Friday 16 February 2024

A possible prehistoric hunting wall off the Baltic coast of Germany.

The seafloor is shaped by geological, biological, and to a certain extent anthropological forces, yet to a large extent is hidden from Human view. It has for some decades been possible to map larger seafloor structures with ship- borne multibeam echosounder systems, and to explore areas where there were known sites of interest with both submersible vehicles and Human divers. Nevertheless, huge areas of the seafloor remain effectively unexplored, hiding much information about the geology, biology, and even Human history of the planet.

In a paper published in the journal PNAS on 12 February 2024, Jacob Geersen of the Institute  of  Geosciences at Kiel University, and the Leibniz Institute for Baltic Sea Research WarnemündeMarcel Bradtmöller of the Heinrich Schliemann-Institute of Ancient Studies and the Interdisciplinary Faculty, Department WKT at Rostock UniversityJens Schneider von Deimling, also of the Institute  of  Geosciences at Kiel University, Peter Feldens, also of the Leibniz Institute for Baltic Sea Research Warnemünde, Jens Auer of the Landesamt für Kultur und Denkmalpflege Mecklenburg- VorpommernPhilipp Held, Arne Lohrberg, and Ruth Supka, again of the  Institute  of  Geosciences at Kiel University,  Jasper Justus Lutz Hoffmann of the Department of Coastal Geology at the Alfred- Wegener- Institute, and the Department of Geosciences, Marine Geology and Seafloor Surveying Group at the University of MaltaBerit Valentin Eriksen of the Schleswig-Holstein State Museums Foundation Schloss Gottorf of the Centre for Baltic and Scandinavian ArchaeologyWolfgang Rabbel, again of the Institute of  Geosciences at Kiel University, Hans-Jörg Karlsen, also of the Heinrich Schliemann-Institute of Ancient Studies at Rostock University, Sebastian Krastel, once again of the Institute  of  Geosciences at Kiel University, David Brandt and David Heuskin of the Institute for the Protection of Maritime Infrastructures at the German Aerospace Center, and Harald Lübke, also of the Schleswig-Holstein State Museums Foundation Schloss Gottorf and the Leibniz  Centre  for  Archaeology of the Centre for Baltic and Scandinavian Archaeology, describe a linear stone structure at a depth of 21 m, in the Bay of Mecklenburg, about 10 km northwest off Rerik in the Rostock District of Mecklenburg-Western Pomerania, Germany, which they interpret as a pre-historic, Human-made wall.

The Bay of Mecklenburg was shaped during the Weichselian Glaciation (i.e. the most recent glacial interval), and since the retreat of the ice has been significantly uplifted by isostatic rebound (the weight of the ice pushed the solid lithosphere down into the liquid mantle, and when removed it bobbed up). At the same time, the sea level in the area has been calculated to have risen significantly, as water locked up in glacial ice was released, from about 40 m below its current level to about 20 m below, between 13 300 and 12 700 years before present, and from about 28 m below the current level to about 10 m below between 8570 and 8000 years before the present. Since the water in the Bay of Mecklenburg current maximum depth of 28 m, this implies an extended period between the retreat of the ice and the area being covered by the sea. The seafloor over much of the bay is covered by Holocene mud and sand (sandier towards the shore), overlying a glacial till, which is exposed in places.

The seafloor of the Bay of Mecklenburg has a high density of archaeological sites, many of which were documented by the Sinking Coasts project between 2002 and 2009/ Much of this work concentrated on the period after the Littorina Transgression (i.e. following the first establishment of a sea within the Baltic Basin), with 23 sites discovered which date to between 8500 and 5000 years ago, which currently lie in waters between two and eleven meters deep. Older sites are known from the shores of Scandinavia and the Eastern Baltic, but most of the known older sites in Germany are further to the south, away from the Baltic Coast. The putative wall discovered by Geersen et al. lies at a depth of about 21 m, and presumably therefore predates the Littorina Transgression.

During recent decades, the seafloor of the Bay of Mecklenburg has been extensively explored by hydroacoustic surveys.  In 2021 one of these surveys recorded a linear structure almost a kilometre long and slightly less than a metre in height. This structure, which Greersen et al. refer to as the Blinkerwall, is located on the southern, landward, side of a northeast–southwest trending bathymetric ridge. At its northeast end, the structure abuts a conical mound which rises to 13.5 m below sealevel. The Blinkerwall runs for 971 m, sinking from a depth of 21 m at the eastern end, to 21.5 m in the west. 

The study area in the Baltic Sea. (A) Overview map of the Western Baltic Sea. (B) Detailed structure of the Bay of Mecklenburg including the location of the Blinkerwall. Greersen et al. (2024).

The majority of the Blinkerwall is less than a metre high, and visits by divers and autonomous underwater vehicles have shown that it is made of a succession of individual stones. 1673 individual stones were identified, with a combined volume of 52.75 m³, and a calculated combined mass of 142 473 kg (based upon the assumption that the average density of the rocks is equivalent to that of granite). Most of the stones have calculated masses below 100 kg, but 288 were identified as likely to be heavier, with the largest, towards the centre of the wall, having a calculated weight of 11 389 kg. This rock marks a turn in the direction of the wall, which runs east-west  to the west of this rock, and southwest-northeast to the east. The next three largest rocks, with masses of 2083 kg, 2506 kg, and 5792 kg are all located towards the western end of the wall, with the largest one marking the end of the wall. The ten heaviest stones are all associated with slight changes in the direction of the wall.

Morphology of the southwest–northeast trending ridge that hosts the Blinkerwall and the adjacent mound. (A) Multibeam bathymetry collected with RV ALKOR (2021) and FK Littorina (2023). (B) Multibeam bathymetry collected with an autonomous underwater vehicle. (C) Side- scan image (bright colors equal high backscatter) collected with RV Elisabeth Mann Borgese in 2020. White arrows point at the Blinkerwall. Greersen et al. (2024)

The heights of the rocks were measured with several different techniques, producing consistent results, which increased Greerson et al.'s confidence in their findings. There are almost no stones of similar size within 10 m on the northward side of the wall, particularly towards its western extent, and the density of such stones is depleted compared to the rest of the bay further north than this, not reaching typical levels till about 50 m north of the wall. No stones of similar size could be found south of the wall.

Bathymetric scans show that the wall runs along a bathymetric ridge, with sedimentary basins to the north and south. Within the basin to the south there are two sedimentary units, the lower of which is 3,5 m thick and has an erosional upper surface, while the upper unit is about 2.5 m thick. A sediment core sunk in this area recorded an upper layer 1.8 m thick, which was comprised of Holocene mad, over a thin peat layer, then an eroded surface from which some wood fragments were recorded. The wood was dated to 10 578 years before the present, leading Greersen et al. to suggest that the eroded layer probably corresponds in time to the Yoldia to early Ancylus Lake lowstand, about 11 700 years ago; similar unconformities have been recorded at this time from other locations in the region. Another piece of wood, recovered from the upper layer of sediment, was dated to 9886 years before the present. Based upon this, Greerson refer to the upper layer as Holocene sediments and the lower layer as Baltic Ice Lake sediments.

North- south- oriented sediment echosounder profile across the east–west trending ridge that hosts the Blinkerwall and the sedimentary basin to the south. The erosional unconformity of the Yoldia to early Ancylus Lake lowstand land surface is located at 1 to 3 m depth in the basin and crops out slightly south of the Blinkerwall. The peat layer (dated to 10 662 to 10 494 years before present) that hindered a deeper penetration of gravity core is visible as a layer with low reflectivity located above the erosional unconformity. The latter is underlain by the sediments of the Baltic Ice Lake. Greersen et al. (2024).

The eroded layer reaches the surface slightly to the south of the Blinkerwall, with till sediments on the  ridge not covered by Holocene muds. This also occurs at the southern margin of the basin, towards the shore at Rerik and the islands of Wustrow and Poel. The bathymetric results suggest that at the Yoldia to early Ancylus Lake lowstand, if  the Blikerwall already existed, then it would have run along an east-west running promontory, connected to the German mainland to the southeast.  To the south of the wall would have been a 5 km wide basin with a maximum depth of 25 m.

The Blinkerwall is a remarkable  topographic feature, unlike anything else which has previously been detected beneath the Baltic Sea, located upon  reworded  till deposits  associated with the Weichselian  Glaciation. The Weichselian ice retreated from the area about 17 000 years ago, leaving an exposed till deposit, which was covered by floodwaters and then exposed several times before the final development of the Baltic Sea in the early Holocene. The Blinkerwall is currently 21 m below sealevel, but would have been exposed above the surface during the Baltic Ice Lake phase, around 12 800 years ago, and the Yolida Sea phase, about 11 700 years ago, both of which fall within the Younger Dryas interval. The wall would also have been exposed on the surface during the Early Holocene Ancylus Lake regression, about 9500 years ago, before finally being covered by the Baltic Sea during the Littorina trans­gression, between 8600 and 8000 years ago.

The deposition of Holocene muds has led to a thick sediment cover over much of the Baltic seafloor, burying many topographic features. The Blinkerwall lies upslope of a basin now largely infilled with stratified muds. Greersen et al. reason that the lowest sediment layers in the basin predate the Yoldia to early Ancylus Lake lowstand unconformity, and probably therefore date from the time of the Baltic Ice Lake or Baltic Ice Lake II. Exposure and erosion of the till layers would have led to the concentration of rocks upon the surface, although south of the Blinkerwall these are now largely covered by subsequent sediments. However, these processes are likely to have contributed to the presence of at least some of the rocks that make up the 971 m long Blinkerwall.

Autonomous underwater vehicle multibeam data from sections along the Blinkerwall. Greersen et al. (2024).

Stones can be moves and emplaced by natural processes. The largest tsunamis are capable of shifting boulders with diameters of several metres, and depositing them in strewn fields. No tsunami, however, has ever been recorded placing rocks in a neat line along a promotory, and it is difficult to conceive of one ever doing so. 

Glaciers are also capable of moving rocks. The retreat of the Weichselian Ice Sheet left a variety of landforms across northern Europe, which are still largely responsible for the landscapes seen today. These include a variety of moraines, linear structures caused by rocks and till being pushed into place by the movement of glaciers, but these take the form of ridges of unconsolidated debris, including gravels and clays as well as rocks and boulders. 

Eskers are linear structures formed beneath glaciers, from material deposited in streams running beneath the glacier. In the Baltic region eskers with high-stone compositions up to 7 m high and a few hundred metres in length have been recorded, and in other parts of the world, eskers up to 50 m high, 150 m in width, and several tens of kilometres long have been recorded. All eskers, however, are comprises of a mixture of sand, gravel, and rocks, and they do not contain neat linear arrangements of rocks. 

The deposition of material by floating ice has been cited as a cause for beach ridges elsewhere around the Baltic, although these are again generally made up mostly of sand and gravel. Furthermore, a large beach ridge would require a stable coastline over a long period of time, so that the material was always placed in the same position. The Blinkerwall is located at a depth of 21 m, and while beaches at 21 m below sealevel undoubtedly existed during the Younger Dryas, the sealevel is known to have fluctuated significantly over this interval, making it unlikely that a particularly large beach ridge would have built up during this interval.

The most plausible natural cause for the Blinkerwall would be ice thrusting associated with the Baltic  Ice Lake, about 13 000 years ago. This could conceivably have formed a shoreline at -21 m, and occurred at a time when the glacial till was not covered by significant amounts of sediments. However, a wall formed at this time would have had to endure several subsequent emersions in high-energy coastal waters during the subsequent transgressions and regressions of the sea, and it seems unlikely that the Blinkerwall, less than 2 m wide and made mostly of rocks weighing less than 100 kg, would have survived this as intact as it is. Nor is the form of the Blinkerwall consistent with such an origin, as such thrust ridges tend to be several, often tens of, metres wide, and contain significant proportions of sand and gravel, whereas the Blinkerwall is less than 2 m wide, and made entirely of large rocks and boulders.

Taken together, these lines of evidence suggest that it is highly unlikely that the Blinkerwall is a natural phenomenon, and that therefore it was most likely formed by the activity of prehistoric Humans.

The Blinkerwall is located at a depth of 21 m, in a location which would have been covered by the waters of the Baltic for a final time between 8570 and 8000 years ago, giving a minimum possible age for its construction. Preserved wood in the basin to the south of the Blinkerwall has been dated to 9100 years before the present, indicating that this basin must have flooded after that time. This is long before the appearance of agrarian societies in Europe, at a time when the Baltic Basin was known to have been inhabited by hunter gatherer societies living in small bands and forming non-permanent camps. The Blinkerwall is made up of 1385 smaller stones with masses of less than 100 kg, which could conceivably have been moved by small groups of Humans, connecting 288 larger stones, which it is unlikely that Humans of the time would have been able to move with the tools available to them. All of the rocks are emplaced on the southern flank of a ridge made of glacial till, where rocks of this size would have been readily available; the absence of such rocks in the zone to the north of the wall supports the idea that the rocks were sourced from close by and deliberately emplaced, and that most of them were moved downslope to get them into position. It has been estimated that about 1281 people were living in Germany and Poland during the latest glacial interval, in bands of 40-45 people. For a group of this size to construct a stone wall almost a kilometre in length would not be impossible, but the project would have had to have been of considerable benefit to them to have been considered worthwhile.

No similar structures are known from the region. Late Mesolithic communities are known to have made extensive use of coastal resources, including fishing with static wooden weirs. These structures would have been at most a few hundred metres in length, significantly smaller than the Blinkerwall, and are generally emplaced on rivers or on coastal sections with a strong tidal range, so while stone Fish weirs have been recorded in other parts of the world, it is unlikely that this was the intended purpose here. 

Groins for sea defence purposes have been recorded in the Neolithic of the Middle East, though the Blinkerwall appears too small to have served such a purpose. Nor is it likely to have formed part of an artificial harbour, since it predates the documented use of boats in the region significantly, making it likely that even if boats did exist, they would have been small structures easily dragged up a beach to safety. Furthermore, both a groin and an harbour wall are structures likely to be built only by people with a permanent claim to the land, which is contrary to our understanding of roaming Mesolithic bands of hunter gatherers, Rather, for such a structure to have purpose, the people who made it would have needed to have been able to leave it unattended for much of the year without any maintenance, and to have been confident it would have been ready to use at any time when they did return.

The purpose that Greersen et al. suggest for the wall is as a piece of hunting architecture, used to control the movements of herds of Ungulates as they were driven by hunters. While we do not today associate hunter gatherer societies with large architectural projects, recent studies in the Middle East, where an arid climate tends to favour preservation of such structres, have shown the presence of numerous long stone walls, and evidence of such stuctures has also been uncovered in western Greenland and the Great Lakes Region of North America. These structures vary in form, and often include features such as multiple drive lanes, enclosures for trapping Animals, and blinds where hunters could have hidden. In the Middle East these walls are presumed to have been used to hunt Gazelles or other Antelopes, and while such Animals would not have been present in the Baltic region, such a wall would have been equally effective in controlling the movements of Bison or Reindeer.

The dating of large stone walls has proven problematic, beyond stating that they are clearly prehistoric, although walls apparently used for hunting Reindeer and now submerged by changes in sea-and/or-lake level, such as the Blinkerwall and the 'Drop 45' structure on the Alpena-Amberley Ridge beneath Lake Huron in North America can be presumed to date from the Late Glacial/Early Holocene transition. The Blinkerwall and 'Drop 45' structures share a number of features, including a location near the top of a slope, but below the crest, a subparallel trending marsh/lakeshore on one side, construction on bedrock, and solid and continuous construction. The 'Drop 45' site is also presumed to be a drive lane used in the hunting of Ungulates, something which in this case has been supported by the recovery of numerous stone tools associated with hunting by divers, which were concentrated at points interpreted as hunting blinds. It is possible that the large erratic blocks which served as part of the Blinkerwall could also have served as hunting blinds, giving a potential target for future archaeological surveys.

In other ways, however, 'Drop 45' differs significantly from the Blinkerwall, comprising two stone lines which narrow towards a funnel through which the hunted Animals were presumably driven, and covering a total area of only about 30 m by 100 m. It is possible that another line of stones is present to the south of the Blinkerwall, which has now been covered by Holocene mud, or it is possible that the second limb of the trap was formed by the former lake in the basin to the south of the wall. Reindeer will enter water, and are fairly good swimmers, but are much slower in water than they are on land, which would make it easy to target them from nearby boats.

Most ancient drive lane traps are believed to have been placed on the migration routes of Ungulates, effectively allowing a herd to be targeted twice a year, and significantly increasing the yield of meat achieved in those hunts. The 'Drop 45' site is thought to have been associated with Caribou, and the most likely candidate for the target of the Blinkerwall would have been Eurasian Reindeer (which are the same species). Reindeer are known to have been targeted as a prey species since at least the Middle Palaeolithic, and would certainly have been a familiar prey species to any hunting population living in the Baltic region. Such hunters would have been familiar with the behaviour of Reindeer, and able to predict how they would react to a structure like the Blinkerwall. Furthermore, the Reindeer population of the period is thought to have migrated east-to-west, making a location on am east-west oriented ridge an ideal location for such a trap.

As the Holocene progressed, the area would have become more forested, changing the seasonal climatic patterns, and causing the Reindeer herds to shift their migrations northward. By around 9800 years ago, they would have been absent from the region, which implies that if the Blinkerwall was used for hunting Reindeer, then it is probably Pre-Boreal in origin, making it one of the oldest known hunting structures in the world, and possibly the oldest known megastructure in Europe. However, direct dating of such structures is notoriously difficult, as datable remains such as carcasses were generally cleared away by the site's users. The oldest known structure for which a direct date exists is a desert kite in Jordan, which has been dated to 10 000 years before the present. The Blinkerwall is potentially older than this structure, though direct dating of it is likely to be extremely problematic.

3D model of a section of the Blinkerwall adjacent to the large boulder at the western end of the wall. The scale bar at the Top- Right edge of the image is 50 cm. Philipp Hoy & Jens Auer in Greersen et al. (2024).

Based upon analysis of sedimentological, geophysical, and archaeological data, Greersen et al. suggest that the Blinkerwall dates from the End Pleistocene or very earliest Holocene. There is an absence of similar structures from this period in Europe, but that does not indicate that they were not being built, rather it is likely that they have simply not survived the to modern times on the densely populated European continent, or that other surviving structures have not been identified. In offshore environments, the most likely environment for such a structure to survive, the identification of walls is at the limit of capability of hydroacoustic surveys. Furthermore much of landscape which would have been exposed at the end of the Pleistocene has subsequently been covered by Holocene mud, so structures would only be visible if they were located on highgrounds which still remain exposed. There are, nevertheless, a number of areas beneath the Baltic where moraines associated with the Weichselian Glaciation are known to still be exposed, at depths of 20 m or more, providing locations which could provide further insights into Late Glacial and Mesolithic cultures during phases of rapid advance of the Baltic Sea.

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