing in the laboratory as the basis for ensuing action. The instant recognition strategy made it possible to discriminate a large number of anomalies as archaeologically uninteresting and to collect detailed information about a number of anomalies of potential archaeological interest. Afterwards it was no problem to continue the original sailing-line on the basis of the navigation- computer’s display. As an example it can be mentioned that the instant recognition strategy made it possible within 30 minutes from Fig. 4: Upper: unprocessed profile showing a section through wreck 4 from the time when it was recognized Hedeby/Haithabu Harbour when it was distinguished in the Chirp-profiles by for the first time to distinguish direct interpretation (Grøn et al. 1998). The broken black line shows the surface the hull of the unknown wreck of the soft sediments. Recording: Ole Grøn and Lars Erik Larsen, 1996. Lower: 4 in Hedeby/Haithabu harbour processed recording of section through ‘Mary Rose’ (Quinn et al. 1997b).

119 as a likely wreck and to determine its approximate dimensions and shape through a number of additional profiles (Carnap-Bornheim et al. 2002; Grøn et al. 1998). Numerous other cases have demonstrated the efficiency of the direct approach. Its main demand is an interpreter specialised in recognition of archaeological features. Its main advantage is that it saves oceans of time. The distance between the lines sailed depends on the size of the features surveyed for. Barrages block a whole inlet or fjord and therefore should appear with a few sailing-lines in and out – of course avoiding artificially dredged areas. Harbour structures and larger wrecks would normally demand 10-15 m between the sailing lines, where a large number of supplementary profiles will be applied with only a few metres between them. Mesolithic settlements and smaller wrecks 5-10 m. The optimal horizontal resolution was obtained with 8 pulses per second with velocity of approximately 1 m/sec (1 knot). To control navigation at such a low speed is, however, impossible in strong wind or current, and must be negotiated. The first features studied in detail were the barrages in Haderslev Fjord and Vordingborg Harbour, which had already been carefully documented by divers. The early exercises were focussed on them to learn how such structures were reflected in high-resolution profiles. This proved to be very productive in the later work, because we had a good idea of what we were looking for. The primary purpose of the project was to develop a method for efficient underwater survey in shallow waters. Therefore the focus was on locating structures by means of the sub-bottom profiler and afterwards to verify their character and, if possible, age by diving. Detailed underwater investigation of structures was outside the scope of the project.

3. General Features Observed

3.1. Single Poles

The verified chirp recordings from a series of barrages and ‘palisade- structures’ in Vordingborg, Jungshoved, Haderslev Fjord, Nakkebølle Fjord, Gudsø Bay, Fjord, Hedeby/Haithabu and a Mesolithic Cultural layer in Roskilde Fjord (fig.1) show that vertical, inclining or horizontal timbers embedded in soft sediments can appear as recognisable anomalies when the instrument it tuned in the right way and there is not too much gas in the bottom sediments (Grøn 1995; Grøn et al. 1998; Quinn et al. 1997a). Vertical poles in soft sediments often appear as linear features or ‘channels’ into the bottom (figs. 3, 5, 6). This type of reflection is not restricted to

120 Fig. 5: Profile from N to S through structure B in Hedeby/Haithabu harbour (see fig. 22). From the left to the third bar is soft sediment. The third bar coincides with an old steep coast line. To the right of this is sandy and hard bottom with an echo of second order (a weaker reflection at the double depth of the primary reflection). Between the 7th and the 8th bar from the left a number of poles (about 30 cm in diameter) from structure B are visible as ‘empty tubes’ - narrow dark linear structures. The horizontal lines mark the depths 2, 4 and 6 m. Recording: Ole Grøn and Lars Erik Larsen, 1996.

Fig. 6: Profile parallel to the shore in the Hedeby/Haithabu harbour area. Zones with concentrations of large poles (sections through piers) marked with horizontal white bands on top of the profile. The outline of one of the poles is marked in white in an enlarged section. Recording: Ole Grøn and Lars Erik Larsen, 1996. vertical poles but is also observed with items tilted beyond the vertical. In sandy sediments, poles with the top covered by a sediment layer of some thickness can sometimes be seen as narrow and dark rather diffuse bands into the bottom. We have not investigated the relation between the inclination of these signals and the actual inclination of the poles. The data- processing used in the Chirp II system (CAP 6600) seems to produce screen pictures with a linear relation between the diameter of the poles and their graphical representation. This facilitates an estimation of the pole diameters on the basis of the recordings. Single poles with diameters smaller than about 10 cm have not been distinguished. The observations mentioned were in an early phase of the project confirmed by systematic recordings of poles from modern structures such as pound nets, duc’dalphes, etc. Later, the

121 verification of seismically distinguished pole structures such as harbours, barrages, piers and ferry landings confirmed the observations of single poles numerous times. If the bottom sediments are not extremely soft, vertical poles of some diameter often appear as water-filled canals into the bottom with a darker tube-like feature around them (e.g. fig. 5). The most logical explanation is that the water-saturated cells in the poles allow acoustic pulses to pass through them in their longitudinal direction as easy as through water, so that it is not possible to discriminate between their inner characteristics and the water (Grøn et al. 1998). The difference between poles’ and tree- trunks’ reaction to longitudinal and transverse acoustic signals is known from experiments (Arnott et al. 2002; Quinn et al. 1997a)

3.2. Total Absorption of Signal /‘White-out’

An interesting feature that actually can be used as a diagnostic feature of structures with concentrations of more or less vertical poles is the total absorption of the signal that can be observed in areas with a certain density of more or less vertical poles. Our working term for the phenomenon is ‘white- out’, because the colour-setting best suited for observation of the features we looked for displayed it as a white zone in the profiles. The existing data do not give any information about which relations between density of poles, thickness of poles, deviations from vertical, and materials (kind of wood) will produce such a total absorption. The basic acoustic rationale of the feature is also unknown to the authors. We have so far observed only that it seems to appear as a general feature where a relatively high density of approximately vertical poles is found. Poles separated by more than one metre will normally appear as independent anomalies. In Kerteminde Fjord the acoustic anomaly reflecting the presence of a barrage was from 15 to 25 m wide. This reflection was produced by an up to 40 cm thick, lenticular accumulation of shell fragments and coarse gravel that had been deposited around the structure by the tidal currents. A 3-4 m wide ‘white-out’ zone Fig. 7: The Late Viking Age/Early Medieval barrage that was was located centrally in this deposit (fig.7, 10). discovered at the innermost part of Kerteminde in the low- A 0.75 m wide ditch excavated through the frequency channel (2-7 kHz - upper) and in the high frequency barrage revealed a corresponding 3.3 m broad channel (8-23 kHz – lower). Between the bars 5 and 6 from the left is a narrow vertical zone down through the barrage- concentration of poles. The area excavated related features where apparently all of the signal is absorbed. contained 12 vertical poles with diameters Recording: Ole Grøn and Lars Erik Larsen, 1996. ranging from 8 to 12 cm. This is equivalent to 122 approximately 5 poles per square metre. The horizontal area taken up by a vertical pole measuring 10 cm in diameter is 79 cm². Thus only 395 cm² or approximately 4% of the area excavated was occupied by poles. If the pole density observed in this ditch is representative for the pole density in the entire structure, it indicates that even a small density of poles may have a surprisingly strong effect. A couple of typical examples are the barrages ‘Æ Lei’ in Haderslev Fjord and the newly discovered one from Kerteminde Fjord. Their central parts show a clear ‘white-out’ (figs. 7 and 8).

3.3. Lenticular Features.

Our experience shows that the tidal movements of the water generally tend to Fig. 8 (above): The Late Roman Iron Age barrage ‘Æ Lei’ in cause deposition of coarser material such as the low-frequency channel (2-7 kHz - upper) and in the high sand, gravel and shell fragments around pole- frequency channel (8-23 kHz – lower). Between the bars 3 and structures. In section such deposits, where 5 from the left is a narrow vertical zone down through the they are of some size, can often seen to have a barrage-related features where apparently all of the signal is lenticular shape and to be symmetrical around absorbed. Recording: Ole Grøn and Lars Erik Larsen, 1995. the barrage (e.g. fig. 9). Smaller lenticular features tend to appear as thin, horizontal hard reflectors. These features do not need to appear as a local heightening of the bottom as in fig. 9 but can appear as a local change of its character – for instance from soft sediment

Fig. 9 (right): Seismic profile through the barrage ‘Æ Lei’ in Haderslev Fjord. The upper profile is channel 1: 2-7 kHz. The lower one is channel 2: 8-23 kHz. A narrow ‘white-out’ zone showing the position of a concentration of poles is located just to the left of the 5th bar from the left. A thick lenticular feature, probably consisting of coarse material deposited around the verified barrage, is visible between the 3rd and the 7th bar from the left. The horizontal lines mark the depths 2, 4 and 6 m. The soft sediments can be seen to be at least 1 metre deep around the barrage. Recording: Ole Grøn and Lars Erik Larsen, 1995.

123 to hard gravel and then back to soft sediment can reflect the local effect of the poles when they were still standing above the bottom. One result of the 1994-96 campaign is the observed relation between pole structures standing in water and the deposition of coarse material around them. Around the barrages in Kerteminde Fjord, Gudsø Vig, Nakkebølle Fjord, Haderslev Fjord (‘Æ Lei’) we observed such smaller or larger accumulations of material coarser than the normal local bottom sediments. According to information from Hugo Sørensen, Langelands Museum, a barrage excavated in the reclaimed area Henninge Nor on Langeland contained a lens of sand deposited on the peat bottom around the poles. In combination with the possibilities for observing single poles and white-out zones Fig. 10: Schematic section through the Kerteminde barrage the lenticular features provide a further useful with lenticular tidal deposit of coarse particles and shell indicator of pole structures of which the remains fragments. are embedded in the bottom sediments. For the interpretation of the characters of the different pole structures their spatial configurations are normally quite important.

4. The Late Viking Age/Early Medieval Barrage in Vordingborg Fjord

The correct character of the feature, that had already in 1791 been claimed to be the remains of an old bridge, was recognised by Crumlin-Pedersen in June 1977. The barrage had protected one or several phases of the castle located in the inner part of Vordingborg Fig. 11: Section through a series of the parallel pole- Fjord, most likely from before 1160 AD. The structures that forms the Vordingborg barrage. The pole- well-known pole-feature was regarded as well structures stand up as highly visible peaks in the very soft bottom sediments. None of them are visible above the top suited for training in seismic recording and of the bottom. The upper profile is channel 1: 2-7 kHz. The interpretation. In 1995 a detailed recording of lower one is channel 2: 8-23 kHz. Recording: Ole Grøn and the structure was carried out by Ole Grøn and Lars Erik Larsen, 1995. Lars Erik Larsen.

124 In the southern side of the fjord, south of the sailing channel, the barrage stood out in the very soft bottom sediments as a surprisingly visible structure consisting of several parallel lines of poles. Possibly these represent at least four phases of repair (figs. 11, 12). In the northern side of the fjord the structure was difficult to observe because of dredging and modern landing piers. The degree of detail that can be observed and the consistency of the horizontal Fig. 12: The spatial configuration of the different parts of the Vordingborg barrage as configurations of the different they could be observed with the Datasonics Chirp II in 1995. Black square: feature parts of the pole structure are interpreted as the remains of a defensive tower. Recording: Ole Grøn and Lars Erik promising. Larsen, 1995.

5. The Late Roman Iron Age Barrage ‘Æ Lei’ in Haderslev Fjord

A systematic registration of the already known Late Roman barrage in Haderslev Fjord, ‘Æ Lei’ was used to check the efficiency of the method (Bonde 1992; Crumlin Pedersen 1975). Transportation of the equipment to the location, launching of the boat from the trailer, mounting of the chirp and the DGPS-navigation, placing of the DGPS basis-station in a known coordinate-point and the recording of several profiles parallel to the barrage and 60 profiles perpendicular to it took less than one day for two persons. Interpretation of the data took a further 3 days for one person. Thus the whole exercise took one day’s use of a chirp and five man-days. The data collected produced a far more detailed picture of the layout of the structure than the earlier diver-investigations had done (figs. 8, 9, 13). It appeared that the barrage’s northern part is double – probably due to repair. Of interest is also that its central part appeared to have an opening that probably had been regulated by additional pole-structures on its eastern side. Because the opening is located between the old sailing channel that was dredged with a steam dredger in 1843 north of it and the later one dredged in the 20th century to the south of it, it is unlikely to have been caused by modern dredging.

125 Fig. 13: Plan of the barrage ‘Æ Lei’ in Haderslev Fjord. The ‘opening’ is placed 150 m from the old natural sailing channel as well as from the dredged one from 1843 (Crumlin-Pedersen 1975). Anomalies around the opening may be reflections of pole structures for controlling the passage through it, as suggested for the barrage in Kerteminde Fjord (fig. 19). The lighter grey zones within the barrage structure show where the pole structure has been damaged by the dredging of the sailing channel from 1843 or where it seems to have a less massive structure than in the remaining parts. The white ‘bands’ show zones that seem to have no poles - and may thus be borders between different building/ repair phases. Recording: Ole Grøn and Lars Erik Larsen, 1995.

Fig. 14: Plan of the structures at Jungshoved. A and B are barrages that block the mouth of the inlet. C is interpreted as the remains of a breakwater that protected a harbour and E and F as ferry landings connected to an old road crossing the inlet. Recording: Ole Grøn and Lars Erik Larsen, 1995.

6. Structures in Jungshoved Cove, Southeastern Zealand

The first survey of more complex structures was carried out in 1995 in Jungshoved Cove, the little inlet to the west of the remains of the Medieval castle at Jungshoved, Zealand (fig. 14). It is assumed that the castle was built around 1200 during the reign of King Valdemar I and at that time held a central position. Probably it burnt down in the 1650s during the ‘Swedish War’ (La Cour 1972:218).

126 In the up to 3.5 m deep inlet open to the south, five anomalies were distinguished (fig. 14). They were verified as 1) two barrages, A and B, 2) a breakwater protecting a harbour, C, and 3) two ferry landings, E and F. None of the structures were visible above the bottom surface. The southernmost barrage, A, is 6-8 m wide and consists of 3-5 m long oak trunks with diameters between 35 and 55 cm (figs. 14, 15). They appear all originally to have been fixed to the bottom with four smaller poles of approximately 15 cm in diameter. The southern border consisted of a continuous line of trunks. In one two of these were observed to lie one on top of the other. This may mean that the barrage may have had a regular ‘log-built’ southern front, and that one possibly in some cases should reconsider the role of the so-called floating timbers. In the northern part of this barrage, the orientations of the trunks varied and most of them were not parallel with the southern front (fig. 15). This structure is radiocarbon dated to the Early Roman Period - 80 AD (20-210 AD) (K-6759) (1). The second barrage, B, consists of a single row of poles 15-20 cm in diameter and spaced 75-80 cm apart centre to centre. In the eastern side of the inlet B joined the eastern end of A (fig.14). A sample from B received a radiocarbon dating of 980 AD (895-1015 AD) (K-6760) (1) placing it in the Viking Age. The breakwater C (fig. 14) consists of a single-line pole construction. The poles are 15-20 cm in diameter and stand side by side with their tops 50-70 cm below the surface of the bottom sediments. According to the acoustic recordings, the structure should end where it turns to the north and meets the southeast edge of the dredged channel leading to a small harbour close to the church. Meanwhile, the diver investigation showed that 1) exactly on the edge of the channel, C has an ‘extension’ to the south-southwest that is at least 5 m long, and 2) that just about 1 m of a similar construction is preserved on the northwest edge of the channel most likely representing the northwest end of C has been severely damaged by the excavation of the sailing channel. The northwest end (fig.13) is connected to, C with a dot- and-dash-line. C is radiocarbon-dated to 1020 AD (990-1155 AD) (K-6763) (1) and thus may be contemporaneous with the Viking Age barrage B. The morphology of C does not give meaning for a barrage, but makes sense for a breakwater protecting a harbour. The two ferry landings, E and F, were radiocarbon dated to 1530-1635 AD (1475-1660 AD) (K-6761) and 1430 AD (1405-1450 AD) (K-6762), respectively (1). Both consisted of a more or less irregular frame of poles with diameters between 15 and 20 cm. E could furthermore be seen to contain layers of branches and timber as part of its construction. The 30-60 cm thick layers of filling inside the structures consisted apart from branches, trunks and worked wooden fragments of a mixture of sand, smaller stones and

127 pieces of larger artificially cloven stones. A survey of the landscape around E and F revealed an artificial earthen ramp leading down a steep slope to the coast on the western side of the inlet. The ramp is directed exactly against E. Soundings with a steel probe revealed a large content of boulders in the ramp. At the top of the steep side - in 1995 the eastern border of a plough-field - and just beside the top of the ramp, a large heap of fist-sized stones removed from the field by the farmer was observed. Most likely they are the remains of road paving. An old road is known to have approached the inlet approximately here. The structures located in the survey point to a central position for the inlet and its surroundings already from the Early Roman Period till the castle burnt down in the 17th century. Especially interesting is that the barrage, A, can be taken to indicate the importance of the landing-place, from the Early Roman Period worth defending. Its geographical position is central in relation Fig. 15: Plan of a section of barrage A from Jungshoved. to the nearby concentration of rich Roman Period graves at Himlingøje on Stevns, where a powerful dynasty seems to have had its base. This dynasty appears to have gained further power on a basis involving international trade in the transitional period B2/C1a around 150 AD. In the Late Roman Period it is suggested that there existed a large-scale network of military alliances around this central kingdom (Lund Hansen 1995:243 ff.,fig.9.1). Apart from a possible role in international trade and military alliances, there can be little doubt that fishing directly or indirectly played a role for the economy at Jungshoved. According to ‘Valdemar’s Cadaster’ the area had its own parcel named after it at the annual Skanør market, the centre of the East Baltic herring trade in the Medieval Period (Aakjær 1926-45).

128 7. Remains of Floating Barrages in Gudsø Vig, Southeastern Jutland

In 1985 a double barrage from late Viking Age/early Medieval (structure H in fig.16) was discovered across the natural sailing channel into Gudsø Vig near Kolding. Furthermore, a possible barrage or a fishing weir found close to it was dated to the Pre-Roman Period (Rieck 1992). In 1996 a seismic survey was carried out to see if more defensive structures could be found in the inlet. The investigation revealed features of a hitherto unparalleled type of structure in Denmark: floating barrages. The seismic recordings revealed five narrow and parallel ‘bands’ (A-E in fig.16) that connect the northeast side of Gudsø Vig with Hovens Odde. A diver investigation showed that the northernmost Fig. 16: Plan of the structures in Gudsø Vig. A-F are one (A) (fig. 16, 17) consisted of a single line of interpreted as traces of floating barrages. G is the land oak trees with felling facets and their branches connection from the fortification ‘Krabborre’ built from intact. The trees lie top to root-end with the latter large caissons, and H marks the two barrages discovered to the south-west in the southwestern end of the in 1985. Recording: Ole Grøn and Lars Erik Larsen, 1996. structure and to the north-east in the opposite end. The bottom consisted of soft mud, and none of the trunks were visible above it. Due to deep mud in the 3 m deep, central part of the inlet, it was impossible to follow the trunks’ orientations here. At the felling facets the trees were 40-80 cm in diameter and they were 5-10 m long. Approximately 50 cm below the present bottom, about the level of the lower side of the trees, was an up to 15 cm thick and lenticular layer accumulated around and up to 0.5 m from the trunks and the bigger branches. The layer consisted of sand, gravel, crushed shells of common mussel and oyster, and a few intact shells mainly of oyster. It is interpreted as coarse material deposited around the structure by the tidal current. Fig. 17: Plan of a registered section of the remains of a The implication is that the trees when they proposed floating barrage, A, from Gudsø Vig, consisting became saturated sank and lay on the top of the of a line of oak trees with coarser material sedimented soft bottom, which was 50 cm below the present around them.

129 bottom level. No poles were found to have kept the trees in position and no remains of rope/line/chain were observed. However, the structure appears meaningless if the trees were not connected in a chain as a floating barrage. A more thorough investigation may reveal more details about this. Where we dived on the parallel structure, B (fig.16, 18), it consisted of an up to 50 cm wide and 10 cm thick band of sometimes several-metre-long concentrations of sand, gravel, and crushed shells 5-15 cm below the bottom surface. In section the concentrations are lenticular. The feature may represent material deposited around a fishing net by the tidal current, but its similarity to A suggests that it represents a similar structure. One difference is that B is narrow and linear. If the interpretation presented here is correct, one may think this reflects the use of trees without branches – easier to remove. Fig. 18: Plan of a registered section of feature B from The restricted depth of structure B may indicate Gudsø Vig proposed to consist of material deposited around a floating barrage. that it is younger than A, but may also reflect local differences in the sedimentation. To explain B as a natural phenomenon is problematic, because it consists of a narrow band perpendicular to the direction of the tidal current. During the diver investigation it was not possible to observe concrete features that matched the recorded acoustic anomalies C, D, and E. This may be because they were too deep in the bottom sediments or because they consisted only of material that was difficult to distinguish in the mud. From the fact that at least C and D consistently appeared in the seismic profiles it must be assumed that they represent some kind of physical anomalies running parallel to two structures it was possible to verify. Structure F (fig. 16), which also transverses the sailing channel, is of the same type as B. It consists of 5-10 m long, straight bands of gravel and crushed shells up to 50 cm wide, 10-20 cm thick, and with lenticular sections. They are located 20-50 cm below the bottom surface. The single bands in F are often at angles to the neighbouring ones. As an acoustic

130 anomaly F could be observed over a stretch of 250-300 m. During the diver investigation no structures were observed that could have served to keep a line of trunks in place. The interpretation is also here that F represents the remains of a floating barrage, possibly one that has drifted out of its original position. In the inlet behind the proposed barrages A-E is a little island with the name Krabborre (Castle of Crabs)(fig. 16). The seismic survey indicated that there were pole constructions connected to it. A closer examination revealed that the remains of a dam (G) that had once connected the island with the mainland consisted of large oak trunks and closer to land of what appears to be regular caissons of oak trunks. The many barrages and the undated remains of a castle found in Gudsø Bay indicate that this natural harbour with an escape route up the Rands Fjord played an important military-strategic role. Probably it also had a function as a landing-place/harbour for trade. The barrages date the activities to a period from the Pre-Roman Period to the 11th and possibly the 14th century, with the main weight in and around the Viking Age (Rasmussen 1989; Rieck 1992). A possible Medieval wreck has been registered approximately a bit to the north of the eastern landings of the barrages A-E.

8. A Barrage in Kerteminde Fjord, Northeastern

Acoustic survey in Kerteminde Fjord revealed a 20-25 m wide and 250 m long structure apparently blocking the central and deepest part of the threshold between Kerteminde Fjord and its inner part: Kertinge Nor (figs. 7, 10, 19). The acoustic recordings indicate a central opening for passage of ships (fig. 19). Since the access to Kerteminde Fjord was blocked for larger ships by the building of a low bridge across its mouth in the middle of the 17th century, the opening in the barrage cannot have been caused by the dredging of a sailing channel (2). Connected to the central opening in the Kerteminde barrage are features interpreted as several narrow rows of poles (fig. 19). The system may have been organized so that the main barrage was maintained through longer time, whereas access through the central opening was regulated by rows of single poles. Since more than 1 metre of mud covers the central part of the barrage, the character of this opening could not be investigated during the following diver-inspection. Radiocarbon datings of poles from its northern end found just below the surface of the mud date the structure to the Viking Age and the Early Medieval Period: 1215 AD, 1010 AD, and 900-960 AD (1).

131 It must be assumed that Kerteminde Fjord and Kertinge Nor has played an important role for the shipment of cargo to and from the central town on Funen: . The name Ladby at the southern coast of Kerteminde Fjord presumably refers to this activity, which may also be reflected in the famous Ladby Grave, a Viking Age mound containing a ship burial dated to the 10th century (Crumlin-Pedersen et al. 1996:62-73; Sørensen 2001:160 ff.). In the Medieval Period Munkebo at the head of Kertinge Nor seems to have been the centre for shipment to and from Odense, Kerteminde taking over its role accordingly (Sørensen 2001:158). There seems thus prior to Munkebo’s decline to have been good reason for protection of the inner part of the water system Fig. 19: Plan of the barrage at the threshold Kerteminde Fjord/Kerteminde Nor. between Kertinge Nor and Kerteminde Fjord. The Because of its qualities as a natural harbour and shaded area marks the accumulation of coarse- grained materials around the pole structure. The the presence of the barrage, Kertinge Nor may well linear anomalies connected to the central opening have served as a naval base - a fleet assembly place in this structure are interpreted as possible pole - in the Early Viking Age for ships of the oldest type, structures used to regulate the traffic through the such as the Ladby ship. It is noteworthy that this area central opening in the barrage. Recording: Ole Grøn - precisely between Odense and Kerteminde - has the and Lars Erik Larsen, 1996. largest number of baun (i.e. ‘beacon’) names on Funen. This indicates that the high positions these names are related to were used for lighting signal beacons (Crumlin-Pedersen et al. 1996:62-73). Kertinge Nor probably had two egresses, one at the mouth of Kertinge Fjord in the east and one towards in the west close to Dræby. The total coast-length of Kertinge Nor is about 10 km. The most suitable gathering-place seems to be its northern shore between Snekkeled (passage/ gate for ships) to the east and Munkebo to the west, a stretch of about 2 km with the Munkebo Hill in its hinterland. The barrage controlled the access to the western half of this area. Odense became a bishop’s seat in 988 AD and is often mentioned in the sources as a political centre and meeting-place. Nonnebakken in Odense, one of the four Danish complexes, is dated to about 980 AD. The city’s political and strategic importance in general has led some researchers to suggest that it was a royal seat and centre of the ancient Odin cult. Kertinge Nor may have played a central role as a naval gathering place for fleets under King Gorm, or Sweyn Forkbeard, and in the period around 1200 AD under Canute VI or Valdemar II (the Victorious).

132 9. A Late Roman Barrage in Nakkebølle Fjord

An acoustic survey in Nakkebølle Fjord revealed in 1996 an approximately 200 m long pole-structure blocking the most narrow part of the original fjord (fig. 20). The structure is located 150 m south of a former small island that now forms part of a dam used to reclaim the inner part of the fjord and the remains of a fortification registered as Medieval on the western shore. Inspection by divers showed that the anomaly consisted of an up to 20 m broad irregular band of oak trunks spread on the bottom around a 1-2 m wide line of poles with diameters of 3-15 cm (fig. 21). The observations confirm the information that oak trunks had earlier been collected from the fjord and used for production of furniture and fence poles. The samples from this site produced a dating of 370 AD with one indicating earlier activity (320 AD) (Daly 1997). A series of borings with a Perchauer- borer perpendicular to the structure showed no observable layer of material coarser than the mud of the bottom. However it became obvious from wading around that the surface of the bottom in the barrage zone was 20-30 cm higher and clearly harder (larger content of sand) than north and south of the barrage. Because the fortification on the west bank of the Fjord where it is most narrow is registered as Medieval, a Medieval dating was expected from the samples taken from the barrage. But the sample from the Nakkebølle Barrage is dendrochronologically - with a growth- ring curve of 215-364 plus sapwood - dated to about 370 AD (Bonde & Daly 1998; Daly 1997). This raises the question of whether the fortification has earlier phases than the Medieval one. The barrage seems to have been part of a local defence. Local defences in southern Funen area were probably useful in the Late Roman Period. The numerous weapon graves, the rich Fig. 20: Plan of the barrage in Nakkebølle Fjord before the inner fjord settlements and even ritual sites on was drained. Recording: Ole Grøn and Lars Erik Larsen, 1996.

133 Funen indicate the existence of a busy trade to and from the island and the importance of the military aspect (Hedeager 1990:344). Attacks from overseas enemies on Funen already in the Early Roman Period and several times in the Late Roman period seem reflected in the weapon sacrifices in Vimose (Engelhardt 1869; Ilkjær 1990:257ff; 319ff; Abb.201). The weapon sacrifice Kragehul in Flemløse parish north-west of Nakkebølle Fjord has deposits from C2 250-310 AD and the transition to C3, as well as later deposits of weapons in D1 375-430/40 or D2 430/40-525/30 AD (Engelhardt 1867; Ilkjær 1990, 338). Most likely the latest Kragehul deposit should be dated to D1 (Vang Petersen 1988:131). The find of a barrage dated to around 370 AD appears interesting in this context.

10. A Harbor in Hedeby/Haithabu

Several archaeological campaigns have contributed to our knowledge about the harbour area of the Viking Age trading centre Hedeby/Haithabu, Schleswig in Northern Germany (Schietzel 1981). In 1953 one of the early underwater investigations resulted in the find of several zones with concentrations of poles (Hingst & Kersten 1955). The most impressive was ‘structure I’, at least 160 m long and with 7-8 parallel lines of poles 20 to 30 cm thick. Several smaller pole concentrations were also located. A burnt Viking ship, structure II, was found in the harbour area. Seismic surveys carried out with a 5 kHz ‘pinger’ in the period 1978-1981 led to further underwater investigations. Apart from a pole structure that

Fig. 21: Schematic section through the Nakkebølle barrage.

134 apparently connects structure I to the northern rampart, two dugouts and a second Viking ship were found (Stümpel & Borth-Hoffmann 1983). In 1995 and 1996 new surveys were carried out with the Chirp II-system, to see if the new technology could reveal a more detailed and coherent picture of the harbour - especially of the areas with extremely shallow water where the modern system is better suited for operation than the ‘pinger’. Immediately, a very clear, straight and narrow acoustic anomaly, A (fig.22), appeared, starting from the end of the southern rampart and proceeding to the north. Inspection with divers confirmed the interpretation of the structure as a more than 200 m long and 4-6 m broad breakwater consisting of poles and stakes standing as close as possible. Its northern 10 metres are made up of an area where broken poles are visible above the bottom surface. One suggestion is that this may be the remains of a defensive structure/tower that has collapsed. A similar anomaly, B (fig.22), 220 m long and 10- 15 m broad ran E-W. While A was embedded in very soft mud, B was in many places embedded in and covered by re-deposited sand. Where the divers had contact with it, it appeared to have much more space between the poles than A. Together these two breakwaters seem to have protected a shallow area well suited for a harbour. Behind the breakwaters a number of somewhat diffuse anomalies interpreted as 21 oblong pole structures more or less perpendicular to the coast were observed. Because three of them coincide with the three landing-piers excavated in 1979- 80 (Crumlin Petersen 1980; 1997), they were interpreted as such. This was supported by the pole zones observed during the diver inspection and by the fact that such anomalies were not observed outside the two breakwaters (figs. 6, 22). The dotted line in fig.22 marks an old eroded coastline related to a sea level about three m below that of today. Between this line and the coast, the Fig. 22: Plan of the harbour of Hedeby/Haithabu. bottom is sand, whereas to the north and east of it The proposed pier zones and the 1979-80 the bottom consists of deep mud. It is suggested that excavation are shown. A and B are interpreted the presence of hard bottom at a reasonable depth as breakwaters. C and D are wrecks. The has regulated the morphology of the harbour basin. easternmost parts of the circular ramparts are Had A been placed in a more eastern position, it seen. Recording: Ole Grøn, Lars Erik Larsen, and might have been a problem to get the poles into solid Gerd Hoffmann-Wieck 1996.

135 bottom. The orientations of the southernmost landing-piers indicate that access to them was not ideal because of the narrow basin. In the northern end there seems to have been no lack of space, and B is placed well ‘inside’ the old coastline. Apart from the pole structures, two anomalies were interpreted as wrecks. C is the already known wreck 3 in the southernmost part of the suggested harbour basin (Stümpel & Borth-Hoffmann 1983). The other one, D, is located in a more southerly position (figs. 4, 22). The seismic data indicated that it had a size of 5 by 20 m. Diver inspection indicated that it was a flat- bottomed barge. Later investigation showed it to be a 14.5 by 2.7 m large barge with a dendrochronological dating to 1184 AD (Carnap-Bornheim et al. 2002).

11. A Mesolithic site at Blak in Roskilde Fjord

In the first phase of the project, before the Mesolithic was excluded from the periods it dealt with, some promising initial results were in 1994 obtained from the Kongemose site Blak II in Roskilde Fjord where a partly excavated cultural layer, documented up to 1.5 m below the bottom, could be observed in the seismic profiles parallel to the deep ditch excavated by Søren A. Sørensen in the years before the seismic survey. It was a positive surprise that the system was able to penetrate the sandy gyttja on the site and produce such a clear image (3).

Fig. 23: Seismic profile through Søren A. Sørensens deep ditch at Blak II after it has been filled in. The cultural layer that is well-documented in the excavation area can be seen to continue outside it. Recording: Ole Grøn and Bo Steen, 1994.

136 12. Discussion

The results demonstrate that high-resolution sub-bottom profilers with success can be applied to systematic archaeological survey in areas with shallow water and a bottom of soft and even of rather sandy sediments. In contrast to side-scanners they can provide detailed information about archaeological features not visible above the top of the bottom sediments. The method described speeds up survey and makes it possible to observe features embedded in sediments not observable on the surface. Because the outlines of the anomalies are automatically registered directly with an accuracy of ±0.4 m, the use of divers for surveys and strategic investigations can be restricted to strategic points where specific problems concerning construction and stratigraphy can be solved. This improves the scientific quality of the results and reduces the total costs of survey considerably. The increasing number of features recognised as useful indicators of archaeological archaeological features such as single poles, white-outs, tidal-deposited lenticular soles of relatively coarse material, Mesolithic cultural layers, etc., already now provide a sound basis for systematic and detailed on-site interpretation of chirp recordings. It is important that the method also allows detailed interpretation of the features recorded. It is interesting that some of the barrages display what looks like a central opening controlled by what appears to be pole-rows forming local labyrinths (figs.13, 19). At Jungshoved, Gudsø Vig and Nakkebølle Fjord, fortifications apparently belonging to the Medieval Period seem to be protected by contemporaneous barrages. It appears that fortifications near the sea must be expected to have defences against naval attacks to an extent that is far beyond what was earlier anticipated, even though that should not be surprising. It is obvious that if the interpretation of the data had been based on processing in the laboratory and not on immediate recognition and detail-profiling it would only have been possible to obtain a small fraction of the verified results outlined in this paper. This is a crucial point if one wishes to develop a methodology that allows cost-effective application of chirp-systems to general survey situations.

137 Norsk sammendrag:

Marinarkeologisk rekognosering med høy-oppløselig penetrasjonsseismikk

1. Introduksjon Dagens fokus på marine ressurser og miljøfaktorer har understøttet en rask utvikling av teknologi til overvåkning av kystsonen. Den stigende bevissthet om betydningen av de historiske og arkeologiske plasser under vann samt deres enorme forskningspotensiale, har gjort det klart at de er et uunnværlig supplement til det arkeologiske materiale fra land. Det har derfor vært logisk å forsøke å anvende den nye teknologi til praktisk rekognosering på lavt vann. Mens ”side-scanner-utstyr” i dag er mye anvendt fordi de relativt raskt kan gi et overblikk over strukturer som er synlig over bunnen, er det kun penetrasjonssystemene som gir informasjon om den store mengde kulturminner som ligger skjult under bunn-nivå samt detaljer i den lokale geomorfologi. Parallelt med denne utvikling tyder forhold på at landbaserte myrer, som i mange land har rommet boplasser med fantastisk bevaring av det organiske materiale, har mistet sin bevarende effekt på grunn av landbrukets drenering og industriell utnyttelse av torv som brensel. Det er derfor sannsynlig at overgangen fra eldre til yngre steinalder best kan studeres i velbevarte myrer og tilstøtende landskaper som ble oversvømmet under havstigningen etter siste istid (fig. 2). Utover dette vil det i de kystnære områder finnes seilsperringer, havner, landingsplasser, broer osv. som ikke ses over bunnen, men er skjult i bunnens sedimenter. Denne artikkelen demonstrerer de såkalte Chirp penetrasjonssystemers potensiale i forbindelse med marinarkeologi. Kun et par av de anlegg som skal omtales ville ha blitt lokalisert med side-scanner, multi-beam eller dykker-survey (fig. 1).

2. Bakgrunn og metode Chirp-systemer er seismisk system som kan brukes til å penetrere bunn- sedimentene. Den sender ut et frekvens-intervall motsatt de tradisjonelle typer, som typisk arbeider med en eller to frekvenser. Den arbeider best på 0.5-10.0 m dybde. Ved større dybder må man senke ”fisken” med hydrofonene, så den er nær bunnen. Det ble benyttet et Chirp II system fra Datasonics/Benthos (CAP-6600) som arbeider med intervallet 2-23 kHz på to kanaler. Posisjonering av fisken ble foretatt med et Ashtech DGPS navigasjonssystem med en presisjon på ± 0.3 m i dynamiske målinger. Dette tillot oss å avbryte seilasen i de planlagte

138 linjer på stedet, for å lage ekstraprofiler som kunne avklare karakteren av en lokal anomali (fig. 4). Deretter var det uproblematisk å fortsette den påbegynte seillinjen. Denne interaktiviteten gjorde det mulig i høy grad å skjelne mellom arkeologisk interessante og uinteressante anomalier allerede i opptaksfasen, og overflødiggjorde etterfølgende prosessering i laboratoriet igjen etterfulgt av avklaring av de interessante lokaliteter. Det var viktig å seile langsomt for å få god horisontal oppløsning, best 1 m/sek. Som trening startet vi på allerede kjente og velregistrerte strukturer, som sperringene i Vordingborg og Haderslev Fjord. I startfasen dykket vi på alle detaljer av interesse for å lære hva vi så på skjermen.

3. Observerbare elementer Det viste seg mulig å observere påler og trestykker ned til ca. 10 cm i diameter i bunn-sedimentene, selv om de ikke kunne ses over bunnen (fig. 3, 5, 6, 11). På steder hvor mange påler står nær hverandre ble det generelt utviklet en total absorpsjon av signalet (’white-out’) som kan brukes som indikator (fig. 7, 8). Det ble også klart at tidevannet generelt avleirer en linse av grovere materiale enn de lokale sedimentene omkring en pålestruktur. Disse linsene er fremragende akustiske reflektorer som øker muligheten betydelig for å lokalisere arkeologiske pålestrukturer (fig. 9, 10, 21).

4. Sen vikingtids-/tidlig middelaldersperring i Vordingborg Fjord Opptakene fra dette allerede kjente testobjektet gjorde det klart at anlegget består av minst fem konstruksjonsfaser. Et sannsynlig forsvarstårn kunne ses i opptakene (fig. 11, 12).

5. Sen romertids sperring ‘Æ Lei’ i Haderslev Fjord Den velkjente romertidssperringen ’Æ Lei’ i Haderslev Fjord ble brukt til å sjekke metodens effektivitet. Det viser seg å være mulig å foreta en detaljert oppmåling og fortolkning ved et totalt forbruk på fem dagsverk (fig. 8, 9, 13). Denne oppmålingen ga mer informasjon om sperringen enn de tidligere dykkertokter hadde gjort, blant annet at dens nordlige del består av to faser og at den ser ut til å ha en sentral passasje med flere pålerekker for å regulere gjennomseilingen.

6. Strukturer i Jungshoved vik, sydøstlige Sjælland I Jungshoved vik ble det lokalisert fem anomalier (fig. 14): 1) de to sperringene A fra romertid (fig. 15) og B fra sen vikingtid/tidlig middelalder 2) en le- molo for en vikingtids havn, og 3) to fergeleier, E og F, fra sen middelalder. Tilstedeværelse av strukturene viser at stedet har hatt en sentral posisjon fra romertid til det 17. århundre, da borgen ved viken brant ned.

139 7. Flytesperringer i Gudsø Vik, sydøst-Jylland I 1985 ble det lokalisert en dobbel sperring fra sen vikingtid/tidlig middelalder tvers over seilløpet inn til Gudsø Vik like ved Kolding, samt en førromersk struktur som kunne fortolkes som en sperring eller et fiskegjerde. Oppfølgende seismisk rekognosering avslørte en rekke parallelle bånd (A-E i fig. 16) hvorav det nordligste viste seg å bestå av en rekke eiketrær med fellingsfasetter (fig. 17) – med all sannsynlighet en flytesperring – mens den neste så ut til å bestå av en linje av det grove materiale som avleires omkring påler/stokker av tidevannsstrømmen (fig. 18). Et mulig middelaldervrak er registrert omtrent hvor denne serie av antatte sperringer møter land i NØ. Struktur F (fig. 16) som også gikk på tvers av seilløpet besto likeledes av strømavleiret grovt materiale som ser ut til å være avleiret omkring 5-10 m lange stokker – altså enda en sannsynlig flytesperring. Innerst i viken ligger den lille øyen ‘Krabborre’ (Krabbeborg) (fig. 16). Demningen som forbinder øya med land består delvis av steinkister av eikestokker. De mange anleggene indikerer at viken har spilt en sentral strategisk rolle i lang tid.

8. En sperring i Kerteminde Fjord, nordøst-Fyn I Kerteminde Fjord ble det observert en 20-25 m bred og 250 m lang struktur som ser ut til å ha blokkert terskelen mellom fjorden og dens indre del ’Kertinge Nor’ (fig. 7, 10, 19). Også her fremstod en sentral åpning for gjennomseiling regulert av pålerekker. Strukturen er i etterkant datert til sen vikingtid/tidlig middelalder. Beliggende i et område som ser ut til å ha vært sentralt for handel til og fra Odense (navnet ’Ladby’ med den kjente skipsgrav fra vikingtid refererer sikkert til dette), og med Kertinge Nor som en opplagt mulig flåtebase har det vært god grunn til å beskytte lokaliteten.

9. En sen romertidssperring i Nakkebølle Fjord I Nakkebølle Fjord ble det observert en ca 200 m lang uregelmessig pålestruktur (fig. 20). Dykkerinspeksjon viste at strukturen besto av et ca 20 m bredt bånd av stokker spredt på bunnen samt et 1-2 m bredt bånd av stående påler (fig. 21). Dette støtter rykter om at det tidligere har vært samlet eikestokker i fjorden til møbelproduksjon. Dendrokronologisk er sperringen datert til ca. 370 f.Kr. Sperringen ligger umiddelbart sør for en middelalder befestning på den vestlige fjordbredd i fjordens indre og nordlige del, som i dag er drenert og oppdyrket. Angrep på Fyn allerede i tidlig romertid synes reflektert i våpenofferfunnet i Kragehul nordvest for fjorden (Engelhardt 1867; Ilkjær 1990, 338; Vang Petersen 1988:131).

140 10. Hedeby/Haithabu havn I 1953 ble det i en av de tidlige undervannsundersøkelser påvist konsentrasjoner av påler under vann i den gamle Hedeby Havn, samt et brent vikingskip (Hingst & Kersten 1955). Seismisk rekognosering ble utført med en 5 kHz ’pinger’ som førte til ytterligere funn av enda et vikingskip og to stammebåter (Stümpel & Borth-Hoffmann 1983). Ny rekognosering ble gjennomført med Chirp II systemet i 1995 og 1996. Dette førte til lokalisering av havnens to le-moloer, A og B, begge mer enn 200 m lange og bestående av tettstilte påler. I havnebassenget ble det lokalisert 21 landingsbrygger (3 av dem allerede utgravd i 1979-80 (Crumlin-Pedersen 1980; 1997) (fig. 6, 22). Undersøkelsen ga også anledning til lokalisering av det nye vrak 4 av en flatbunnet pram fra 1184 AD (Carnap-Bornheim et al. 2002) (fig. 22).

11. En Mesolittisk boplass ved Blak i Roskilde Fjord På lokaliteten Blak i Roskilde Fjord, hvor en mesolittisk boplass fra Kongemosekulturen tidligere er delvis utgravd, lykkes det å påvise resten av det dokumenterte men ikke utgravde kulturlag som lå flere meter under bunnen (fig. 23) (3).

12. Diskusjon Det intensive arbeidet med Chirp II systemet I perioden 1994-1996 har vist systemets marinarkeologiske anvendelighet. Kontrollerte gjennomseilingsåpninger i seilsperringer og kraftig sjøforsvar av middelalder- befestninger er ikke overraskende, men viktige å være oppmerksom på. Det er viktig at systemet tillater direkte fortolkning av de strukturer som observeres.

141 Notes

1) The datings obtained until now are (Bonde & Daly 1998; Daly 1997; Rasmussen 1998, 1999): Barrage A, Jungshoved, calibrated and with ±1 standard deviation: 80 AD (20-210 AD) (K- 6759). Barrage B, Jungshoved, calibrated and with ±1 standard deviation: 980 AD (895-1015 AD) (K- 6760). Break-water C, Jungshoved, calibrated and with ±1 standard deviation: 1020 AD (990-1155 AD) (K-6763). Western ferry landing, E, Jungshoved, calibrated and with ±1 standard deviation: 1530-1635 AD (1475-1660 AD) (K-6761). Eastern ferry landing, F, Jungshoved, calibrated and with ±1 standard deviation: 1430 AD (1405-1450 AD) (K-6762). The barrage in Kerteminde Firth, callibrated and with ±1 standard deviation: 1215 AD (1050-1265 AD) (K-6829); 1010 AD (970-1030 AD) (K-68230); 900-960 AD (880- 990 AD) (K-6831).

2) This information has been given by Ole Dam Mortensøn, Langelands Museum, who is a specialist in the ship history of the Funen area.

3) Søren A. Sørensen is thanked for supplying unpublished data from his excavations at Blak.

References

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