Geo-archaeological aspects of the Modena plain (Northern Italy)

This paper traces the various stages of geomorphological evolution of the plain area around Modena from the VIth millennium B.C.E. (Neolithic) to the Present, through a reconstruction of the ancient landscape and human settlements. By means of a GIS platform, geomorphological investigations led to the implementation of a Microrelief Map, a Digital Elevation Model (DEM), a Map of Surface Deposits and a Geomorphological Map. The most striking altimetric features in the study area are the morphological changes of the Rivers Secchia and Panaro: south of Modena they run deep in the alluvial plain whereas north of the town they flow elevated over the plain. The surface lithology consists mainly of silt, with bands of sand and clay; mainly gravel deposits crop out only near southern tracks of the main Apennine watercourses. The geomorphological landscape is mainly characterised by alluvial fans, fluvial ridges paleo-riverbeds fluvial scarps, natural springs and some depressed areas; worth of note are also forms connected with human activity. There have been several attempts to cross–date geological and geomorphological evidence with archaeological data, on both detailed and wide territorial scales. A further advancement can now be attempted by comparing data obtained from systematic research on fluvial forms with data contained in the archives. In the area studied, some 800 archaeological sites were identified and catalogued. Such a high number of archaeological sites can give a great deal more information than is found in any other place in northern Italy. The research took into account the overlapping of archaeological and geomorphological data, with the implementation into a GIS (ArcGis 8.3) of geoarchaeological maps divided in main periods and here represented from the Neolithic to the to Iron Age and from Roman period to the Early Middle Ages. The dating thus obtained for fluvial forms was mainly based on the relationship between these forms and archaeological evidence. The upper layers of stratigraphical deposits of the urban center of Modena are quite well-known. These layers can be divided macroscopically into two large stratigraphic sequences, one belonging to the ancient Roman town of Mutina and the other to the medieval and modern town, separated by a substantial alluvial layer up to 2 meters thick in places. The archaeological strata of Modena go down to about 7-8 meters below the present surface level, reaching a depth of 11 meters in places. In this work, a three-dimensional model indicates the thicknesses of the large stratigraphic levels of the Roman Age, the alluvial deposits and the Medieval/Modern Age. The comparison of geomorphological and archaeological data allowed many fluvial forms to be dated. In particular, the formation of some alluvial fans and ridges south and south-west of Modena took place before or during the period dating from the Neolithic to the Iron Age. The investigations have confirmed more in detail the shift to the east of the R. Secchia and to the west of the R. Panaro in the area downstream of Modena starting from the Bronze Age. It was also possible to reconstruct the ancient soil levels in the Modena city centre starting from the Roman Age. Similarly, it was possible to measure the thickness of both archaeological stratigraphies and the various alluvial levels. Therefore, starting from the end of the Roman Age, an aggradational-type model of fluvial evolution was hypothesised for the first time thanks to this reconstruction. According to this model, the watercourses, even the minor ones, passed from a runoff occurring in deep riverbeds to one hanging over, or at least at the same level as, the surrounding plain. The plain aggradation from the Roman Age could be ascribed to different causes, such as previously occurring processes (northbound shift of the R. Po) or coeval events (climatic changes, subsidence, deforestation, abandonment of the countryside, degradation of the water flow system set up by the Romans). After the hydrological degradation which reached its peak at the end of the VIth century C.E., the morphological evolution of the study area in the Middle Ages and Modern times shows more stability, even if the area of Modena is still characterised by the risk of floods. In the last centuries an important factor of geomorphological changes in the Modena plain has been human interventions. In conclusion, in the Modena plain, as in other parts of the Po, the dominant geomorphic drivers switched from natural processes to processes strongly influenced by anthropogenetic activities.