Jordi Revelles

Matthieu Ghilardi

CONTRIBUTIONS TO THE EUROPEAN POLLEN DATABASE Piantarella (South Corsica, France)

Site details

The study site, l'étang de Piantarella, is located in Bonifacio, on the southern coast of Corsica (France). Three cores were extracted: Piantarella 1 (630 cm, WGS84: 41°22'29.34"N/9°13'12.62"E, 0.36 m a.s.l.); 2 (315 cm, WGS84: 41°22'23.61"N/9°13'14.64"E, 0.33 m a.s.l.); 3 (420 cm, WGS84: 41°22'31.10"N/ 9°13'11.32"E, 0.19 m a.s.l.). The climate of South Corsica is Mediterranean and the region is included within the meso/thermo-Mediterranean vegetation belt, under a semi-arid bioclimate according to Emberger's classification [START_REF] Emberger | La végétation de la région méditerranéenne: essai d'une classification des groupements végétaux[END_REF], with an annual precipitation of ca. 542 mm and a mean annual temperature of 16.4 °C. Corsica is one of the most wooded islands in the Mediterranean and has a high diversity of environments, from the littoral zones to the mountain summits [START_REF] Reille | New pollen-analytical researches in Corsica: the problem of Quercus ilex L. and Erica arborea L., the origin of Pinus halepensis Miller forests[END_REF]. The lowest elevations are characterized by the predominance of sclerophyllous evergreen oak forest (Quercus ilex, Q. suber) and Mediterranean scrubland (Erica arborea, Pistacia lentiscus, Cistus monspeliensis and Juniperus phoenicea). At intermediate elevations, mesophyllous pine forest (Pinus pinaster) is widespread, and mixed deciduous forest (Quercus pubescens, Q. petraea, Ostrya carpinifolia, Alnus cordata, Castanea sativa) is locally abundant. At the local scale, l'étang de Piantarella is surrounded by Mediterranean maquis (C. monspeliensis, J. phoenicea, P. lentiscus, Olea europaea, Arbutus unedo) and marshland.

Sediment description

Bio-sedimentary units were defined based on granulometry, LOI and molluscs analysis: Piantarella 1 (pollen analysis was done on 20 samples from the basal part of the core, 515-610 cm): Unit I (620-515 cm; 7.4-7.2 kyr cal BP): gray clays rich in organic matter, abundant Posidonia oceanica and large specimens of lagoonal Cerastoderma glaucum. Unit II (515-300 cm; 7.2-5.8 kyr cal BP): medium to coarse gray sands containing layers of P. oceanica and shallow marine to lagoonal bivalve species such as C. glaucum and Loripes lacteus. Unit III (300-170 cm; 5.8-3.4 kyr cal BP): coarse sands containing shallow marine molluscs (L. lacteus, Cerithium sp., Bittium reticulatum and C. glaucum) alternating with layers of P. oceanica. Unit IV (170-100 cm; 3.4-1.7 kyr cal BP): alternating coarse gray sand layers with the remains of P. oceanica. No shells recognized. Unit V (100-0 cm; 1.7 kyr cal BP-present): greyish white silty clays with a low organic matter content (modern coastal swamplands).

Piantarella 2 (pollen analysis was done on 9 samples from core sections 5-69 cm, and 130-145 cm): Unit I (315-300 cm): clayey sediments containing the abundant remains of P. oceanica. No shells. Unit II (300-160 cm): gray to greyish white fine to medium sands with a generally low organic matter content (<5%). Layers of P. oceanica are intercalated and abundant molluscs are representative of shallow marine to lagoonal species such as L. lacteus, C. glaucum and Cerithium sp.). Unit III (160-115 cm; 4.7-3.9 kyr cal BP): alternating medium to coarse sandy layers with intercalated gray clay layers containing the remains of P. oceanica and small intact shells, mainly Cerithium sp. Unit IV (115-90 cm; 3.9-1.8 kyr cal BP): medium to coarse gray sands with the intact gastropods B. reticulatum and Cerithium sp. Unit V (90-0 cm; 1.8 kyr cal BP to present): gray clayey layers alternating with fine sandy laminae and layers of P. oceanica. Molluscs are not abundant, but intact specimens such as Cerithium sp.

Piantarella 3 (pollen analysis was done on 28 samples from the basal part of the core, 250-381cm): Unit I (420-400 cm; >6.7 kyr cal BP): coarse gray sands containing intact specimens of B. reticulatum. Unit II (400-250 cm; 6.7-5.3 kyr cal BP): black organic clays with numerous plant remains, wood fragments and specimens of C. glaucum. Unit III (250-50 cm; 5.3-1.2 kyr cal BP): medium to coarse gray sands containing numerous layers of P. oceanica, intact marine gastropods (Rissoa ventricosa, Nassarius reticulatus and B. reticulatum).

Dating

Age-depth models for the 3 cores were based on twelve Accelerator Mass Spectrometry (AMS) radiocarbon dates (Table I). Calibration to years cal BP was made using Clam 2.2. [START_REF] Blaauw | Methods and code for 'classical'age-modelling of radiocarbon sequences[END_REF] based on the data set Marine13.14C [START_REF] Reimer | IntCal13 and Marine13 radiocarbon age calibration curves 0-50,000 years cal BP[END_REF], applying a regional ΔR correction of 27± 32 (averaged values for this sector of the Mediterranean Sea, [START_REF] Reimer | Marine radiocarbon reservoir corrections for the Mediterranean and Aegean seas[END_REF]:

Interpretation

Pollen Sum includes all pollen taxa except those of Cyperaceae, Typha latifolia, Typha/Sparganium, Amaranthaceae and other aquatic taxa in order to avoid over-representation by local taxa. Six pollen zones were defined, two for each core (Fig. 1): -Piantarella 1-A (7.4-7. Erica was dominant in the landscape from ~7.4 kyr cal BP to the present (maximum of 64% at ~7.4 kyr cal BP), with phases of regression during 7.3-7.2 kyr cal BP in Piantarella 1, and during 6.6-6.4 kyr cal BP in Piantarella 3. The original Erica arborea forest would have been progressively affected by human impacts, resulting in its conversion into Mediterranean scrubland. Thus, maquis vegetation dominated by Erica would have existed since the mid-Holocene in southern Corsica as a result of human impacts since the early Neolithic (earliest evidence of cereal cultivation at 7.35 kyr cal BP).

  3 kyr cal BP): Low values of AP (10-25%), dominated by Pinus. Highest values of Erica and continuous curve of Cistaceae, increasing trend of Poaceae and Amaranthaceae and high values of Asteraceae liguliflorae. Cerealia-t continuously present at the end of the zone. -Piantarella 1-B (7.3-7.2 kyr cal BP): Decrease in Erica and expansion of herbs (A. liguliflorae, A. tubuliflorae, Poaceae). Decreasing trend in Amaranthaceae and almost continuous curve of Cerealia-t. -Piantarella 3-A (6.6-6.4 kyr cal BP): low values of AP, showing increasing trend, similar representation of Quercus ilex-coccifera and Q. deciduous and increasing trend of Olea. High values of A. liguliflorae, A. tubuliflorae, Cirsium-t and low values of Amaranthaceae. Occurrence of Cerealia-t. -Piantarella 3-B (5.9-5.3 kyr cal BP): decrease in AP (Q. deciduous, Corylus, Olea) and increases in Erica and Cistaceae, especially at the end of the zone (peak of Erica and Cistaceae at 5.3 kyr cal BP). Increase in AP values due to Olea expansion in ca. 5.5 kyr cal BP. Continuous representation of Cerealia-t at 5.6-5.4 kyr cal BP. -Piantarella 2-A (4.7-4.6 kyr cal BP): high values of AP (29-48%) dominated by Q. ilex-coccifera and Pinus. Landscape dominated by Erica and Cistaceae and high representation of Amaranthaceae. -Piantarella 2-B (0.4 kyr cal BP-present): decreases in AP and Q. ilex-coccifera, high values of Amaranthaceae, peaks in Poaceae and A. liguliflorae at 0.25 kyr cal BP, peak in Erica at the top.

Acknowledgements

JR is member of the research group GAPS (2017 SGR 836) and acknowledges postdoctoral fellowship support from the Spanish "Juan de la Cierva (FJC2017)" program (MICINN, Spain). This research was undertaken through the PCR project "Approche géoarchéologique des paysages de Corse à l'Holocène, entre mer et intérieur des terres «Trà Mare è Monti», funded by Ministère de la Culture (France).


Dr Stéphane Desruelles
email: stephanedesruelles@gmail.com

Matthieu Ghilardi

Christophe Cloquier

Doriane Delanghe

Yannick Buchot

Daniel Hermitte

Jean-Claude Parisot

Jean-Marc Hoeblich

Dr Alain Trentesaux
email: alain.trentesaux@univ-lille.fr

Pr

Dr Pierre Stephan
email: pierre.stephan@univ-brest.fr

Dr INRAP Sylvie Coutard
email: sylvie.coutard@inrap.fr

Mid-to Late Holocene landscape reconstruction of the Maye Estuary (Picardy, Northern France) and its implications for human occupation

Keywords: geoarchaeology, palaeogeography, Sedimentology, Relative sea level rise, Coastal plain, Holocene Geoarchaeology, Palaeogeography, Sedimentology, Relative sea level rise, Coastal plain, Holocene

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

15/12/2019

Thijs van Kolfschoten Editor in Chief Quaternary International Dear Editor: I wish to submit a research paper for publication in Quaternary International titled 'Geoarchaeology of the Maye estuary (Picardy, Northern France): Mid-to Late Holocene landscape reconstruction and its implications for human occupation'. The paper was coauthored by Matthieu Ghilardi, Christophe Cloquier, Doriane Delanghe, Yannick Buchot, Daniel Hermitte, Jean-Claude Parisot, and Jean-Marc Hoeblich.

This study reconstructs the geoarchaeology of the Maye estuary in Northern France, combining geophysical and chronostratigraphical investigations with the analysis of historical sources. We provide a detailed description of the palaeoenvironment during the last 5000 years and make inferences about human settlement and land reclamation in the area. We believe that our study makes a significant contribution to the literature because a detailed palaeoenvironmental investigation of the Maye estuary has not been performed before, and our study additionally provides new information about the probable position of the medieval harbour of the town of Rue, the precise location of which was unknown.

We believe that this paper will be of interest to the readership of your journal due to its broad audience of scientists dealing with physical and natural issues in the Quaternary.

Please consider, as potential referees, Pr. Dr. Alain Trentesaux (Faculté des Sciences et Technologies, Université de Lille; alain.trentesaux@univ-lillefr) and Dr. Pierre Stéphan (UMR6554 LETG, CNRS; pierre.stephan@univ-brest.fr). This manuscript has not been published or presented elsewhere in part or in entirety and is not under consideration by another journal. We have read and understood your journal's policies, and we believe that neither the manuscript nor the study violates any of these. There are no conflicts of interest to declare.

Thank you for your consideration. I look forward to hearing from you.

Sincerely, Dr. Stéphane Desruelles UFR de Géographie et Aménagement, FRE 2026 ENeC, Faculté des Lettres, Sorbonne Université, CNRS 191 rue Saint-Jacques, 75005 Paris, France +33663068802 stephanedesruelles@gmail.com

Introduction

Over the last decades, particular attention has been paid to the study of palaeoenvironmental dynamics combined with the human settlement of the French coastal plains of Brittany, Normandy, and Picardy [START_REF] Lespez | Evolution des paysages et anthropisation depuis le Néolithique dans la péninsule de La Hague[END_REF][START_REF] Gandouin | Analyse en haute résolution de la transgression holocène dans un secteur subsident du littoral français : le bassin-marais de St Omer (Pas de Calais, France)[END_REF][START_REF] Lespez | Middle to Late Holocene landscape changes and geoarchaeological implications in the marshes of the Dives estuary (NW France)[END_REF][START_REF] Pailler | Evolution des paysages et occupation humaine en mer d'Iroise (Finistère, Bretagne) du Néolithique à l'Âge du Bronze[END_REF][START_REF] Stéphan | Colmatage sédimentaire des marais maritimes et variations relatives du niveau marin au cours des 6 000 dernières années en rade de Brest (Finistère)[END_REF][START_REF] Lequint | Apports de la géoarchéologie à l'étude de la baie de Wissant (Pas-de-Calais, France)[END_REF][START_REF] Daire | Ancient maritime communities and the relationship between people and environment along the European Atlantic coasts[END_REF]. However, the Bay of Somme has been little investigated from a geoarchaeological perspective, despite its long term history of human activity.

The town of Rue is located on the Picardy coastal plain, approximately 7 km from the presentday coastline (Fig. 1). It was one of the most important coastal harbours of Picardy during medieval times, from the beginning of the 12th century to the end of the 15th century CE [START_REF] Labrecque | La chapelle du Saint-Esprit de Rue, Picardie, étude historique, architecturale et iconographique d'un monument de la fin du Moyen Age[END_REF]. It is generally accepted that this harbour gradually silted up due to a natural infilling of the estuary with sediment, reinforced by structures (artificial levees in particular) built for agricultural land reclamation purposes on the coastal plain from the 12th century CE (e.g., [START_REF] Demangeon | La Picardie et les régions voisines. Artois, Cambrésis, Beauvaisis[END_REF]. However, no prior palaeoenvironmental or geomorphological studies have involved the scrutiny of ancient sketches, maps and plans in the endevour to reconstruct shoreline displacements during medieval times. This paper aims to reconstruct the landscape evolution surrounding the medieval town of Rue and discusses relative sea level change and related morphological impacts on the evolution of the former Maye Estuary over the last 5000 years.

Physical setting and historical context

2. 1. Topographic,geological,and

tectonic settings

The town of Rue is situated in the south of the Picardy coastal plain, which extends between the Bay of Somme to the south-west and Authie Bay to the north. This flat land is characterized by an average height of about +4.50 m above mean sea level (a.m.s.l.). It developed to the west of a relict cliff bordering a plateau composed of Upper Cretaceous chalk with flint (BRGM, 1981) and was built up primarily from wet or drained lowlands [START_REF] Demangeon | La Picardie et les régions voisines. Artois, Cambrésis, Beauvaisis[END_REF][START_REF] Munaut | Recherches palynologiques et datations 14C dans les régions côtières du nord de la France : Phases transgressives et stabilisations dunaires flandriennes dans l'estuaire de la Canche (Pas de Calais)[END_REF][START_REF] Fournier | La sédimentation Holocène en bordure du littoral picard et sa signification dynamique[END_REF], 1980) which surrounded relict Pleistocene coastal spits composed of rounded pebbles and blocks (called 'Formation de Rue'; [START_REF] Agache | Le Quaternaire de la basse Somme : tentative de synthèse[END_REF][START_REF] Ters | Sur l'évolution géodynamique du Marquenterre (Basse-Somme) à l'HoIocène et durant le Weichsélien ancien[END_REF]BRGM, 1981;Fig. 1). These spits (locally called 'foraines') tend to form in a north/south direction and have a maximum elevation of between +9 and +13 m a.m.s.l. The medieval town of Rue is located at about +7 m a.m.s.l. and is built on the Rue foraine, which was locally incised by small river channels and filled with Holocene marine and continental sediments [START_REF] Ters | Sur l'évolution géodynamique du Marquenterre (Basse-Somme) à l'HoIocène et durant le Weichsélien ancien[END_REF]. The coastal plain is bordered to the west by a coastal dune ridge rising to +36 m NGF which has established over the last 5500 years [START_REF] Fournier | La sédimentation Holocène en bordure du littoral picard et sa signification dynamique[END_REF], 1980;[START_REF] Meurisse | Complexes dunaires et tourbeux holocènes du littoral du Nord de la France[END_REF][START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique : le rôle des tempêtes[END_REF].

According to BRGM (1981), the relict cliff probably formed through faulting (Fig. 1). In addition, the coastal plain was likely divided into pieces by a system of faults and flexures whose movements contributed to the sedimentary filling in of the former undated 'Gulf of Rue'. BRGM (1981) estimates that these movements have caused the Rue area to uplift by about 5 m since the 18th century CE. According to [START_REF] Goffé | Le contrôle tectonique de la morphologie de la baie de Somme. Ses conséquences sur la sédimentation[END_REF], an active northwest-southeast strike-slip fault north of the Bay of Somme may have raised the northern compartment, corresponding to the Rue area. A series of earthquakes occurred during medieval times, for example in 1467 CE, when such an event damaged the church of Saint-Wulphy [START_REF] Labrecque | La chapelle du Saint-Esprit de Rue, Picardie, étude historique, architecturale et iconographique d'un monument de la fin du Moyen Age[END_REF], and in 1580 CE when another earthquake affected the town of Abbeville (Louandre, 1844(Louandre, -1845)).

Conditions along the coastline are megatidal with tidal range varying from 5 m to 8.65 m at Cayeux-sur-Mer, in the south of the Bay of Somme (SHOM, 2017). The mean high-water springs (MHWS) are situated at +5.41 m NGF (4.36 m a.m.s.l.) and the level of the highest astronomical (extreme) tide is +6.13 m NGF (5.09 m a.m.s.l.). Highest sea level can reach approximately +6.45 m a.m.s.l. with a 100-year return period [START_REF] Sogreah | Étude sédimentologique de la Baie de Somme[END_REF]. According to [START_REF] Goslin | Reconstitution de l'évolution du niveau marin relatif holocène dans le Finistère (Bretagne, France) : dynamiques régionales, réponses locales[END_REF], the tidal range has undergone only slight changes since 6000 BCE (a maximum 0.70 m increase ) in Brittany (western France). The coastal plain is exposed to storm-surge flooding caused by dune breaching. The prevailing wind is in the west-southwest direction and the longshore drift moves sediments northward [START_REF] Bastide | Morphodynamique et enjeux d'aménagement des franges littorales d'un estuaire macrotidal tempéré : la baie de Somme[END_REF][START_REF] Michel | Morphodynamique et transferts sédimentaires au sein d'une baie mégatidale en comblement (Baie de Somme, Manche Est). Stratégie multi-échelles spatio-temporelles[END_REF].

South of the study area, the Bay of Somme is a hyper-tidal (Trenteseaux et al., 2012) and mixed estuary [START_REF] Dalrymple | Estuarine facies models: conceptual basis and stratigraphic implications[END_REF]) which has filled in with marine sands (BRGM, 1981(BRGM, , 1985;;[START_REF] Michel | Morphodynamique et transferts sédimentaires au sein d'une baie mégatidale en comblement (Baie de Somme, Manche Est). Stratégie multi-échelles spatio-temporelles[END_REF].

Holocene sea level changes and coastal evolution in north-western Europe

Holocene sea level changes and related coastal evolution are generally considered to be influenced by the interplay between several factors (e.g. [START_REF] Goslin | Holocene relative sea-level changes in western Brittany (France) between 7600 and 4000 cal. BP: Reconstitution from basal-peat deposits[END_REF]: (1) eustatism, (2) isostatic processes (Vink et al., 2007;[START_REF] Shennan | Relative sea-level changes and crustal movements in Britain and Ireland since the Last Glacial Maximum[END_REF], (3) tectonic activity, (4) local effects controlling the sedimentary record, particularly sediment compaction (the effect of postdepositional consolidation; [START_REF] Long | Driving mechanisms of coastal change: Peat compaction and the destruction of late Holocene coastal wetlands[END_REF][START_REF] Horton | Compaction of Holocene strata and the implications for relative sealevel change on the east coast of England[END_REF][START_REF] Marion | Short-term (<2 yrs) estuarine mudflat and saltmarsh sedimentation: High-resolution data from ultrasonic altimetery, rod surfaceelevation table, and filter traps[END_REF][START_REF] Brain | Compression behaviour of minerogenic low energy intertidal sediments[END_REF] and changes in tidal regime, and (5), other unspecified, random, and hardto-quantify factors which influence relative sea level reconstruction, particularly erosion of sedimentary sequences [START_REF] Goslin | Holocene relative sea-level changes in western Brittany (France) between 7600 and 4000 cal. BP: Reconstitution from basal-peat deposits[END_REF]. According to several recent papers, local factors (4 and 5) play a key role in the effect of sea level change on coastal system morphologies [START_REF] Bungenstock | The high-resolution Holocene sea-level curve for Northwest Germany: global signals, local effects or data-artefacts[END_REF][START_REF] Baeteman | Reconstructing middle to late Holocene sea-level change: A methodological review with particular reference to 'A new Holocene sea-level curve for the southern North Sea' presented by K.-E. Behre[END_REF]. The general geomorphological evolution of the north-western European coastal plains can be regarded as the cross combination of natural forcings (such as sea level change, variations in sediment supply and sediment distribution; e.g., [START_REF] Costa | Géomorphologie des littoraux français[END_REF][START_REF] Pierik | Late Holocene coastal-plain evolution of the Netherlands: the role of natural preconditions in humaninduced sea ingressions[END_REF] and, for roughly the last 2000 years, a mixture of natural and anthropogenic factors [START_REF] Pierik | Late Holocene coastal-plain evolution of the Netherlands: the role of natural preconditions in humaninduced sea ingressions[END_REF].

Previous research conducted in the southern Picardy coastal plain did not attempt to precisely reconstruct the geomorphological evolution of the Rue area, despite sediment cores being drilled in the vicinity of Rue in the 1980s [START_REF] Lefèvre | Alternances de sédimentation marine et continentale durant l'Holocène en plaine maritime picarde[END_REF][START_REF] Ters | Sur l'évolution géodynamique du Marquenterre (Basse-Somme) à l'HoIocène et durant le Weichsélien ancien[END_REF]Figs. 1 and 3).

However, the goal of those studies was not the reconstruction of the former shoreline position west of the town so as to locate its medieval coastal harbour. The published 14 C dating results were recalibrated by [START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique : le rôle des tempêtes[END_REF] using the IntCal13 calibration curve [START_REF] Reimer | IntCal13 and MARINE13 radiocarbon age calibration curves 0-50000 years cal[END_REF]. In addition, [START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique : le rôle des tempêtes[END_REF] considered the influence of sediment hydrocompaction on her chronostratigraphies to reconstruct the Holocene sea level evolution.

However, the role of sediment hydrocompaction remains difficult to quantify (e.g., [START_REF] Allen | Holocene coastal lowlands in NW Europe: Autocompaction and the uncertain ground[END_REF][START_REF] Goslin | Holocene relative sea-level changes in western Brittany (France) between 7600 and 4000 cal. BP: Reconstitution from basal-peat deposits[END_REF][START_REF] Brain | Compaction[END_REF][START_REF] Stéphan | Évolution du niveau marin relatif à l'Holocène le long des côtes françaises de l'Atlantique et de la Manche : réactualisation des données par la méthode des "sea-level index points[END_REF][START_REF] Shennan | Relative sea-level changes and crustal movements in Britain and Ireland since the Last Glacial Maximum[END_REF].

Along the Atlantic and English Channel coasts of France, some Holocene marine transgressions and regressions were identified by [START_REF] Ters | Les variations du niveau marin depuis 10 000 ans, le long du littoral atlantique français[END_REF][START_REF] Ters | Variations in Holocene sea level on the french Atlantic coast and their climatic significance[END_REF]. Many papers have since adopted this description of Holocene transgression-regression cycles. For example, [START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique : le rôle des tempêtes[END_REF] reconstructed the coastline evolution of the Picardy coastal plain over the last 6000 years (see the red curve in Fig. 2):

 Around 4000 BCE, the sea level maximum was +2 m/+3 m a.m.s.l. with storm surges at ca. +6 m a.m.s.l.. The sea reached the foot of the present relict cliff. According to the stratigraphies published by [START_REF] Lefèvre | Alternances de sédimentation marine et continentale durant l'Holocène en plaine maritime picarde[END_REF] and [START_REF] Ters | Sur l'évolution géodynamique du Marquenterre (Basse-Somme) à l'HoIocène et durant le Weichsélien ancien[END_REF] and recalibrated by [START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique : le rôle des tempêtes[END_REF] date recalibration and consideration of sediment compaction), silts were deposited at the extreme limit of high waters at ca. + 3 m a.m.s.l. in the area of core Rue 3 (ca. 1.5 km west of the present town; Fig. 1).

 A marine regression occurred between ca. 4000 and 3100 BCE, with an increase in rainfall, peatland growth and development of coastal swamps. According to [START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique : le rôle des tempêtes[END_REF], the relative mean sea level was ca. -10 m a.m.s.l. at around 3100 BCE.

 A second marine transgression occurred from ca. 3100 BCE to 1400 BCE, during which shoreline impact varied greatly due to local geomorphological features. At the end of this phase, mean sea level was ca. -2 m/-3 m a.m.s.l.. Finally, around 2500-2000 BCE, the coastal plain was wider than it is today and consisted of a vast floodplain between bays and estuaries.

 From ca. 1400 BCE onward, the influence of aeolian dynamics and climate change exceeded the marine forcings. Mean sea level revealed few deviations from its present-day height. Storm-surge flooding caused by coastal barrier breaching played a greater role in landscape evolution, particularly between 450 BCE and 600-700 CE. However, shoreline stabilisation facilitated multi-stage processes of dune accretion.

Around 1000 BCE, a peatland lay southwest of Rue, behind a sandbar (according to a dating published by [START_REF] Ters | Sur l'évolution géodynamique du Marquenterre (Basse-Somme) à l'HoIocène et durant le Weichsélien ancien[END_REF] and recalibrated by [START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique : le rôle des tempêtes[END_REF]. A more recent (831-1264 CE) freshwater peatland (according to a dating published by [START_REF] Lefèvre | Alternances de sédimentation marine et continentale durant l'Holocène en plaine maritime picarde[END_REF] and recalibrated by [START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique : le rôle des tempêtes[END_REF]), was identified southeast of Rue in core M21 (Fig. 1). This outline of Holocene transgression-regression cycles is strongly challenged by recent works which take into account that for the last 10 kyr, there have been only small-scale fluctuations of global sea level in tectonically stable areas (e.g. [START_REF] Stéphan | Évolution du niveau marin relatif à l'Holocène le long des côtes françaises de l'Atlantique et de la Manche : réactualisation des données par la méthode des "sea-level index points[END_REF]. Along the Atlantic and English Channel coasts of France, relative sea level is thought to have risen continuously during the last 8000 years, with a major deceleration of sea level rise around 4000 BCE (e.g. [START_REF] Mrani-Alaoui | New data and a morphodynamic perspective on midto late-Holocene palaeoenvironmental changes in the French Flanders coastal plain, southern North Sea[END_REF][START_REF] Goslin | Holocene relative sea-level changes in western Brittany (France) between 7600 and 4000 cal. BP: Reconstitution from basal-peat deposits[END_REF][START_REF] Goslin | Reconstitution de l'évolution du niveau marin relatif holocène dans le Finistère (Bretagne, France) : dynamiques régionales, réponses locales[END_REF][START_REF] Stéphan | Évolution du niveau marin relatif à l'Holocène le long des côtes françaises de l'Atlantique et de la Manche : réactualisation des données par la méthode des "sea-level index points[END_REF][START_REF] Garcia-Artola | Holocene sea-level database from the Atlantic coast of Europe[END_REF]Fig. 2). The glacial isostatic component played a major role in increasing the rate of sea level rise between 4000 and 2000 BCE [START_REF] Garcia-Artola | Holocene sea-level database from the Atlantic coast of Europe[END_REF]. During the last 4000 years, according to the data published by [START_REF] Shennan | Holocene land-and sea-level changes in Great Britain[END_REF] for south-west England, relative sea level rise due to glacial isostatic adjustment was around 0.4 mm yr-1. Regarding the eustatic component, model ICE-5G (VM2a) published by [START_REF] Peltier | Global Glacial Isostasy and the Surface of the Ice-Age Earth: The ICE-5G (VM2) Model and GRACE[END_REF] indicates a relative sea level rise of approximately 1.1 mm yr-1 over the last 4000 years in our study area (Fig. 1).

While recent papers agree that relative sea level has continued to increase, they attribute to climate change the main effect on the geomorphological evolution of the coast of north-western Europe during the last 3500 years. Increased storminess and pluviometry were recorded at around 1000 BCE in north-western Europe [START_REF] Gandouin | Enregistrement paléoclimatique de la transgression Holocène ; signature paléoenvironnementale des Chironomidae (Diptères) du bassin de Saint-Omer (France)[END_REF][START_REF] Dark | Climate deterioration and land-use change in the first millennium BC: perspectives from the British palynological record[END_REF]Clark and Rendell, 2009;[START_REF] Sorrel | Persistent non-solar forcing of Holocene storm dynamics in coastal sedimentary archives[END_REF]Van Vliet-Lanoë et al., 2014a, 2014b;[START_REF] Pierik | Late Holocene coastal-plain evolution of the Netherlands: the role of natural preconditions in humaninduced sea ingressions[END_REF]. Major changes in coastal sedimentary systems have been observed for this period along the coasts of both the English Channel [START_REF] Long | Mid-and late-Holocene evolution of the Dungeness foreland, UK[END_REF][START_REF] Lespez | Middle to Late Holocene landscape changes and geoarchaeological implications in the marshes of the Dives estuary (NW France)[END_REF][START_REF] Tessier | Infilling stratigraphy of macrotidal tide-dominated estuaries. Controlling mechanisms: Sea-level fluctuations, bedrock morphology, sediment supply and climate changes (The examples of the Seine estuary and the Mont-Saint-Michel Bay, English Channel, NW France)[END_REF] and the Atlantic [START_REF] Pontee | Morphosedimentary evidence of Holocene coastal changes near the mouth of the Gironde and on the Medoc Peninsula, S.W. France[END_REF][START_REF] Tastet | Morpho-chronology of coastal dunes in Medoc. A new interpretation of Holocene dunes in Southwestern France[END_REF]Clave et al., 2001;[START_REF] Moura | Holocene sea level fluctuations and coastal evolution in the central Algarve (southern Portugal)[END_REF]. [START_REF] Pouzet | Chronology of Holocene storm events along the European Atlantic coast: new data from the island of Yeu, France[END_REF] distinguish four main 'European Atlantic storm events' (EASEs) over the last 3500 years: 1300-1600 CE (EASE 1), 200-800 CE (EASE 2), 900-400 BCE (EASE 3) and 1500-1300 BCE (EASE 4). During these events, marine deposits may have been eroded by drainage network incision, increased river discharge may have favoured continental sediment supply, and coastal breaching may have caused river courses to shift [START_REF] Goslin | Reconstitution de l'évolution du niveau marin relatif holocène dans le Finistère (Bretagne, France) : dynamiques régionales, réponses locales[END_REF].

For the last 1000 years, the geomorphological evolution of the Picardy coastal plain has been strongly influenced by human activities and land reclamation for dike construction [START_REF] Demangeon | La Picardie et les régions voisines. Artois, Cambrésis, Beauvaisis[END_REF][START_REF] Dobroniak | Géomorphologie, hydrodynamique et écologie d'un estuaire tempéré macrotidal : l'Authie, Manche orientale[END_REF]. The first dike was likely built in the 12th century CE between 'Le Muret' and 'La Petite Retz' [START_REF] Briquet | Le littoral du Nord de la France et son évolution morphologique[END_REF], approximately 7 km north-northwest of Rue (Fig. 1). Dikes continued to be built until the 1860s [START_REF] Briquet | Le littoral du Nord de la France et son évolution morphologique[END_REF] north of the coastal plain, linked with the shifting of Authie Bay. In the south of the coastal plain, the former mouth of the Maye ('Val de Maye'; Fig. 1) is protected from coastal flooding by a dike built in 1835.

However, storm-surge flooding still occurred [START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique : le rôle des tempêtes[END_REF], probably linked with episodes of increased storminess which took place in north-western Europe from 100-950 CE and from1300-1700 CE [START_REF] Sorrel | Persistent non-solar forcing of Holocene storm dynamics in coastal sedimentary archives[END_REF][START_REF] Pierik | Late Holocene coastal-plain evolution of the Netherlands: the role of natural preconditions in humaninduced sea ingressions[END_REF]. [START_REF] Lefèvre | Alternances de sédimentation marine et continentale durant l'Holocène en plaine maritime picarde[END_REF] report marine flooding deposits in the area of core M21 (Fig. 1); these deposits covered the upper peat layer (Fig. 3) and may have occurred around 1300 CE.

Rue: a major harbour in the northwest of France from the beginning of the 12th century to the end of the 15th century CE

The Rue foraine, like all the Pleistocene bars of the Picardy coastal plain [START_REF] Rougier | Rue (Somme), La foraine de Hère, Carrière d'Hère-lès-Rue, Rapport de diagnostic[END_REF], was favourable for human settlement at least from the Bronze Age [START_REF] Rougier | Une fouille préventive à Quend « Le Muret » (Somme) : questions sur l'organisation et le rôle d'un site côtier au Bas-Empire[END_REF][START_REF] Gapenne | Foraine de Hère[END_REF]). An Early Bronze Age cemetery [START_REF] Baray | Archéologie funéraire du Bronze ancien au Bronze final en Bourgogne, Franche-Comté[END_REF] and a rural peopling dating from the Iron Age (2nd-1st century BCE; [START_REF] Rougier | Un établissement rural de La Tène Dl à Rue « Le Chemin des Morts » (Somme)[END_REF] were discovered together at the top of a Pleistocene bar, at a location called 'Le Chemin des Morts', northeast of Rue. A coin hoard dated to the 3rd century CE [START_REF] Ben Redjeb | Découverte d'un dépôt monétaire gallo-romain à Rue (Somme) ; premières données[END_REF] and the archaeological remains of a Gallo-Roman villa were discovered in the Bleue foraine (at ca. +9 m a.m.s.l.) south of Rue [START_REF] Notte | Rue. La Foraine bleue[END_REF]. Other remains have been unearthed at the top of Pleistocene bars (the Hère foraine; [START_REF] Rougier | Rue (Somme), La foraine de Hère, Carrière d'Hère-lès-Rue, Rapport de diagnostic[END_REF], indicating a human occupation since the Bronze Age, even the Neolithic [START_REF] Gapenne | Foraine de Hère[END_REF].

Data also lack for the Early Middle Ages. It is generally accepted that the town flourished in the 12th century CE [START_REF] Labrecque | La chapelle du Saint-Esprit de Rue, Picardie, étude historique, architecturale et iconographique d'un monument de la fin du Moyen Age[END_REF]. This development can be attributed to a legend claiming that in 1101, a local villager of Rue discovered a boat washed up on the shore, in which was found a large wooden cross originating in Jerusalem. The wealth of the city was thereafter built on pilgrimages and trade (transport of goods by sea). As early as the 13th century, the harbour of Rue, which is thought to have been located in the Maye Estuary, was as important as those of the Bay of Somme [START_REF] Delaporte | Quelques mots sur l'histoire de Rue[END_REF]. A municipal charter, recognised in 1210 by the Count of Ponthieu, testifies to the importance and diversity of production and trade (e.g. fishing, agriculture, farming, textiles; [START_REF] Labrecque | La chapelle du Saint-Esprit de Rue, Picardie, étude historique, architecturale et iconographique d'un monument de la fin du Moyen Age[END_REF]. This suggests that the Count imposed high taxes on each ship entering the harbour.

At the end of the 13th century, a canal project to re-establish access to the sea was proposed to the inhabitants, which implies that the harbour had silted up. That project was never begun, but in 1277, a canal was dug to bring the Authie's flow towards the harbour, probably because of the increase in siltation [START_REF] Labrecque | La chapelle du Saint-Esprit de Rue, Picardie, étude historique, architecturale et iconographique d'un monument de la fin du Moyen Age[END_REF]. However, the westward shift of the shoreline seems to have gradually prevented access to the harbour by heavily laden ships arriving from the Authie. Thus, harbour activity strongly decreased as early as the 14th century CE, even if commercial maritime activity continued during the 15th century CE [START_REF] Labrecque | La chapelle du Saint-Esprit de Rue, Picardie, étude historique, architecturale et iconographique d'un monument de la fin du Moyen Age[END_REF].

Literary sources indicate that the town of Rue was naturally protected by the Maye to the north and west, and by marshes to the south and east [START_REF] Delaporte | Quelques mots sur l'histoire de Rue[END_REF]. Following the Charter of 1210, fortification works (protecting walls) were built. These essentially took the form of ditches, on the edges of which stone houses and walls were built. The ramparts were likely widened in the 15th century before being pierced by five gates, with 'la porte de la grève' (the shore gate) to the west [START_REF] Poiret | Essai sur la topographie et l'aspect de Rue au milieu du XVe siècle[END_REF]. The fortifications were destroyed in 1670, under the reign of Louis XIV, by order of Jean-Baptiste Colbert, Secretary of State of the Navy. The earliest map of Rue found in historical archives dates from 1640. A previous excavation near 'le bastion de Saint-Jean' [START_REF] Flucher | Direction régionale des Affaires Culturelles Picardie[END_REF]) identified an embankment under the ravelin. This embankment diverted the course of the Maye. However, artefacts (mainly ceramics) discovered there provide only an imprecise chronology of the fortification works.

Present day landscape configuration does not enable to significantly demonstrate that Rue was a coastal town. Moreover, archaeological evidence is scarce, and the few historical sources fail to provide a robust and indisputable reconstruction of landscape evolution surrounding the medieval town of Rue. In an attempt to do so, we have conducted a palaeoenvironmental study.

Material and methods

Field investigations (geophysical survey and coring) were carried out northwest of Rue, particularly around 'la porte de la grève', where the medieval port is thought to have been located in the upper part of the Maye Estuary. Field investigations were completed by laboratory analyses (sedimentological analysis and radiocarbon dating of drill core samples)

and by the study of historical sources. All these data, as well as altimetry data 'RGE 1 m' provided by the 'Institut Géographique National', were integrated into a Geographic Information System database.

Electric resistivity tomography (ERT)

To evaluate both the geometry and thickness of surficial deposits and bedrock depth, and to locate archaeological structures, eight electrical resistivity tomography (ERT) profiles were established around the town of Rue (Figs. 4,5a,5b,and 5c).

ERT has been successfully used in various subsurface field studies, especially in geomorphology [START_REF] Beauvais | Ultramafic rock weathering and slope erosion processes in a south West Pacific tropical environment[END_REF][START_REF] Ghilardi | Reconstructing the landscape evolution and the human occupation of the Lower Sagone River (Western Corsica, France) from the Bronze Age to the Medieval period[END_REF] and archaeology [START_REF] Siart | Karst depressions as geoarchaeological archives: the palaeoenvironmental reconstruction of Zominthos (Central Crete) based on geophys-ical prospection, mineralogical investigations and GIS[END_REF][START_REF] Quesnel | Geophysical signatures of a Roman and Early Medieval Necropolis[END_REF][START_REF] Ghilardi | Dating the Bridge at Avignon (South France) and reconstructing the Rhone River fluvial palaeo-landscape in Provence from Medieval to Modern times[END_REF]. The 2D subsurface profiles were obtained using the ABEM Lund Imaging System (Terrameter LS/4 channel) with an array of 64 electrodes with a Schlumberger-Wenner reciprocal layout protocol. A computer inversion program (Res2Dinv; [START_REF] Loke | RES2DINV, Rapid 2-D Resistivity and IP Inversion Using the Least-Square Method (Geotomo doftware User's Manual)[END_REF] that includes topography and generates images of the resistivity distribution was used.

To satisfactorily obtain data on the thickness of the surficial deposits and their organisation [START_REF] Ritz | Electrical imaging of lateritic weathering mantles over granitic and metamorphic basement of eastern Senegal, West Africa[END_REF], ERT profiles were conducted with an electrode distance ranging from 0.60 m (ERT6, with a maximum depth of 5.17 m) to 2.50 m (ERT3, with a maximum depth of 21.50 m).

Because different materials can have similar electrical properties, the ERT method does not allow for unequivocal interpretation. Therefore, two reference profiles were established on sites where the stratigraphy and/or lithology can be readily seen: ERT7, on an extraction site of Pleistocene gravels and pebbles, and ERT8, on a chalk plateau near Rue.

Sedimentological coring

Vibracores with a diameter of 50 mm were drilled to a maximum depth of 8.40 m below the surface (Fig. 4; Table 1) in May 2012 northwest of Rue using a Cobra TT vibrocorer with hydraulic extractor (Figs. 6 and7). The position of the cores was chosen in accordance with the results obtained from the geophysical survey. Each core was precisely levelled using a theodolite and local IGN geodetic benchmarks were used as references.

Sedimentological analyses

The grain-size distribution of the fine fraction (<2 mm) was measured by laser diffraction granulometry at CEREGE. Preparation of the samples was based on [START_REF] Ghilardi | Mid-to Late Holocene shoreline reconstruction and human occupation in Ancient Eretria (South Central Euboea, Greece)[END_REF]. The grain-size distribution was then measured using a Beckman Coulter LS 13 320 laser granulometer with a range of 0.04 to 2000 µm.

Loss-on-ignition (LOI) measurements were made at CEREGE, following [START_REF] Dean | Determination of carbonate and organic matter in calcareous sediments and sedimentary rocks by loss on ignition: Comparison with other methods[END_REF], and [START_REF] Bengtsson | Chemical analysis[END_REF]. Sediment samples of approximately 1 g were taken at 5 cm intervals throughout the core. After drying at 105 °C to a constant weight, the samples were heated to 550 °C for 7 h to estimate their organic matter content. A second heating phase, to 925 °C for a further 7 h, was undertaken to assess the proportion of carbonates.

Mollusc identification was conducted on all cores, but analyses were limited due to the paucity of these samples along the cores, and to the small sizes of thematerials which were retrieved.

AMS 14 C dating method

The chronostratigraphy of the cores was analysed at CEREGE using a series of 14 AMS radiocarbon determinations derived from charcoal, plant remains, wood samples and peat samples collected from the cores (Table 2). These analyses were performed at the Poznan Radiocarbon Dating Laboratory (Poland). Subsequent calibration of 14 C ages was conducted using the Calib Software Version 7.1 [START_REF] Stuiver | Extended 14 C data base and revised CALIB 3[END_REF] and the IntCal13 calibration curve [START_REF] Reimer | IntCal13 and MARINE13 radiocarbon age calibration curves 0-50000 years cal[END_REF].

Examination of historical sources

Document sources provide diverse information on coastal landscapes, including physical environment (e.g. nature of the foreshore), constructions, and various human activities such as fish species caught, quantities of fish due for rent, goods transported by boats, places of loading and unloading, peat extraction, and relationships between coastline users and the authorities exercising rights.

For the study area, document sources regarding the medieval coastal landscapes are scarce, but those available are relatively varied and rich in information. Originally produced by municipalities or fiefdoms, they are today stored in various collections and series of the National Archives (Paris), the Departmental Archives of the Somme (Amiens), the National Library of France (BnF, Paris), and the Abbeville Municipal Library. A large part of the archives has unfortunately succumbed to the destruction provoked by successive wars. For example, the documents of the old series of the Abbeville Municipal Archives were lost as recently as 1940.

We implemented a regressive method to study the document sources, beginning with the analysis of the most recent documents. Unfortunately, for the study area, documentary sources prior to the 13th century are relatively rare and provide no historical maps or figures until the early 16th century.

Results

Borehole chronostratigraphy

Pleistocene gravels, including rounded flint pebbles and marine shells in a matrix of fine sand [START_REF] Ters | Sur l'évolution géodynamique du Marquenterre (Basse-Somme) à l'HoIocène et durant le Weichsélien ancien[END_REF], were found in the lowermost part of all cores situated west of Rue with a top bed depth that varied between 2 m and 7 m (Figs. 6, 7 and8). This reveals the lateral heterogeneity and extension of these units. These formations were also identified in various ERT profiles (see § 4.2).

Above this first layer, four main morphosedimentary units were distinguished in cores R1, R2, R4, R5, R6, and R7 (Figs. 6, 7 and 8):

 Unit A (recorded in R4, R5, R6 and R7) contains medium grey silty sands. It can be divided into two subunits: from the base layer (-1.15m in R5) to ca. +0.70 m a.m.s.l., Subunit A1 comprises medium to fine sand (mean grain-size of ca. 210 µm and modal index of ca. 230 µm) and Subunit A2, from ca +0.70 m a.m.s.l., is composed of very fine to silty sand (mean of ca. 70 µm; modal index of ca. 65 µm and ca. 60% of particles <62 µm). This unit has a low carbonate content (less than 2.5%) and a significant number of macrocharcoals and plant debris are observed. The thickness (up to 2.50 m in R5) and the surface of this unit are irregular (between +0.90 m a.m.s.l. in R6 and +2.50 m a.m.s.l. in R2). The uppermost part of Unit A dates roughly from the beginning of the 4th millennium BCE 7 and 8).

Because of the texture, colour, and chronology of the sediments in this unit, we determine that they were deposited by the sea, in a calm environment, particularly in depressions bordered by Pleistocene coastal spits ('foraines').

 Unit B (recorded in R4, R5, R6 and R7) consists of a peat accumulation rich in plant remains and exhibiting low CaCO3 content (less than 5%). Large pieces of charcoal are visible but molluscs are absent. The height of the base of this formation varies according to the geometry of the underlying marine deposits, while the top of the unit is almost flat, at about +2.60 to +2.90 m a.m.s.l. Generally, this unit dates from the early 4th to the late 2nd millennium BCE (3824-3695 cal. BCE in R6 and1134-1004 cal. BCE in R7).

The characteristics of Unit B differ from core to core. R4 and R7 are homogeneous and rich in organic matter (around 60%). Between +1.25 m a.m.s.l. and +1.50 m a.m.s.l., R5 reveals a sandy layer between the peat accumulations. In R6, above +1.90 m a.m.s.l., relatively young (834-752 cal. BCE) dark grey peaty silts were identified.

The combined sedimentological and geochemical proxies described above lead us to conclude that this unit provides evidence of coastal swamp environments with sizable peat accumulation.

 Unit C, identified in cores R1, R2, R4, R5, R6 and R7, between about +2.60 m a.m.s.l. and +5.20 m a.m.s.l., from the 11th century to the beginning of the 13th century (989-1052 cal.

CE and 1039-1210 cal. CE in R4), is not homogeneous from core to core.

Up to about +4 m a.m.s.l., the unit mainly comprises very fine to silty sands (mean at ca. 75 µm and mode at ca. 65 µm) in almost all cores (R1, R2, R4, R5, R6, and R7). However, the clayey-silty fraction (<63 µm) was generally higher in the deposits found in R4 -and to a lesser extent in R7 -than that identified in R5 and R6. This difference indicates that the deposits occurred in calmer water in the areas from which R4 and R7 were taken.

Between about +2.70 m and +3.20 m a.m.s.l., R5 and R6 show an intercalation of thin sandy layers with thin clayey beds that correspond to tidal rhythmites (Fig. 9). Such features were observed by [START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique : le rôle des tempêtes[END_REF] in the muddy saltmarsh areas of the Canche Estuary (located north of the Picardy coastal plain).

In the uppermost part of Unit C (above +4 m a.m.s.l.), the stratigraphy differs from core to core:

-Up to +5.20 m a.m.s.l., R4 consists of very fine oxidised sands (mean at ca. 67 µm and mode at ca. 76 µm), including marine to brackish bivalve fragments (Cerastoderma edule) typical of a sandy or muddy sand area of an estuary and enclosed bay [START_REF] Bellamy | Croissance de la coque (Cerastoderma edule) en baie de Somme : Morphométrie et Marquage[END_REF], and charcoals dated to 989-1210 cal. CE. R1 and R2 contain coarser sediments (fine to medium sand; mean at ca. 250 µm and mode at ca. 260 µm) than that recorded in R4, R5, R6, and R7. Based on sedimentological and faunal (cockles) evidence, we surmise that these sediments (Subunit C1) were deposited in a marine environment.

-Subunit C2, identified in R1, R2, R5, R6, and R7, contains fine material (very fine sands or silty sands; mean at ca. 122 µm and mode at ca. 75 µm). It exhibits a relatively higher calcium carbonate content (>35%) in R1, R2, and R7. Between +4.40 m and +4.90 m a.m.s.l., the very fine sands recorded in R7 contain unidentifiable land snail debris. In R1, R2, and R5, this fine material is covered by a peat layer containing plant debris, dated to 380-536 cal. CE (in R5). We surmise that Subunit C2 were deposited in a riverine or swamp environment.

-In R6, we found a layer of fine sands between +4.40 m and +5.10 m a.m.s.l., characterised by a multimodal particle size distribution with secondary medium to coarse sand modes situated at 250 µm and 1000 µm. Even if they contain orangecoloured archaeological remains; it is difficult to determine which anthropogenic influence prevailed on these deposits (Subunit C3), and whether they are of marine or continental origin.

Unit C includes coarser sand beds that may correspond to lag deposits, recorded in R4 and R6 at ca. +4.30 m a.m.s.l. In R4, the bed is 0.05 m thick and is characterised by D90 at 805 μm. In R6, this layer is thicker (0.20 m) and is covered by finer marine deposits (Subunit C1) above a clear contact (erosional surface). These finer deposits are characterised by a D90 ranging from 403 to 660 μm. This bed is covered by fluvial deposits (Subunit C2), with a bottom layer that is relatively rich in CaCO3.

Coastal processes of sedimentation are clearly attested in Unit C (open-to semi-enclosed marine bay) with local continental influence on the sedimentary processes (rivers or wetlands).

 Unit D, above +4.90 m to +5.20 m a.m.s.l., contains sandy-silty deposits, highly disturbed by anthropogenic activities. Deposition began in the 13th century (after 1221 cal. CE in R6), and these layers contain fragments of marine to lagoonal shells, some pebbles, and occasionally potsherds. Thin coarse sand beds (main mode around 60 µm with one or more coarser secondary modes) are recorded in R4 (at +5.40 m a.m.s.l. and +6.10 m a.m.s.l.). Units A and B are not recorded in R3, which is located higher in the Rue foraine. Nor are they found in R8, which was drilled into what was likely an embankment at the foot of the medieval town fortifications of Rue, according to the map dating from 1640 (Fig. 10b).

ERT profiles and subsoil resistivity

No archaeological structures were identified in any of the ERT profiles measured northwest of Rue. However, the geophysical survey allows to distinguish three main units, described as follows, from bottom to top (Fig. 5a, 5b, and 5c):

 The geophysical Unit Ge1, with a top depth that varies from 1 m (in ERT4) to ca. 7 m (in R5), has highly variable resistivity values, generally greater than 60 Ωm. According to the reference profiles ERT7 and ERT8, Upper Cretaceous chalk and Pleistocene gravels and pebbles have resistivity values ranging from 40 Ωm to more than 180 Ωm, depending on the degree of weathering of the chalk and the presence (or absence) of groundwater.

Comparison of the ERT profiles with the stratigraphy of the cores indicates that the uppermost part of Ge1 consists of Pleistocene gravels and pebbles. The lowermost part is probably composed of chalk. The geometry of this unit is irregular, particularly in ERT4, with a greater depth to the west.

 Unit Ge2, whose thickness varies from a few centimetres (north-northeast of profile ERT4) to about 6 m (southeast of profile ERT3), is characterised by low resistivity (<40 Ωm) related to the finest sedimentary particles (very fine sands of marine or fluvial origin, or organic matter, according to the stratigraphy of R4, R7 and R8). In addition, the conductivity of this unit may have been enhanced by the presence of shallow groundwater, as has been observed in similar sedimentary contexts [START_REF] Ghilardi | Dating the Bridge at Avignon (South France) and reconstructing the Rhone River fluvial palaeo-landscape in Provence from Medieval to Modern times[END_REF][START_REF] Ghilardi | Reconstructing the landscape evolution and the human occupation of the Lower Sagone River (Western Corsica, France) from the Bronze Age to the Medieval period[END_REF]. Ge2 probably corresponds to stratigraphic Units A, B and C.

 Unit Ge3 can be recognized at a depth of ca. 2 m in most of the profiles, especially in profiles ERT4, ERT5 and ERT6. It is characterisezd by high resistivity values (from 80 Ωm to ca. 300 Ωm). In R4, R7 and R8, the sediments corresponding to this unit are sandy-silty and strongly anthropised (Unit D).

Discussion

Mid-to Late Holocene landscape reconstruction of the Rue area

Palaeogeographical knowledge of the Maye Estuary remains incomplete for the first half of the Holocene. However, the cross combination of geophysical and sedimentological results derived from boreholes helps to partly reveal the palaeorelief. Pleistocene deposits composed of rounded pebbles and blocks ('Formation de Rue'; BRGM, 1981) are observed on most of the geophysical (ERT) profiles. Above this presumed marine formation, fine material accumulated (Unit A) within a regional context of marine ingressions, related to the rapid post-glacial sea-level rise until around 4000 BCE [START_REF] Garcia-Artola | Holocene sea-level database from the Atlantic coast of Europe[END_REF]. Our interpretation is strengthened through comparison with core Rue 3 (situated southwest of Rue) as analysed by [START_REF] Ters | Sur l'évolution géodynamique du Marquenterre (Basse-Somme) à l'HoIocène et durant le Weichsélien ancien[END_REF] (Fig. 1 and3), who identified and dated foreshore deposits to 4030 ± 240 cal. BCE (after recalibration by [START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique : le rôle des tempêtes[END_REF] in the lowermost part of the sedimentary sequence, below a peaty layer. According to [START_REF] Gapenne | Foraine de Hère[END_REF], the coastal plain was covered by an oak grove around 3000 BCE. These trees slowly disappeared, especially due to the development of bogs.

 From ca. 3500 BCE to ca. 1000 BCE Around 3500 BCE, Unit B, defined by the presence of continental peat in several cores, reveals a major environmental change. Back-barrier marshes extended west of the Rue foraine. The eastern boundary of this area is influenced by the morphology of the Pleistocene gravel and pebble bar. The ERT profiles performed in the northwest of Rue delineate the upper limit geometry of the deposit of Pleistocene gravels and pebbles, from its outcropping at the top of the Rue foraine (eastern ends of ERT6 and NNE of ERT4) to the bottom of adjacent valleys (profile ERT2). Until the beginning of the 3rd millennium BCE, the landscape configuration combined bogs and woodland [START_REF] Gapenne | Foraine de Hère[END_REF].

The chronology of peat shows a hiatus between the lowermost part, dated to ca. 3500 BCE, and the uppermost part, dated to the late 2nd millennium BCE (Fig. 12a). In addition, the R5 peaty deposit, whose top was dated to ca. 1500 BCE, is disrupted by a sandy layer (between +1.25 m a.m.s.l. and +1.50 m a.m.s.l.), probably deposited by storm-surge flooding (see § 5.2).

The upper limit of the peat varies in height, and shows a greater thickness in R4, R5, R6 and R7 than in cores M21 and Rue3 [START_REF] Lefèvre | Alternances de sédimentation marine et continentale durant l'Holocène en plaine maritime picarde[END_REF][START_REF] Ters | Sur l'évolution géodynamique du Marquenterre (Basse-Somme) à l'HoIocène et durant le Weichsélien ancien[END_REF], where it was identified around 0 m a.m.s.l. (Figs. 1 and3). Yet, peat thickness in these cores is very close to that of cores SC1 and SC3, drilled in a small valley south of the Rue foraine [START_REF] Ters | Sur l'évolution géodynamique du Marquenterre (Basse-Somme) à l'HoIocène et durant le Weichsélien ancien[END_REF]. The cores drilled in 2012 reveal conditions that were more sheltered from the sea than those evidenced in cores M21 and Rue3. The local geomorphological context of cores SC1 and SC3 is more similar to that of R4, R5, R6 and R7 (Figs. 1 and4). These differences confirm the role of the local geomorphological features highlighted by [START_REF] Lefèvre | Alternances de sédimentation marine et continentale durant l'Holocène en plaine maritime picarde[END_REF] and [START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique : le rôle des tempêtes[END_REF], even if sediment compaction probably strongly disturbed the stratigraphy of core Rue3.

Comparison between R4, R5, R6 and R7 shows a slight variation in the altitude of the upper limit of the peat. From ca. +2.60 m to ca. +3 m a.m.s.l., R4 and R7 record the top of Unit B, with finer sediments, including black patches perhaps indicative of old roots, while R5 and R6

show tidal rhythmites similar to those currently observed in the muddy saltmarsh [START_REF] Smith | Clastic Tidal Sedimentology[END_REF]. Given these features and their location to the southeast of R5 and R6, the area from which R4 and R7 were taken would have been submerged only during high tides and was probably covered by saltmarsh vegetation in ca. 1000 BCE (Fig. 12b). In addition, the peat recorded in R6 laminations (Fig. 9) suggests that the area around the core was at the boundary of the high marsh (schorre). In addition, the influence of saltwater inputs on the top of the peaty deposits is proved by a recent analysis of diatoms near the Hère foraine [START_REF] Gapenne | Foraine de Hère[END_REF].

The relatively recent date (834-752 cal. BCE) and the characteristics of the deposits (dark grey peaty silts) at +1.75 m a.m.s.l. in R6 suggest the influence of human activities on the peatlands at the beginning of the Iron Age. For comparison, large-scale peatland reclamation in the coastal plain of Holland began between 600 and 250 BCE, according to [START_REF] Pierik | Late Holocene coastal-plain evolution of the Netherlands: the role of natural preconditions in humaninduced sea ingressions[END_REF].

 Around 1000-1250 CE (Fig. 12c) Human impact (ditches, dikes, fortifications, embankments) on the landscape intensified from at least the 12th century. The sediments recorded between ca. +3.90 m and +5 m a.m.s.l. in R4, R5, and R6 (Figs. 6 and7) continued, nevertheless, to be influenced by marine processes. In these three cores, the sands contain cockle (Cerastoderma edule) valves, while the sands in R7 contain land snail fragments that indicate a continental influence of sedimentation. In addition, the thin beds of coarser sand recorded in R4 and R6 at ca. +4.30 m a.m.s.l. (Figs. 6 and7) can be defined as storm surge deposits. The erosional surface (clear contact) between the coarser bed and the underlying marine deposits in R4 testifies to the suddenness of the event [START_REF] Chaumillon | Storm-induced marine flooding: lessons from a multidisciplinary approach[END_REF][START_REF] Pouzet | Chronology of Holocene storm events along the European Atlantic coast: new data from the island of Yeu, France[END_REF]. Moreover, this kind of massive increase in coarser grains (30% of sand and a D90 ranging from ca. 400 to ca. 800 μm) as compared to underlying sediments, has occurred in a similar manner in saltmarsh sediments deposited during recent storm surges [START_REF] Swindles | Sedimentary records of coastal storm surges: evidence of the 1953 North Sea event[END_REF]. The overlying fluvial deposits (Subunit C2) suggest that a fluvial flooding of the area then followed the storm event, which likely occurred in the 12th/13th century BCE, according to the sample dated in R6.

The fine sands (Subunit C3) in R6 between +4.40 m and +5.10 m a.m.s.l. were mainly influenced by human activities. At the same height, the sediments recorded in R4 and, in particular R5, were thinner and even peaty (R5), indicating low energy environments. Mottled sediments (marked by groups of red, yellow and grey patches), between +4.10 m and +4.50 m a.m.s.l. in R5 and between +4.30 m and +5.20 m a.m.s.l. in R4, indicate periodic waterlogging.

All these observations suggest that R6 is located near a former river channel that may have been artificially closed after silting-up. About 100 m to the southeast, R4 is likely situated at the former interface between fresh and saltwater, in the upper part of the muddy flat (slikke) and in an environmental context favourable to cockles (Cerastoderma edule). In the area from which R1 and R2 was taken, and where fluvial deposits are recorded, freshwater probably flowed into a natural or artificial channel (ditch).

Wetlands and bogs under continental influence were probably located north and northeast of the former river channel. Freshwater peatlands were identified in sediment cores taken in the 1980s at similar altitudes in other areas to the west, south and southeast of Rue [START_REF] Lefèvre | Alternances de sédimentation marine et continentale durant l'Holocène en plaine maritime picarde[END_REF][START_REF] Ters | Sur l'évolution géodynamique du Marquenterre (Basse-Somme) à l'HoIocène et durant le Weichsélien ancien[END_REF]. The peat dating performed in R5 at +4.88 m (380-536 cal. CE) conflicts with the datings (989-1052 cal. CE and 1039-1210 cal. CE) obtained for R4 between +4.89 m and +5.12 m a.m.s.l. In addition, the upper peat layer of core M21 analysed by [START_REF] Lefèvre | Alternances de sédimentation marine et continentale durant l'Holocène en plaine maritime picarde[END_REF] was dated to 1070 ± 200 CE (Figs. 1 and3). These results probably indicate that the peat dating obtained for R5 is unreliable, whereas the other two results (in R4) were obtained using charcoal.

The fluvial sediments identified in R1 and R2 (Subunit C2) between +4.50 m and +5 m a.m.s.l.

were most likely deposited by the Maye River. The latter, which initially flowed south of the Rue foraine in 'le Val de Maye' (Lefevre et al., 1980;[START_REF] Ters | Sur l'évolution géodynamique du Marquenterre (Basse-Somme) à l'HoIocène et durant le Weichsélien ancien[END_REF]Fig. 1), moved northwards, probably due to sedimentary deposition at the end of the medieval period. This riverbed change may have been caused by the silting-up of a former pond (called 'étang de Rue' or 'vivier de Rue') southeast of Rue, which existed at least from the 13th century CE [START_REF] Demangeon | La Picardie et les régions voisines. Artois, Cambrésis, Beauvaisis[END_REF] and which had been maintained by dikes downstream. Its siltation may have caused the shift of the Maye towards the northwest, to a channel crossing the Rue foraine.

In the Late Middle Ages, the channel planform of the Maye was thus similar to today's. The tidal harbour of Rue was established in the upper estuary of this river.

 Since the 13th century CE Continental processes and human activities have played a major role in the aggradation of the plain since the 13th century. The construction of dikes in the 12th century CE north-northwest of Rue [START_REF] Briquet | Le littoral du Nord de la France et son évolution morphologique[END_REF] assisted this aggradation (Unit D).

According to early maps (dating from the late 16th century), the Maye River ended in a closed gulf and Rue was located inland (Fig. 10a). A map from 1773 shows a lock at the mouth of the Maye, at a place called 'la Haie Pénée' approximately 3 km west-southwest of Rue (Fig. 10c), at the boundary of a probable high marsh (schorre). This map also indicates a destroyed dyke ('digue rompue') at the boundary between the saltmarshes and the spring tide line, which probably corresponds to the limit between high marsh and tidal mudflat. The destroyed dyke also features on the 1758 map by Cassini (Fig. 10d), which confirms the risk of coastal flooding due to dune or dike breaching at that time. Moreover, the Maye canal has carried water from the river towards 'Le Crotoy' since the 1780s, aiding the drainage of the wetlands around Rue.

Relative sea level changes in the area of Rue during the last 5000 years

The basal part of the peat deposit (Unit B) that overlies an incompressible basement, especially in R5 and R6, can be taken as free basal sea level index points (SLIP), which allow the estimation of past relative sea levels [START_REF] Brain | Compaction[END_REF][START_REF] Hijma | A Protocol for a Geological Sea-Level Database[END_REF]. Analyses of the peat collected in these cores do not allow us to accurately reconstruct the depositional environment (salinity, in particular) of the peat. Thus, the indicative meaning (the quantitative altitudinal relationship that connected the SLIP with the reference tidal level at the time of deposition; [START_REF] Hijma | A Protocol for a Geological Sea-Level Database[END_REF] of the basal peat of R4, R5, and R6 remains difficult to determine. However, the low concentrations of CaCO3 (<5%) of the peat and the lack of molluscs suggests a predominantly continental influence. This assumption is confirmed by a recent analysis of diatoms collected at the bottom of the peat layer, near the Hère foraine [START_REF] Gapenne | Foraine de Hère[END_REF]. Indeed, we propose to define these indicators as upper limiting data points [START_REF] Shennan | Holocene land-and sea-level changes in Great Britain[END_REF][START_REF] Hijma | A Protocol for a Geological Sea-Level Database[END_REF]: under normal conditions, the sea does not reach this limit except, perhaps, during astronomical tides and storm surges. Based on R4, R5, and R6, three limiting points can be defined: (i) 3824-3695 BCE (at +0.90 m a.m.s.l.), (ii) 3798-3654 BCE (at +1.50 m a.m.s.l.), and (iii) . Given that the present highest astronomical tide (HAT) is at +6.13 m a.m.s.l. (5.09 m a.m.s.l.), we estimate that:  at 3824-3695 BCE, the HAT was at ca. 0.14 m below the present mean sea level, and the mean sea level was likely around 5.23 m below the present mean sea level.

 at 3798-3654 BCE, the HAT was at ca. 0.45 m above the present mean sea level, and the mean sea level was likely 4.63 m below the present mean sea level.

 at 3532-3368 BCE, the HAT was at ca. 1.05 m above the present mean sea level, and the mean sea level was likely 4.03 m below the present mean sea level.

These three limiting points and the corresponding probable past mean sea level are in good agreement with the relative sea level evolution over the last 5000 years in Brittany, as reconstructed by [START_REF] Goslin | Reconstitution de l'évolution du niveau marin relatif holocène dans le Finistère (Bretagne, France) : dynamiques régionales, réponses locales[END_REF] , even if the HAT indicated by the first limiting data point seems low compared to the second point. The reconstructed relative sea levels are lower than values estimated by the Ice-5G model [START_REF] Peltier | Global Glacial Isostasy and the Surface of the Ice-Age Earth: The ICE-5G (VM2) Model and GRACE[END_REF], and higher than values calculated by the models of [START_REF] Bradley | An improved Glacial Isostatic Adjustment model for the British Isles[END_REF] and Kuchar et al. (2012) (Fig. 11). Our results indicate that the role of tectonic movements in the relative sea level changes over the past 5000 years was insignificant.

The faults of the Rue area seem to be inactive.

The sandy layer recorded at +1.25 m a.m.s.l. and +1.50 m a.m.s.l. in R5 may be explained by storm-surge flooding that was likely linked with major climatic changes identified in northwestern Europe (see aforementioned references), particularly 'European Atlantic storm events' (as defined by [START_REF] Pouzet | Chronology of Holocene storm events along the European Atlantic coast: new data from the island of Yeu, France[END_REF]. The shorter second phase of peat accumulation at ca. 1300 BCE may have been enhanced by the closure of the beach barrier complex. This phase may, in turn, have been stopped by another 'European Atlantic storm event' period that began at ca. 1100 BCE [START_REF] Pouzet | Chronology of Holocene storm events along the European Atlantic coast: new data from the island of Yeu, France[END_REF], as suggested by the clear contact (erosional surface) identified between Unit B and Subunit C1 in R5 at +2.50 m a.m.s.l. (Figs. 7 and8).

Regarding the present limit between the slikke and the schorre at ca. +4 m a.m.s.l. [START_REF] Michel | Stages of sedimentary infilling in a hypertidal bay using a combination of sedimentological, morphological and dynamic criteria (Bay of Somme, France)[END_REF], the laminated marine sediments (Subunit C1) situated above the peat sequence in R5 and R6, and deposited close to the limit between slikke and schorre at ca. 2.70 m a.m.s.l., are probably linked to a sea level, at the beginning of the 1st millennium BCE, of ca. 1.30 m below the present one.

The coarser sand beds identified in R1, R2, R4, and R6 reflect a greater frequency of marine incursions since ca. 1100 CE. Some of these storm surge deposits can be related to the 'EASE 1' period [START_REF] Pouzet | Chronology of Holocene storm events along the European Atlantic coast: new data from the island of Yeu, France[END_REF] during the early 'Little Ice Age' (in ca. 1450 CE), even after land reclamation (as suggested by [START_REF] Lefèvre | Alternances de sédimentation marine et continentale durant l'Holocène en plaine maritime picarde[END_REF]) in the area of core M21.

Consequences for human occupation from Neolithic times to the medieval period

Our results suggest that the shoreline position was probably located at the foot of the Rue foraine at the end of the Middle Neolithic period (ca. 3500 BCE). According to [START_REF] Gapenne | Foraine de Hère[END_REF], at the end of the Neolithic, a forest covered the area. From the end of the Neolithic to the Middle Bronze Age (1600-1350 BCE), much of the lowlands in the area was covered by peatland. Human settlements were probably preferentially located at the top of the Pleistocene bars, at least from the Bronze Age [START_REF] Rougier | Rue (Somme), La foraine de Hère, Carrière d'Hère-lès-Rue, Rapport de diagnostic[END_REF][START_REF] Gapenne | Foraine de Hère[END_REF]. The lowlands were probably highly exposed to storm surges which, however, did not affect the settlements located on the Pleistocene bars.

From the end of the Bronze Age (ca. 1000 BCE) to the Late Middle Ages (987-1492 CE), sediments deposited in the lowlands in the upper part of an estuary were mainly of marine origin. Saltmarshes, including muddy flats (slikke) and high marshes (schorre), developed at the bottom of the Pleistocene bars. During the Iron Age, peatland reclamation may have begun (see § 5.1), but human activities in the lowlands were subject to storm surge flooding.

With the aggradation of the plain and the westward migration of the coastline, the drainage network was shaped.

Finally, despite the lack of archaeological evidence, our results agree well with different historical sources and confirm the likelihood of a coastal harbour at Rue during the 12th and 13th centuries CE; this harbour was probably situated northwest of the town, near 'le Moulin de Saint-Jean'.

Conclusion

Sedimentological analyses derived from boreholes and geophysical investigations conducted in the Rue area, combined with the scrutiny of historical sources have allowed us to reconstruct the Mid-to Late Holocene evolution of the Maye Estuary. The geoarchaeological approach we employ allows to identify: (1) the prevalence of marine environments at the beginning of the 4th millennium BCE, (2) the development of peat accumulation from ca. 3500 BCE to the late 2nd millennium BCE, interrupted by a period (between ca. 3050 cal. BCE and 1550-1290 cal. BCE) of marine deposition (probably due to storm surges), (3) an estuarine depositional environment around 1000-1250 CE, including a foreshore composed of both mudflats (slikke) and high marshes (schorre) as well as a river channel (Maye), and (4) land reclamation in the former Maye Estuary since the 13th century.

This palaeoenvironmental reconstruction agrees well with recently published sea level curve reconstructions for the Holocene. A continuous Mid-to Late Holocene sea level rise does not conflict with our reconstruction of the Picardy coastal plain evolution.

Based on our results, in around 1000-1250 CE, the town of Rue was bordered to the west by the Maye Estuary, thus confirming literary sources and local legends. The inner estuary was located near 'le Moulin de Saint-Jean', where a boat carrying a sacred wooden cross from Jerusalem is reported to have beached during the 12th century. Thus, we suggest that the harbour of Rue was situated northwest of the town, at the upstream boundary of the Maye

Figure and table captions

Fig. 1 :Fig. 2 :Fig. 3 :

 123 Fig. 1: Location and Geological maps of the study area 1: Spot height (in meters); 2: 'Plateau Picard': Upper Cretaceous chalk with flints; 3: Flint pebble spit; 4: Holocene dunes and sand bars; 5: 'Formation du Marquenterre': Holocene clay and sands; 6: Holocene alluvial deposits; 7: Holocene peat; 8: 'Formation de Rue': Pleistocene pebble spit; 9: Fault; 10: Hidden fault; 11: River; 12: Location of corings undertaken by Lefèvre et al. (1980) and Ters et al. (1980) (source: BD ORTHO® IGN, 2017; BRGM)

Fig. 5b :

 5b Fig. 5b: ERT1, and ERT3 profiles (2D view) and identified geoelectric units (Ge)

Fig. 5c :Fig. 6 :Fig. 7 :Fig. 8 :Fig. 9 :Fig. 10 :

 5c678910 Fig. 5c: ERT7, and ERT8 profiles (2D view) and identified geoelectric units (Ge)

Fig. 11 : 1 :Fig. 12 :

 11112 Fig. 11: Comparison between the past mean sea level reconstructed in the Rue area, the sea level index point identified in Brittany (France) by Goslin (2014), and glacial isostatic adjustment models 1: Basal peats identified by Goslin (2014); 2: Intercalated sea level index points identified by Goslin (2014); 3: Limiting points identified by[START_REF] Goslin | Reconstitution de l'évolution du niveau marin relatif holocène dans le Finistère (Bretagne, France) : dynamiques régionales, réponses locales[END_REF]; 4: Limiting points defined in our study (HAT: reconstructed highest astronomical tide; msl: probable mean sea level); 5: Current sea level (HAT: highest astronomical tide; msl: mean sea level); 6: Ice-5G model[START_REF] Peltier | Global Glacial Isostasy and the Surface of the Ice-Age Earth: The ICE-5G (VM2) Model and GRACE[END_REF]; 7:Bradley et al. (2011) model; 8: Kuchar et al. (2012) model.

Table 1 : ERT profile locations Table 2: Radiocarbon dating results

 1

Acknowledgements

This work was funded by the Regional Council of Picardy from 2011 to 2014. The authors acknowledge Anaïs Doyen, Amandine Gac, Geoffrey Gueit, Rémi Lequint and Mathieu Wawrzyniak for their assistance in field investigations, and thank Rémi Dupuits for his bibliographic study. Laboratory facilities provided by Philippe Racinet (UFR d'Histoire et de Géographie, Université de Picardie Jules-Verne) and Karine Bellart (Service du patrimoine historique de la ville de Rue) were greatly appreciated.

Estuary. The creation of an artificial pond southeast of Rue ('étang de Rue') the diversion of the river towards the medieval town. The discharge of water from the pond to the town was likely caused by anthropic and natural filling in of the wetland. We suggest that this natural barrier was dredged to remove sediments from the harbour and/or to maintain the navigability of the river, as is commonly done today.

Further investigation (coring and geophysical surveys) is required to reconstruct the Holocene evolution of the estuarine landscape to the west and south of Rue. Pollen analyses may also help reconstruct the landuse of the area, in particular during the period of harbour activity of the town. Van Vliet-Lanoë, B., Penaud, A., Henaff, A., Delacourt, C., Fernane, A., Goslin, J., Hallegouët, B., Le Cornec, E., 2014b. Middle-to late-Holocene storminess in Brittany (NW France):

Part II -The chronology of events and climate forcing. The Holocene 24 (4), 434-453. 10.1177/0959683613519688 Vink, A., Steffen, H., Reinhardt, L., Kaufmann, G., 2007. Holocene relative sea-level change, isostatic subsidence and the radial viscosity structure of the mantle of northwest Europe (Belgium, the Netherlands, Germany, southern North Sea). Quaternary Science Reviews 26, 3249-3275. 10.1016Reviews 26, 3249-3275. 10. /j.quascirev.2007.07.014 .07.014