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LOU SCHMITT, STEPHAN LARSSON, CORINNA SCHRUM, IRINA ALEKSEEVA,MATTHIAS TOMCZAK AND KRISTER SVEDHAGE

‘WHY THEY CAME’; THE COLONIZATION OF THE COAST OFWESTERN SWEDEN AND ITS ENVIRONMENTAL CONTEXT ATTHE END OF THE LAST GLACIATION

Summary. In this paper we will bring into view new aspects of LatePalaeolithic and early Mesolithic research on the west coast of Sweden. Indoing so, we make use of oceanography and tidal modelling, in conjunctionwith basic research in the fields of archaeology and palynology. The focus of research concerns the Hensbacka culture group in central Bohuslän, a group of hunter-gatherers which visited the area between c.10,300–9300bp(10,200/10,000–8500cal BC). Recent investigations indicate that the frequencyof Hensbacka sites in the archipelago of central Bohuslän, which at that timehad a total land area of c.500sqkm, might well represent the highest sitedensity area in northern Europe during a c.1000-year period of time at theclose of the Late Glacial and beginning of the early Post Glacial. In the pagesthat follow, we will discuss how, and why, this ‘seasonal colonization’ waspossible.

introduction

During the past ten years, researchers working with the Older Stone Age of westernSweden have become aware that an early phase of a local culture group known as the Hensbackais, for the most part, synonymous with the Ahrensburgian culture group found on the NorthEuropean Plain (Schmitt 1991, 1994, 1999; Nordqvist 1995; Kindgren 1996). Althoughnumerous publications have dealt with technological and anthropological similarities betweenthese two cultural groups, we have neglected to address the various palaeogeographic changesthat made this (as we see it) seasonal colonization possible. In this paper, we will attempt tocorrect this oversight. In doing so our primary focus concerns the carrying capacity of centralBohuslän (Fig. 1) between 10,300–9300bp (10,200/10,000–8500cal BC) since both field andliterature studies indicate that archaeological sites from this period of time are extremelynumerous. Consequently, the question we ask is: what made this particular area exceedinglyattractive to Late Glacial/early Mesolithic hunter-gatherers? In short, can we find a plausibleexplanation as to why the colonization of western Sweden almost certainly started in centralBohuslän? In order to answer this question, we have elicited expertise from three academicdisciplines: archaeology, oceanography, and palynology.

OXFORD JOURNAL OF ARCHAEOLOGY 25(1) 1–28 2006© Blackwell Publishing Ltd. 2006, 9600 Garsington Road, Oxford OX4 2DQ, UKand 350 Main Street Malden, MA 02148, USA. 1

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the palaeogeographic and environmental background

The North Sea basin

During the Late Middle Weichselian, the entire North Sea basin was ice-free and aeriallyexposed until the sea began to rise at about 20,000bp (Oele and Schüttenhelm 1979). In otherwords, when the ice front had reached its maximum extension, present-day Continental landareas extended into what we today know as the North Sea (Fig. 2). After this time, and by 15,000bp, the ice front had receded, but was located on the south side of the Norwegian trench; by13,000bp, however, the ice had retreated to the north side of the trench, thereby allowing‘warmer’ Atlantic water to reach the Swedish west coast (Van Weering 1982). A consequentresult of this massive ice recession is seen in dramatic changes of sea-level within the basin,from, for example, −130 at c.20,000bp (22,000cal BC), to about −90 at 12,000bp (12,000calBC) (Jelgersma 1979).

Following on from when the Weichselian ice front started to recede, sedimentation viacirculation currents in the restricted North Sea basin and Skagerrak played an essential role inthe redistribution of detritus derived from the melting ice as well as from former aerially exposedmorainic sediments (Van Weering 1982). Although palaeogeographic changes in the land area

COLONIZATION OF THE COAST OF WESTERN SWEDEN

OXFORD JOURNAL OF ARCHAEOLOGY2 © Blackwell Publishing Ltd. 2006

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Figure 1Map showing the area referred to in the text as central Bohuslän.

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LOU SCHMITT ET AL.

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Figure 2Map showing the maximum extension of the ice margin in the North Sea basin during Weichselian glaciation. Dashed

line indicates that the exact position of the margin is uncertain. After Jansen et al. 1979.

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between Denmark and England during the period 20,000–12,000bp were insignificant in termsof total area inundated, the changes after 12,000bp were considerable (Fig. 3). Sediment corestaken from the bottom between Denmark and Sweden indicate that sedimentation rates increasedin this area after 10,000bp (ibid.). Furthermore, Van Weering points out that during the period



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Figure 3Reconstructed bottom relief maps showing the various shore lines in the North Sea basin during the close of the LateGlacial period, and the beginning of the early Post Glacial period. Numbers in parenthesis indicate the approximate

depth of these shore lines below present day sea level. Redrawn from Jelgersma, 1979.

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13,000–10,000bp, winnowing through wave and current actions on former land areas to thenorth of Dogger Bank must have resulted in the removal and transport of material to other partsof the Skagerrak basin (Van Weering 1982). This being the case, and assuming that the anti-clockwise current system in the Skagerrak was the same then, as it is today (Fig. 4), we can bereasonably sure that material, in the form of sediment and dissolved nutrient salts removed fromthe progressively inundated North Sea basin, was deposited along the west coast of Sweden tothe north of Göteborg.

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Figure 4The present day circulation pattern in the Skagerrak Sea and adjacent areas. Unfilled current trajectories representwarm Atlantic water; solid black arrows indicate colder sea water. Note the anti-clockwise pattern in the Skagerrak.

Redrawn from Svansson, 1975.

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Many of the perspectives being offered here are compatible with archaeologicalresearch that addresses the close of the Late Glacial period. Illustrative of this point is theinteresting environmental situation that was brought to a close by the inundation of the basin tothe west-north-west of Denmark. The land areas seen in Figures 2 and 3, for example, wereaerially exposed to various extents between 20,000–9000bp. Within this time span of about11,000 years, it is more than probable that a soil layer could, and did, develop as the result ofthe seasonal cycle of vegetation (Schmitt 1991). Indeed, during warmer periods the presence oftrees should also be taken into account since wood has been recovered from the bottom of theNorth Sea and C-14 dated to 13,000bp (Sejrup et al. 1994). This probable vegetational coverin the basin is further substantiated by pollen studies carried out in south-west Norway in anarea that was located about 100km due north of the nearest shore line in the North Sea basinland area at the close of the Late Glacial (Fig. 5). In addition, there is general agreement betweenthe Norwegian studies and similar studies relating to north-west Denmark during the sameperiod of time (Bennike, pers. comm. 2004). It should be borne in mind that present-dayDenmark actually represents the eastern end of the area which constituted the North Sea landarea during the close of the Late Glacial. Assuming that the studies are correct, we find no reasonto believe that the former land area in the North Sea basin did not have a high degree ofvegetational productivity and was, therefore, a likely habitat for hunter-gatherers.

The Vänern basin

During the Late Weichselian, Bohuslän had been depressed about twice as much as thesouthern parts of Halland (Eriksson 1979). The reason for this is that the depression made inthe earth’s crust by a glacier is directly proportional to the weight (i.e. thickness) of the ice.With the removal of the weight, areas that were depressed more than others would have regainedtheir former crustal equilibrium faster. Since the earth’s crust is not fully elastic, this gave riseto faults and fissures.

In this paper we are primarily concerned with the environmental effects of a regressiveshoreline brought about by the regaining of crustal equilibrium in central Bohuslän. The isostaticland uplift that was taking place in this area (Svedhage 1985) created a situation in which glacialmelt-water within the Vänern basin became constricted (Fig. 6). Consequently, the velocity ofthe stratified reaction current, entering and leaving the basin, was progressively increased owingto greater energy factors within the stratified water column. It should also be noted that thenumerous, and increasingly larger, islands that were situated in the archipelago at the western



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Figure 5Simplified drawing illustrating the vegetational development during the Late Glacial on the SW coast of Norway.

Redrawn from Fuglestvedt, 2001.

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Figure 6Maps showing the development of the archipelago in central Bohuslän brought about by isostatic land uplift during,

and after, the recession of the ice margin. After Fredén, 1988.

end of straits oriented towards the Vänern basin in the north-east, almost certainly caused a greatdeal of turbulence within the reaction current in the vicinity of islands (Fig. 7). Thesehydrodynamic features within the basin, and in the archipelago to the west, ceased to exist atabout 9300bp when straits which separated them dried up as the result of continued isostaticland uplift (Fredén 1988) (see also Figure 16).

Judging from the sub-fossil record, these hydrodynamic features before 9300bpprovided an exceedingly rich environment for shellfish, sea mammals and even their predators.

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For example, the shell deposits in central Bohuslän are the best known shell-banks in Sweden(Fredén 1988). The reasons for this are numerous since they have been exploited since theeighteenth century for various purposes. However, particular notice must be taken of their size.For example, 23 shell-banks, just to the east of the city of Uddevalla, have an estimated volumeof about one million cubic metres – or slightly more (ibid.). These particular shell-banks arelocated in one of the straits that ran between the archipelago in the west, and the Vänern basinin the east. Fortunately, many of the sub-fossil finds recovered from these shell-banks, and othershell-banks in the area, have been C-14 dated. Accordingly, we know that seals were present inthe area between 11,600–9800bp along with white whales at 10,180bp and 9910bp (ibid.). Polarbears were also present at 10,380bp and 10,170bp (Berglund et al. 1992). In brief, there is noreason to doubt that this region abounded with both marine and terrestrial wildlife at the closeof the Late Glacial.

In addition to the marine and terrestrial forms of wildlife, palynological studies indicatethat the climate, in our area of investigation at the close of the Late Glacial and beginning ofthe early Post Glacial, was relatively warm. The pollen record, for example, indicates thatEmpetrum (crowberry) was present in Bohuslän at 12,000bp (Huntley and Birks 1983; Svedhage1985), along with Juniperus (juniper) at 11,800bp (Svedhage 1985). Further on in time, we findCalluna (heather) at 10,000bp (ibid.), as well as Corylus (hazel) at 10,000bp (Huntley and Birks1983) and at 9700bp (Svedhage 1985). Lastly, Quercus (oak), Ulmus (elm) and Alnus (alder)are all present by 9900bp (ibid.). Indeed, Tilia (lime) make their appearance at 9500bp (ibid).It should be noted that the appearance of Empetrum in Bohuslän (12,000bp) is approximately600 years earlier than its appearance in southernmost Sweden at 11,430bp (Björck 1979). Thesame holds true for Corylus, but here the difference is reduced to 500 years in that Corylus ispresent in Bohuslän at 10,000bp, while it first appears in southern Sweden at 9500bp (Huntleyand Birks 1983). It is also interesting to note that the Empetrum II maximum, during the Younger



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Figure 7Concept drawing showing the tentative circulation pattern and turbulence within a stratified water column in thearchipelago of central Bohuslän during the close of the Late Glacial. Unfilled arrows indicate brackish water. In part,

after Freden 1988, and pers. comm. N.A. Mörner, University of Stockholm.

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Dryas chronozone, is dated to 10,100bp in southernmost Sweden (Björck 1979), while the samemaximum is dated to 10,250bp in western Sweden just to the north of Göteborg (Svedhage1985). While one cannot draw definite conclusions from these apparent differences in regionalchronology, they seem to suggest that the climate zones might have been inverted during theclose of the Late Glacial. That is to say, the climate that prevailed along the west coast of Swedenwas milder in the north than it was in the south.

a new model in an old context

In seeking an explanation for the concentration of activity in central Bohuslän (Fig. 1),we envisage a pattern of seasonal rounds. Long-term field studies conducted by the authors(Larsson and Schmitt), as well as a review of the literature (see for example Kindgren 1995),indicate the existence of more than 1000 archaeological sites in the relatively limited area ofcentral Bohuslän. A site, in this case, means a small area usually with a diameter of between2–5m in which numerous chipped flints, such as flakes, blades, and diagnostic tools, are foundand/or a previous excavation has been carried out. The colonization seems to have taken placein various phases during a c.1000-year period between 10,300–9300bp and, during recent years,this regional culture group, referred to as the Hensbacka, has been correlated with theAhrensburgian on the North European Plain (see, for example, Schmitt 1994; Nordqvist 1995;Kindgren 1996). From our point of view, these sites can only be understood from their LateGlacial/early Post Glacial environmental context.

It is our contention that the ecological affluence found in the archipelago of centralBohuslän at the close of the Late Glacial would not have been possible if the land area whichexisted between western Denmark and eastern England had not been there or, having been there,had not been progressively inundated over a long period of time. In brief, had either of theseinstances been true, the archipelago in central Bohuslän would have possessed the sameecological conditions as any other archipelago into which glacial melt-water empties. Clearly,this was not the case in Bohuslän which was regionally unique and cannot be paralleledanywhere along the Swedish west coast to the south of Göteborg.

We believe that the inundation of the North Sea basin acted, together with incomingAtlantic water, as a catalyst in a developing ecological system in the archipelago of centralBohuslän. Accordingly, nutrient salts in the form of phosphates, nitrates, potassium, calciumand magnesium were mobilized and removed from the progressively transgressed land areabetween Denmark and England, this easterly moving nutrient-rich sea current arriving at acomparative ‘end station’ in the archipelago of central Bohuslän (see Figure 4). Furthermore,and even more important, nutrient-rich Atlantic water masses arrived as a bottom current whichcontinued eastwards into the Vänern basin since a stratified reaction current had been formedin which glacial melt-water flowed towards the west as a surface current. We have mentionedearlier that turbulence, i.e. between the outgoing glacial melt-water and incoming sea water,probably occurred in the vicinity of islands situated at the eastern end of straits leading out fromthe basin (Fig. 7). In this situation, mixing in the water column would have been enhanced bysignificant fluctuations of tidal movement. This mechanism and the consequent increase inbiological productivity have been described previously, e.g. by Simpson et al. (1982). In orderto establish if this has been the case, a tidal model reflecting topographic conditions in the NorthSea basin at c.10,300bp has been constructed. Before moving on to our model, a few wordsmust be said concerning present-day conditions in the North Sea.

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oceanographic insights concerning the present-day north sea and the past situation

The early Post Glacial North Sea, like the present-day North Sea, is characterized byintense exchange with the North Atlantic Ocean. Winds and density-driven currents result inaverage nutrient-rich inflows in the order of 1SV (106 m3/s), which is compensated by riverrunoff and an outflow in the order of 1.03SV (e.g. Rohde 1998). The incoming North Atlanticwater mass is characterized by high nutrient contents. Because of its enormous volume, theNorth Atlantic nutrient source is by far the most important nutrient source for the North Sea(e.g. Reid et al. 2001), creating there a potential for high biological productivity. However, theoverall nutrient availability is less important. More important are the nutrient resources in theeuphotic zone. Nutrient availability in surface waters is favoured by intense vertical mixing.The local turbulent vertical mixing regime of a region can in the first instance be expressed asthe difference between current shear (favouring vertical mixing) and density stratification(suppressing vertical mixing). Strongest shear stress is produced by highly energetic currents,with the tides having the largest potential to produce mixing. Stable stratification can be causedby fresh water runoff.

In the North Sea, the specific topographic characteristics favour high nutrientavailability in the euphotic zone. Tidal currents in the North Sea, which are generated in theopen ocean and propagate into the North Sea, are highly energetic. Decreasing water depthsresult in increasingly energetic tidal currents towards the continental coast, which can reach upto 2.5m/s. In the present-day North Sea, tidal stirring prevents stable seasonal stratification in the shallower parts of the sea and creates tidal mixing frontal systems (as described by, e.g., Simpson and Hunter 1974). Such systems are well known as areas of high biologicalproductivity (e.g. LeFevre 1986; Munk et al. 1995). Recently, increased productivity wassimulated by application of a 3-D hydrodynamic ecosystem model. Hydrodynamic frontalparameters have been used to explain observed aggregation patterns in higher trophic levelspecies (e.g. Floeter et al. 2005), predator–prey overlap (Floeter et al. 2004a), and recruitmentvariability (Floeter et al. 2004b). The unique frontal ecosystems are characterized by near-bottom onshore transport of saltier and nutrient-rich Atlantic water masses. Intense verticalmixing in the frontal zone, created by tidal stirring, brings nutrient-rich water to the euphoticzone resulting in local continuous primary production. The resulting increased secondaryproduction and increased zooplankton availability attract higher trophic level predators. In thepresent-day environment, the tidal forcing is intense in the North Sea, with amplitudes wellabove 1m, but is drastically reduced in the Skagerrak/Kattegat system. Present tidal amplitudesat the Swedish Baltic Sea coast are well below 30cm (Fig. 8). Kattegat tidal stirring is presentlynot strong enough to create characteristic frontal ecosystems. Local freshwater-inducedstratification, caused by, for example, continental runoff and Baltic Sea outflow, suppresseslocally vertical mixing and thus reduces the local biological productivity.

In contrast to present-day conditions, at about 10,000bp, central Bohuslän did notreceive the outflow from the Baltic Sea catchments. Instead, it received runoff from glacial melt-water which, if the glacier in the Vänern basin had a thickness of 1–2km, is estimated to havebeen between 10,000–20,000 cubic km during a 600-year period of time (see, for example,Fredén 1988, figs. 53–6) and thus comparable to present-day runoff of the Elbe. Although thisis only about 1/10 of the present-day Baltic Sea runoff, it was a sufficient amount of freshwaterto stabilize the water column vertically and suppress vertical mixing locally. Because of the



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balance of earth rotation and freshwater-driven density currents, the freshwater-driven flowwould have followed a counter-clockwise pathway along the Swedish and Norwegian coast,similar to present-day fresh water outflow. Together with additional continental runoff, whichwould have travelled along the south-eastern coastline towards the Swedish coast, a strongstabilization of the water column due to the freshwater signal could be assumed along the pastSwedish coastline. Accordingly, a mechanism is needed to explain apparent increases in localmixing to serve as a potential explanation for local high productivity in central Bohuslän.Significantly, increased tidal amplitudes could provide such a mechanism for the creation of aunique ecosystem along the coast of Bohuslän, even if strong stabilizing effects from the glacialmelt-water were present. However, whether changes in local tidal currents could havecontributed to significant changes in vertical turbulent mixing around 10,300bp is still unclear.With the present investigation, we aim to study the past tidal conditions of the Late Glacial/earlyPost Glacial North Sea–Skagerrak–Kattegat system. In order to investigate the tidal situation atabout 10,300bp, we will make use of a numerical hydrodynamic model.

the hydrodynamic model

The model that was selected for the present investigation is the HAMSOM (HAMburgShelf Ocean model) (Schrum and Backhaus 1999). HAMSOM is a non-linear free-surface

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Figure 8Present day time series of sea surface elevations (SSE) in a full spring-neap period for sites 1–6. See figure 10 for

site locations.

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model, which has been employed frequently to study hydrodynamics (e.g. Janssen et al. 2001;Harms et al. 2004) and ecosystem dynamics in shelf sea systems (Schrum et al. 2004). In thepresent configuration it is used as a barotropic 2-d tidal model with a spherical grid resolutionof Δϕ = 6′ and Δλ = 10′ (approximately 10km).

The model uses, as input data, the North Sea bathymetry and the tidal-induced seasurface elevation at the model’s open boundary. The North Sea bathymetry for the LateGlacial/early Post Glacial situation was constructed from the present-day bathymetry under the assumption of a global and regional sea-level drop of 65m. Additionally, a regionalpostglacial uplift, reconstructed after Flint (1971) (Fig. 9), was taken into account for the newbathymetry in order to provide for a realistic representation of bathymetric conditions at 10,300bp (Fig. 10).

At the open boundaries, the model receives boundary conditions for tidal elevationswhich consider the three most important tidal constituents, M2, S2 and O1. The tidal elevationsare taken from a global present-day tidal model (Zahel et al. 2000) which ignores changes intidal amplitude caused by topographic changes in the global ocean and in the northern deeperpart of the North Sea. This was necessary since global tidal elevation data were not available,for the period under investigation, to force the regional model. Ignoring changes in the tidalforcing at the open boundary of the North Sea can be justified since changes in tidal amplitudesare formed by basin form changes. At the northern entrance of the North Sea the topographic



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Reconstructed uplift/subsidence.

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differences caused by a lower global sea-level do not result in significant basin form changes.Thus, it can be assumed that their influence on tidal elevations near the northern model boundarycan be disregarded.

model results

The calculated tidal amplitudes for 10,300bp conditions are presented in Figure 11 andcompared to the present-day conditions. The location of the amphidromic point off theNorwegian coast has changed only a little. However, the past tidal amplitudes in the eastern partof the Skagerrak and the Kattegat show different magnitudes especially in the region underconsideration. In contrast to the present-day situation (Fig. 8), the amplitudes were highest along

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Figure 10Reconstructed bottom relief for 10,300 b.p. Positions 1–6 indicate the locations where time series of sea surface

elevations (SSE) have been calculated and presented.

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the western Swedish coast. In the central and southern part of the North Sea, amplitudes wereless than 0.3m. Values above 0.4m are reached only along the British coast and along theSwedish coast.

A comparison of local sea surface elevations at six different sites (see Figure 10) givesan indication of local tidal elevations. The time series in Figures 12 and 13 illustrate four outof six selected points, and clearly indicate an increase in tidal amplitude towards the Kattegat.Site 5, located in the central part of the former North Sea basin, shows amplitudes around 0.4m. Towards the east, amplitudes decrease, with an amphidromic point off the Norwegian southcoast. At site 6, which is located near to this amphidromic point, the tidal amplitudes are only



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Figure 11The modelled M2-tidal amplitudes (m) and co-tidal lines at time intervals of the moon’s meridian passage at

Greenwich.

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(a.)

(b.)

Figure 12SSE (grey line) and spectrum of SSE (black line). Results depicted from Fast Fourier Transform for a full spring-neap

period, 29.5 days. Site 3 is seen in fig. 12a, and site 4 in fig. 12b.

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(a.)

(b.)

Figure 13SSE (grey line) and spectrum of SSE (black line). Results depicted from Fast Fourier Transform for a full spring-neap

period, 29.5 days. Site 5 is seen in fig. 13a, and site 6 in fig. 13b.

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about 0.2m. The tidal wave travels further as a Kelvin wave into the Skagerrak/Kattegat areawith once again increasing amplitudes towards the coast. At site 4, which in our model resolutionis the point closest to the Bohuslän region, the maximum amplitudes increase up to 0.8m. Inthe case of spring tides, the difference between the high and low water-level in this area is about1.6m. These amplitudes are exceptional along the former Swedish west coast. Towards the north,tidal amplitudes already decrease significantly. The calculated amplitudes at sites 1–3 (onlyshown for site 3 in Figure 12) are all very similar and clearly about 20cm below the calculatedamplitudes at the central Bohuslän coast.

archaeological evidence in support of our environmental model

In order to illustrate the effects of environmental conditions 10,000 years ago, anarchaeological approach has been employed. There are numerous Hensbacka sites in centralBohuslän (see Figure 14b), which can be dated to between c.10,300–9300bp. Our questionswere: why are there so many sites, and can we be a little more precise as to the total populationof sites from this c.1000-year period? It should also be mentioned that there has been a long-

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(a)

(b.)(a.)

Figure 14Figure (a.) represents an orientations view showing the various areas discussed in the text. (A) Area is the area referredto as central Bohuslän; area (B) delimits a well investigated section of central Bohuslän, and (C) is an area of specialstudy. Figure (b.) is a map of central Bohuslän at c.9300 b.p. showing both the location of, and approximate number

of, Hensbacka sites that are presently known.

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standing academic belief that many Hensbacka sites exist to the north of central Bohuslän, aswell as to the south. The latter, it was argued, were ‘invisible’ because they were beneath present-day sea-level, or covered by tons of transgressional sediments, while the former, or those to thenorth, have simply not been found due to a lack of detailed investigations in the area, as wellas an apparent lack of natural flint nodules in the moraine above the 50m level (Kindgren 1995).We do not reject either of these propositions; instead we have chosen to work with what we cansee, and seek a plausible explanation for the existence of this concentration of archaeologicalsites in central Bohuslän.

For the purpose of our investigation, three parameters have been utilized in order tolocate Hensbacka sites in the terrain. They are as follows: diagnostic artefacts, such as unifacialopposed platform cores, flake axes, tanged points, single-edged points, Zonhoven points, andsimple lanceolates, that can be attributed to the assemblage; blades and/or chipped flint thatdisplay a white patination and, overriding these two parameters, the level of the terrain in whichthey are found. The altitude of a site above present-day sea-level is important in that the rate ofisostatic rebound in central Bohuslän has been different when one compares the northern limitof our area of investigation with the southern limit. For example, a Hensbacka site that datedto c.10,000bp at 70m.a.s.l. in the north, would be found at 50m.a.s.l. in the south. In short, achronological tilt exists that must be taken into account when shore displacement dating is usedfor various sites found in the terrain along a north–south axis. (See, for example, Svedhage(1985) and Larsson (1997) for more detailed information.) A second point of note is that c.95per cent of all known, that is to say excavated, flint artefacts, that can be attributed to theHensbacka assemblage in central Bohuslän, display a white patination.

The presence of this white patination is one of the three criteria for recognizingHensbacka sites. This phenomenon has been discussed by Fredsjö (1953) in connection withhis excavations at Tosskärr. The cause of this white patination is uncertain, but may be relatedto biological productivity in the sea. The process, as we understand it, has its start in the sea;recently chipped flints, that is to say artefacts that are not patinated, are left lying on a beachnot too distant from the edge of the sea. These flints then become coated with a layer of seasalt. Plankton in the nearby highly biologically productive sea produce a gas known as DMS;the gas in turn rises into the clouds where it is oxidized and forms, as an aerosol, sulphuric acid.Precipitation from these clouds, referred to as ‘acid rain’, dissolves the sea salt coating on thechipped flints creating a small amount of hydrofluoric acid (HF) which etches the surface of theflint. This process, when repeated over hundreds of years, would be sufficient to produce a whitepatina. Since the chemical process that we have described is plausible (Prof. R. Kiene, Dept. ofMarine Sciences, University of South Alabama, pers. comm. 2005), laboratory experiments werecarried out and the results analysed through the use of secondary ion mass spectrometry (SIMS).The two analysed patinas, one from an original artefact with a white patina, and another witha white patina that had been artificially produced through exposure of a newly chippedunpatinated flint in an atmosphere of hydrofluoric acid, showed no significant differences. Whileour results do not rule out other possible causes for the formation of a white patina, they dohave a direct bearing on artefacts that are associated with the Hensbacka assemblage inBohuslän.

Within our area of investigation (Fig. 14a:A), topographical features are quite distinct.Tectonic movements in the bed rock have produced faulting and fissures that have resulted inpresent-day valleys with, in many cases, gently sloping sides that extend upwards from 75m.a.s.l. at the bottom of the valley, to100/150m.a.s.l. at the top (Jansson et al. 1979). The



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surrounding tops of these valleys are usually barren bed rock, but along the sides of the variousvalleys, we find many coves that would have provided ideally sheltered areas for encampment.

Hensbacka sites are not always easy to locate in that they are usually situated in highterrain which is unsuitable for farming. Consequently, we have had to rely upon intensivefieldwork carried out by individuals with a particular interest in high-lying archaeological sites.While this has not been possible throughout the entire area of investigation, where such detailedinvestigations have been conducted, the results have been astounding. This becomes self-evidentwhen one compares Figure 15a with Figure 15b. Intensive and long-term field surveys in theLassehaga/Gullmarsskogen (Fig. 15b:C) area resulted in 26 certain Hensbacka sites (HK), sevenuncertain (HK?), and ten (?) sites in which diagnostic artefacts were not found. It should benoted here that experience has proved that ‘uncertain Hensbacka sites’ can usually be classifiedas ‘certain’ after further investigation. Nevertheless, these hitherto unknown Hensbacka sites inthis area, as well as previously known sites in area (B), have led to the concentration of sitesthat we see in Figure 14b:B. While this concentration of 544 Hensbacka sites in area (B) isunique, it reflects the results of long-term investigations. Even so, the area is not fullyinvestigated since higher regions still require additional fieldwork.

It should also be mentioned that the flint artefacts recovered from area (C), with theexception of those from one particular site, all display a white patination. This is usually the

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(b.)(a.)

Figure 15Figure (a.) illustrates the density, or frequency, of Hensbacka sites in area (B), a well investigated section of centralBohuslän. Dashed lines indicate the present day shore line; continuous lines, the shore line at c.9300 b.p. Figure (b.)represents the results of a long term detail study in area (C); notice that Hensbacka sites do not occur below the

50 m.a.s.l. contour which is equivalent to a shore line at c.9300 b.p.

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case with 95 per cent of all chipped flints recovered in central Bohuslän that can be attributedto the Hensbacka assemblage. It should also be noted that unsheltered sites situated in the outerarchipelago at this time (at the close of the Late Glacial and beginning of the early Mesolithic)are not usually orientated towards the west because that is the direction of the dominant windregime. Exceptions, however, do occur during the later phase of the Hensbacka group, that isto say after 9700bp (Kindgren 2002). This is also true of sites in the Lassehaga/Gullmarsskogen(Fig. 15b:C).

Taking the area of investigation as a whole (Fig. 14a:A), we can conclude that manyparts are not as well investigated as area (B) (compare Figure 14b:A with Figures 14b:B and15a:B). If we take into account that prior to our investigation, 536 certain Hensbacka sites werepreviously known in the entire area (A) (in part, Kindgren 1995), and assume that more detailedinvestigations were to be carried out in the area, there is no reason to believe that the results ofthese detailed investigations would not reveal the same pattern that we witnessed in area (B).In brief, if the ratio between certain Hensbacka sites (HK) and uncertain Hensbacka sites (HK?)in area (B) is, for all practical purposes, 1 :1, i.e. 180 certain sites to 190 uncertain sites, it isreasonable to assume that this ratio is the same in the remaining section (see Figure 14a:A) ofour area of investigation. The same holds true for sites that have not produced diagnosticartefacts (?). The ratio here is also 1 :1. Accordingly, in area (B) there are 180 certain sites to174 sites without diagnostic artefacts, and a total of more than 500 certain Hensbacka sites inarea (A) to more than 500 sites without diagnostic artefacts. Since we have already pointed outthat both uncertain Hensbacka sites (HK?), and Hensbacka sites without diagnostic artefacts (?)usually result in certain Hensbacka sites (HK) after further investigation, it is not far fetchedwhen we suggest that there are more than 1000 Hensbacka sites in central Bohuslän. Indeed,this is a conservative figure – as we shall soon see.

While mathematical computations based on the frequency of observed Hensbacka sitesin area (C) (see Figure 15b) can be used as a statistical method for predicting the total numberof Hensbacka sites in central Bohuslän, the results can only be seen as a hypothetical possibility.Nevertheless, if we take into account the 26 certain Hensbacka sites that have been observed inarea (C) and correlate these sites with the total surface area on which they are found, which is170ha, we arrive at a result of 15 sites per sq km. This, in turn, can be correlated with the totalland area that was available in the archipelago at 9300bp which was c.500sqkm (see Figure16). This suggests that we had 7500 (500sqkm × 15 = 7500) Hensbacka sites in the archipelagobetween 10,300–9300bp. Using simple algebra, we can include sites that can be expected to befound on the mainland since we already know that 75 per cent of the total population of sitesare found in the archipelago and the remaining 25 per cent are found on the mainland. Thesimple equation can then be expressed as: 75% is to 7500 sites, as 25% is to (X); (X) = 2500sites. Thus, the implication is that we might have 10,000 Hensbacka sites in central Bohuslänwhen we combine sites in the archipelago (7500) with sites on the mainland (2500). Clearly thepotential of central Bohuslän in research concerning the transition between the Late Glacial andearly Mesolithic in northern Europe is very considerable.

discussion

Earlier in this paper we asked the question: what was the basis for the seasonalcolonization of central Bohuslän? Let us see if we are any closer now to answering that question.



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Although we have stated that the relationship between the Hensbacka group and theContinental Ahrensburgian will not be discussed in this paper, a few words of explanation arenecessary. Firstly, the chronological period in which we find the earliest phase of the Hensbackagroup is 10,300–9700bp. This has been fairly well established through shore displacementcurves, one of which, derived from the excavation of a Hensbacka site on south-west Orust(Svedhage and Schmitt 1995), an island in the archipelago of central Bohuslän, has been C-14dated. The other C-14 date which is of interest derives from the upper level of the Ahrensburgiansite of Stellmoor, which indicates that the site was occupied at various periods betweenc.10,200–9800bp (Fischer and Tauber 1986). Consequently, chronological contemporaneityexists between the early phase of the Hensbacka group and the Continental Ahrensburgian.

One of the technical anomalies that has caused a certain amount of scepticism, whenwe speak of the early Hensbacka group as being coastal sites of the Ahrensburgian, is thepresence of flake axes, and single-edged points (Fig. 17) made by the microburin technique.Although it is known that both single-edged points and microburins are present in theContinental Ahrensburgian material (Schmitt 1999), there still remain doubts about thecorrelation of Ahrensburgian and Hensbacka sites in Bohuslän, because single-edged points,flake axes, and microburins are few in number at Ahrensburgian sites on the Continent (Kindgren2002). There may be many reasons as to why both the latter, as well as the former, reflect acultural reality. Firstly, flake axes are, for the most part, only found on coastal sites; this suggeststhat it is a tool that has been used in conjunction with marine-related activities. Secondly, theuse of the microburin technique in the production of single-edged points might well be aregional/seasonal cultural manifestation. The implication here is, for example, the Nunamiut

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Figure 16Maps illustrating the effects of isostatic uplift in central Bohuslän. Two significant features can be noted; the numeroussmall islands in figure (a.) become fewer, but larger, in figure (b.). In addition, the straits at Väne-Ryr and Uddevalla

are, for all practical purposes, closed by 9300 b.p. Compare, in this regard, figures (a.) and (b.).

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Figure 17Drawings showing the various tools discussed in the text. (a) A flake axe recovered at Lassehaga, Stångenäs; (b) asingle-edged point found at Gullmarsskogen, Stångenäset; and (c) a Lerbergs axe excavated at Morlanda, Orust.

Drawings: (a) and (b) Larsson 1997; (c) Sjögren, Gustafsson and Strinnholm 1996.

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concept of ‘winter things’ and ‘summer things’ (see Binford 1978; Mauss 1979); technologyneed not be excluded from these notions. In many archaeological discussions, we tend to relateto pragmatic concepts, such as ‘bulk reduction’, but very little has been said regarding mentalcontingencies which are, in themselves, a mainstay of that which we study – culture and whyit changes. This is most likely due to the fact that in ethnocentric terms we understand the former,while the latter is more difficult to relate to. In brief, if we only relate to empirical culture groupsthat we create ourselves, we will most certainly miss the very thing that we are trying to study– cultural change (see Schmitt in press 2005).

A further question to consider is where did the Hensbacka group come from? Althoughmention has been made of similarities between Continental and Hensbacka typologies (Fredsjö1953), the question of where the Hensbacka came from has not really been addressed since 1963when Troels-Smith suggested that the group of hunter-gatherers from Stellmoor might be thesame group – while on a seasonal round – found in central Bohuslän, which we refer to as theHensbacka. This suggestion has been broadly accepted (Schmitt 1994).

There are currently three schools of thought. Firstly, we have the ‘direct route theory’,i.e. hunter-gatherers, at the close of the Late Glacial, travelled directly to Bohuslän from north-central Europe via the Danish Belt and/or the Öresund strait (Troels-Smith 1963; Schmitt 1994,1999) (see also Petersen and Johansen 1996; Johansson 2003). Secondly, the ‘indirect routetheory’ wherein hunter-gatherers travelled from the now submerged North Sea continent tosouth-west Norway (Fuglestvedt 1999, 2001) and then, after a period of time, continued to thewest coast of Sweden (Kindgren 2002). This has been referred to as the ‘Atlantis theory’(Johansson 2003). Thirdly, there seems to be a ‘let’s wait and see theory’ that adheres to thenotion that the Hensbacka represents a regional group of hunter-gatherers and their past culturalaffiliations remain to be seen.

Turning our attention to the frequency of Hensbacka sites in central Bohuslän, ourinvestigation and calculations suggest that in addition to a definite population of c.1000 sites,it is reasonable to assume that there may be as many as 10,000 sites. It has also beendemonstrated that 75 per cent of the sites are located in the archipelago, and the remaining 25per cent on the mainland. If we only take into account the known Hensbacka sites, it can besuggested that we had c.750 sites in the archipelago at a time when the land area available foroccupation was no larger than the present-day Isle of Man (572sqkm). Between 10,300–9300bp, the available land in the archipelago of central Bohuslän was only 500sqkm. Indeed, it mightbe difficult to find such a delimited area in north-central Europe with so many archaeologicalsites that derive from the same c.1000-year period of time. If we made the same comparisonwith our extrapolated figures, this would mean 7500 sites within an area of 500sqkm!

While Danish and Norwegian pollen analyses indicate that the now-submerged landarea in the North Sea basin probably had a soil cover, similar pollen analyses in western andsouthern Sweden suggest that it was warmer earlier in Bohuslän than it was in southern Sweden.Hazel, for example, requires a mild climate and its appearance is about 300 years earlier on thewest coast than it is in southern Sweden. It is also noteworthy that the apparently mild climate,in combination with continual seasonal visits by hunter-gatherers, may have contributed to therapid expansion of hazel. Oak and elm are also early in western Sweden and clearly indicate amild climate and relatively long period of growth during the summer.

Having established the early existence of an extraordinary number of Hensbacka sitesin central Bohuslän, as well as a surprisingly mild climate at an early date, the question aroseas to how, and why, this was the case. Accordingly, our hydrodynamic model demonstrated that

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due to Late Glacial and early Post Glacial topographic features within the North Sea basin, atidal mixing frontal zone had been formed along the coast of Bohuslän, and that this was broughtabout by a spring tide, with a range of 1.60m. When we compare the present-day situation withthe situation at 10,300bp, our model indicates local changes in tidal amplitude, and thus tidalcurrents, for a small region at Bohuslän. The past tidal situation created an exceptionalenvironment. Increased tidal stirring could provide an explanation for the existence of the uniqueecosystem in the archipelago of central Bohuslän characterized by high primary productivity.Consequently increased secondary production would have taken place with an impact on highertrophic level productivity. The existence of an ideal feeding environment for larvae, and juvenileand adult fish could explain the locally increased abundance of fish in coastal environments.The increased abundance of fish created an ideal habitat for predators such as seals and polarbears (Freden 1988; Berglund et al. 1992). This unique and rich coastal ecosystem providedideal conditions for hunting and fishing during the open water phase and thus could explain theseasonal colonization of this region by the Hensbacka group.

Furthermore, our results provide an explanation for the apparent lack of sites in northernBohuslän. Locally, decreasing tidal amplitudes along the Swedish coast north of centralBohuslän could have been too small to destabilize the water column and to mix the stratifiedflows. Thus, the region to the north of central Bohuslän would have been less productive. Inaddition, this explanation might also hold true for the region south of Göteborg, as well as alongthe southern coast of Norway where only a relatively small number of Fosna sites are known(Fuglestvedt, pers. comm. 2004). Fosna sites are the Norwegian counterpart of SwedishHensbacka sites.

We believe that central Bohuslän has been a central seasonal catchment area for hunter-gatherer groups travelling from north-central Europe at the close of the Late Glacial. This doesnot imply that other groups were not here earlier than this coastal version of the Ahrensburgianwhich we refer to as an early phase of the Hensbacka group. One or two earlier sites haverecently been investigated (see for example Nordqvist 2003), but their cultural affiliations remaindubious. Thus by the end of the Younger Dryas, it is possible many groups within theAhrensburgian community on the Continent were aware of the advantageous environment andbiological productivity in the archipelago of central Bohuslän. We see no other explanation asto why the region, at an early date, was of such great interest.

After c.9700bp, a change can be noted not only in the composition of the tool-kit, butalso in the size of sites. Core axes, in the form of Lerberg axes (Fig. 17), become a standardinclusion in the kit, and sites are approximately three to four times as large as those we find inthe early phase of the Hensbacka. We conclude, therefore, that seasonal movements back to theContinent have been discontinued, and a more short-distance seasonal round has begun. In short,the Hensbacka has become a regional group that, after the straits at Uddevalla close at 9300bp(see Figure 16), eventually develop into the cultural group which followed – the Sandarna. Thisgroup, in contrast to the Hensbacka, is found along most of the Swedish west coast suggestingthat a lifestyle that had begun during the late Pre-Boreal phase of the Hensbacka was continuedand, in terms of territory, expanded during the Boreal chronozone.

conclusions

On the preceding pages we have brought into view new aspects of Late Palaeolithicand early Mesolithic research on the west coast of Sweden. In doing so, we have used



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information from oceanography and tidal modelling, in conjunction with basic research in thefields of archaeology and palynology. This has proved to be most fruitful and has emphasizedthree areas of specific interest. Firstly, it is most likely that central Bohuslän has the highestdensity of sites that can be dated to the end of the Late Palaeolithic and the beginning of theearly Mesolithic in Europe, if not the world. Secondly, the reason for this is due to the uniquetopography that existed in the North Sea basin during the close of the Late Glacial whereby atidal mixing front was formed along the coast of central Bohuslän. Consequently, thishydrodynamic feature provided for enhanced biological productivity, and a significantlyincreased carrying capacity, in the archipelago of central Bohuslän. Thirdly, there is a strongpossibility that the current regime and tidal input were such that this enhanced carrying capacitycould only be realized in this particular area. Accordingly, areas to the north and south of centralBohuslän were at a disadvantage in regard to marine productivity. This might be the reason whyHensbacka sites are either lacking, or few and far between, in these areas.

In addition, we have reviewed problems that concern the white patination found onchipped flints associated with the Hensbacka assemblage. The results of our investigationsuggest that the cause of the patination can be dependent upon the degree of biologicalproductivity in the sea on a relatively local scale. Further investigations, with a higher degreeof resolution, might prove to be of considerable interest in regard to not only Stone Agechronologies, but also environmental and climatological research.

Lastly, our oceanographic model has afforded us the opportunity to gain a betterunderstanding as to why certain forms of vegetation appear earlier on the west coast of Sweden,than they do in southern Sweden. Here again, incoming Atlantic water has played a key role inthe amelioration of an otherwise Late Glacial environment. In this regard, it should be notedthat the employment of coupled hydrodynamic-ecosystem models with increased spatialresolution to estimate and explain the potential locations of sites could provide new insights inthe field of archaeology.

(LS and SL) Dept. of ArchaeologyUniversity of Göteborg

SWEDEN

(CS) Danish Institute of Fisheries ResearchCopenhagenDENMARK

(IA) Institute of Hydrobiology and Fisheries ResearchCenter for Marine and Climate Research

University of HamburgGERMANY

(MT) School of ChemistryPhysics and Earth Sciences

Flinders University of South AustraliaAUSTRALIA

(KS) Institute of ConservationUniversity of Göteborg

SWEDEN

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