Water matters. Geoarchaeology of the city of Adria and palaeohydrographic variations (Po Delta, Northern Italy)

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Highlights

  • The anthropic mound (from 6th c.BC) is set to the top of the Po alluvial succession

  • The ‘crypt’ was the original church: it was buried only by subsequent flood deposits

  • The first alluvial event took place between 9th and 11th century AD

  • Between the 11th and the 15th century a wide marshy area surrounded the city

  • After the 15th c. AD, another sedimentary event created the present alluvial plain

Abstract

Adria is located in the northern Po delta at the edge of the Venice Lagoon, a sector of the Po Plain that was built by the late Holocene geomorphic activity of the Po, Adige and Tartaro rivers. Early settlements are dated to the Middle Bronze Age, and the city developed as a major urban centre during the Iron Age and Roman times. In the early Middle Ages, Adria survived as a local religious centre in the Venetian area, as evidenced by the existence of a 9th century AD church. The superimposition of archaeological deposits led to the upbuilding of an anthropogenic mound rising up to 5 m above the surrounding plain. The aim of this investigation is to understand the evolution of the city of Adria from the 1st millennium BC to the Middle Ages, and its relationships with the rearrangement of the river network and connected palaeogeographic setting.

The area was studied through the analysis of historical cartography, aerial photographs, a digital terrain model, archaeological data from the archives of the Soprintendenza Archeologia del Veneto, geological corings and a cone penetration test. New stratigraphic data have been acquired through the execution of 21 manual boreholes up to 5 m in depth with an Edelman combination-type auger. Radiocarbon datings were carried out on selected peat samples.

The analysis of the elevation above sea level of different stratigraphic data has exposed the stages of the evolution of the city, starting from the 6th century BC through Roman times and post-Antiquity, revealing the extension of the archaeological mound in the southern part of the modern city. The growth of the mound took place in four phases, dated to the 6th–5th centuries BC, 4th–3rd centuries BC, 1st century BC–1st century AD and post-Antiquity. The presence of a buried soil at the top of the Po deposits, coupled with radiocarbon dating of extensive peat layers, has allowed the reconstruction of the stratigraphy of the alluvial succession, and to correlate it with the archaeological record. The buried soil at the top of the Po deposits lies below the 6th century BC archaeological levels in the lowest part of the archaeological mound. On this same palaeosurface the first 9th century AD church was built. Two alluvial events buried this church under about 2 m of overbank fines, turning it into an underground hall, known as the ‘crypt’ of the modern San Giovanni church. These alluvial events are attributed to the Tartaro River, with a probable contribution by the Adige River. The first alluvial event took place between the 9th and 11th centuries AD. In the 11th–15th centuries AD, a marshy area extended north, west and east of the city centre, as evidenced by an extensive peat layer that was buried by the second alluvial event later than the 15th century AD.

Introduction

Hydrography is a key element in constraining land use dynamics in alluvial plains (Kidder, 1996, Brown, 1997), and understanding its modifications over time can greatly enhance our knowledge of the rise, development and fall of single settlements or complex societal systems from Prehistory to historical periods. The integration of archaeological and geomorphological research in a comprehensive geoarchaeological approach provides the best clues for deciphering, in a diachronic perspective, the multifaceted relations between man and rivers (Brown, 2008, Challis and Howard, 2006, Ninfo et al., 2009).

In the Venetian plain, watercourses have always been important elements of production and trade between the Alpine area and the Adriatic Sea (Fig. 1). Such was the case of Padua from the early Iron Age to the Middle Ages, where the reconstruction of the palaeohydrographic evolution of the Brenta and Bacchiglione Rivers provided clues for understanding settlement strategies and the urban structure (Mozzi et al., 2010, Mozzi and Gamba, 2012, Ninfo et al., 2015). Similarly, river shifting through avulsion appears to have greatly affected the dispersal of late Bronze Age sites along the northernmost branch of the Po River (Piovan et al., 2010), as well as the development of the major Iron Age urban centres of Frattesina on the Po River (Peretto, 1986, Arenoso Callipo and Bellintani, 1994, Pearce and Dela Guio, 1999, De Guio, 2005) and Este on the Adige River (Ruta Serafini, 2002).

Changes in the palaeohydrographic network and coastline migration in the Po Delta (Fig. 2; Amorosi et al., 2008, Rizzetto et al., 2003, Simeoni and Corbau, 2009) evidently affected the evolution of the Etruscan and Roman city of Spina (Rebecchi, 1998, Berti and Guzzo, 1993) and the early Medieval town of Comacchio (Gelichi et al., 2007, Gelichi et al., 2012, Rucco, 2015). Palaeoenvironmental proxies in alluvial successions can be analyzed in order to solve critical transitions in the archaeological record, such as the passage from Antiquity and post-Antiquity in the Emilian plain (Cremonini et al., 2013) and in the city of Modena (Bosi et al., 2015). As post-Antiquity studies in the Po Plain demonstrate, the relation between cities and water management is essential to the understanding of landscape transformations (Campopiano, 2013). An example of how the evolution of a fluvial system can affect urban growth is the city of Ferrara (Stefani and Zuppiroli, 2010), where the site of Corso Porta Reno highlights the interplay between flood deposits from the Po River and human occupation during Medieval times (Cremaschi and Nicosia, 2010). Recent studies of the southern Venetian Lagoon during the Middle Ages suggest that, in order to maintain a strategic position in the river network, anthropogenic landscape transformations can be dramatic, as demonstrated in the case of Sant' Ilario monastery (Corrò et al., 2016, Corrò et al., 2015).

Adria, located at the northernmost edge of the Po Delta, about 20 km south of the Venice Lagoon, was one of the most important Greco-Etruscan settlements of the Venetian area (Colonna, 2003, Baldassarra, 2013) and then became a strategic Roman city (Bonomi and Robino, 2007, Malacrino, 2009). The Adriatic Sea was probably named after this city, as Greeks from Syracuse (Sicily) expanded their hold on Adria in the 4th century BC, and called the whole sea north of the Ionian Islands the “Gulf of Adria”, the Adrias Kolpos (Braccesi, 2001, p. 12). On the other hand, the evolution of the city in the early Middle Ages is almost entirely unknown. Only a few written sources suggest the persistence of a settlement in the urban centre, while archaeological data are relatively poor and do not supply any further information. Nevertheless, the early Medieval remains of a religious centre in the northern part of the city, and of a cemetery in the southern part, prove that Adria was still populated in the early Middle Ages.

Our research aims to understand the evolution of the city of Adria from the 1st millennium BC to the Middle Ages, and the rearrangement of the river network and palaeogeographic setting, through shallow corings, radiocarbon datings, and the analysis of archaeological and historical data. Focusing on the alluvial stratigraphy and landforms around the city we developed a stratigraphic framework of Adria. The superimposition of archaeological deposits led to the upbuilding of an anthropogenic mound that rose above the aggradation of the alluvial plain. This three-dimensional perspective of the city's buried layers allowed us to partly reconcile and coordinate a rather fragmented archaeological record, mostly deriving from scattered excavations, some of which date back to the 18th century (Lodo, 1993). With this multidisciplinary approach it was also possible to increase our knowledge of the archaeology and environmental setting of Medieval Adria, in order to better understand the role of the city in Adriatic trade, while Venetian power was rising in the north and the ancient city of Ravenna was prospering in the south (Gelichi, 2010, Cirelli, 2010).

Section snippets

Material and methods

Archaeological data were provided by unpublished reports of archaeological campaigns and old excavations from the archives of the Soprintendenza Archeologia del Veneto. The elevations above sea level of floors, roads and planking levels were sought and defined for each chronological period. These represent the main archaeological data that have been used to identify the growth of the anthropic mound at different periods. The digital terrain model (DTM) used for the detection of landforms was

Geomorphological setting

The Po River alluvial plain around Adria is characterized by a gently rolling topography due to the presence of wide, continuous alluvial ridges, formed by natural levees and channel bars of suspended channel belts (Piovan et al., 2010, Piovan et al., 2012). The last phase of major sedimentation in this northernmost portion of the Po alluvial system was dated to between 5 and 3 kyr cal BP. Two main river branches, known as the Saline – Cona Po (P1 in Fig. 2) and the Adria Po (P2 in Fig. 2),

Ca' Cima-Amolara transect

The Ca' Cima transect is located east of the urban area and cuts an alluvial ridge detected in the DTM (Fig. 5). Core AD-22 is located about 300 m west of AD-23 (Fig. 5B). The bottom part of the succession is dominated by clay silt with lenses of silty sand related to overbank sedimentation in a floodplain. A 30-cm thick calcic horizon, characterized by a clayey-silt matrix with common CaCO3 nodules, vertical roots and traces of oxidation, indicates the presence of a buried soil layer between − 

Discussion

The presence of the buried soil in the Ca' Cima transect indicates the existence of a sedimentary hiatus in the sedimentary succession. The buried soil extends between − 2.5 and − 3.2 m asl (Fig. 5), an elevation which allows the correlation with the alluvial surface on which the archaeological levels of the 6th century BC lie (Fig. 9). This implies the existence of a stable alluvial palaeosurface which extended throughout the area of Adria.

In the settlement, the archaeological levels were

Conclusions

This research has allowed us to obtain significant new information on the evolution of ancient Adria. The archaeological mound in the city centre developed from the 6th century BC on an alluvial palaeosurface that corresponds to the top of the Adria Po alluvial succession. The mound is entirely anthropogenic, due to the addition of archaeological layers up to Roman times. The area was characterized by rather stable geomorphic conditions during the late Iron Age and Roman times, allowing soil

Acknowledgments

This research is part of a Ph.D. project started in 2012, that builds on geoarcheological research carried out within project “Parco Archeologico dell'Alto Adriatico – PArSJAd the web link is: http://parsjad.regione.veneto.it/”, funded by Regione Veneto and cross-border cooperation program Italy–Slovenia (2007–2013).

The authors thank Prof. Sauro Gelichi, supervisor of this research, Dr. Maria Cristina Vallicelli, supervisor of Adria district for the Soprintendenza Archeologia del Veneto, Dr.

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