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

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.