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Mariana (Corsica). Integrating GPR in Roman urban survey*

Cristina Corsi Dipartimento di Filologia e Storia

University of Cassino Cassino, Italy

c.corsi@unicas.it

Lieven Verdonck and Frank Vermeulen

Vakgroep Archeologie University of Ghent

Ghent, Belgium lieven.verdonck@ugent.be; frank.vermeulen@ugent.be

Abstract— In 2000 a new phase of archaeological field activities started on the abandoned city site of Mariana, located south of Bastia (NE-Corsica). Within the on-going international research project “Projet Collectif de Recherche: Mariana et la vallée du Golo” a joint team of the Universities of Cassino (I) and Gent (B) studies the topography of the ancient city in a diachronic framework. In this paper we will investigate the crucial role of GPR prospections integrated in a whole set of more traditional methods of non-invasive city survey and stratigraphic excavations.

Keywords – Roman Archaeology, Three dimensional GPR visualisation; Roman Corsica (France)

I. INTRODUCTION After limited excavations in the 1930s and the 1950s-1960s, a new phase of archaeological research has started in 2000 on the site of Mariana. The buried remains of this ancient city are located around the Romanesque church of La Canonica (Municipality of Lucciana) and near the mouth of the river Golo, some 20 km south of Bastia, on the east coast of Corsica (figg. 1 and 2). Within the framework of the ongoing international “Projet Collectif de Recherche”, entitled “Mariana et la vallée du Golo de l’Age du Fer à la fin du Moyen Age” and directed by Philippe Pergola (Maison Méditerranéenne des Sciences de l’Homme, C.N.R.S., Aix-en-Provence), the Roman town and its territory are being investigated in a longue durée perspective. A joint team of the Universities of Cassino (Italy) and Gent (Belgium) is involved here in studying the ancient topography of the city in a diachronic approach [1]. Main aims of this research team are: defining the urban layout, delineating the defensive system, outlining the urban and peri-urban street network, identification of the public and private spaces and individualization of the harbor and landing places along the coast and/or possibly along the river that bordered the southern portion of the town. Part of our research is devoted to the territory of the colony, mainly the land division and the road network, and to the town/country relationships in Antiquity. At this stage of research, a new 2D map of the town is being composed, thanks to topographical investigations, based mostly on aerial photography, historical cartography, field survey, geophysical prospections and other non-destructive

approaches, and trial excavations. But the chronological dimension and the three-dimensional comprehension of the developments of the town are still not clear. As we will see more in detail, mostly the first colonization phase is still shady because of the lack of archaeological finds datable to the period of the town foundation, traditionally attributed to Marius (ca. 100 BC).

Figure 1. Location of Mariana in Northern Corsica.

II. THE “FOUNDATION” PHASE AT MARIANA It is only thanks to Seneca, who was exiled in Corsica, that we learn that Mariana was a colonia civium Romanorum (SEN, Cons.Helv., VIII, 8), and that it took its name from the founder, Gaius Marius himself. Indeed, no historical, annalistic, juridical or epigraphic source mentions explicitly the deduction of Mariana, and it is possible that the registration of the deduction of the colony is part of the "lacuna" in the tradition concerning the lex Appuleia of 100 BC, mentioning the deductions in Sicily, Achaia and Macedonia (SUET., Vir.Ill., LXXIII, 5), and Gallia Cisalpina (APP., Bell.Civ., I, 130-134). The on site Late Republican evidence is at the moment very unclear. Only a few sherds of Black Glazed Pottery have been recorded in the stratigraphic excavations until now, and only one structure (a “U”-shaped wall made with cobbles from

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sector 11 south of La Canonica 1 , fig. 4, n. 2) has been attributed to the pre-Augustan phase. During the first “random” excavations in the area of the Roman town, carried out by the French archaeologists Leschi and Chauvel in the 1930s [2, 3], finds of numerous Republican coins were mentioned in the excavation of their “Chantier C”, which according to our reconstruction is located northeast of our trench opened in parcel 81, north of the church of La Canonica (sector 12; fig. 3, n. 2 and fig. 4, n. 3; see further). The dating of the structures excavated in the 1930s remains, however, problematic and so does the character of the first Roman settlement in the area.

III. SKETCHING OUT THE TOWN LAYOUT The delimitation of the urban surface of Mariana has been reconstructed on the basis of the very poor archaeological evidence and the historical aerial imagery interpretation, with the support of data from 19th century cadastres, GIS processing and geomorphological observations [5, 6]. It is currently hypothesized that the city walls, with a trapezoidal plan possibly limited on its southern side by the river Golo, enclosed a surface of more or less 30 hectares (fig. 3, nn. 4-5). The pre-existing axis of communication, linking the foothills of the mountain range to the mouth of the river Golo, probably imprinted the general layout of the street network: the colonnaded “rue à boutiques” has probably to be considered the “generating axis” (fig. 3, n. 6), considering that the three known necropoleis of the town are located along it in the western (Palazzetto and Murotondo) and eastern (I Ponti) suburbia (fig. 3, nn. 7-9) [7]. This slightly declined E-W orientation (12/13º WNW-ESE) was respected by most of the buildings archaeologically excavated so far [3]. The well known Augustan phase of the commercial district is a clear proof of it.

Figure 2. Aerial view of the church of La Canonica and the archaeological area of Mariana in the Lower Golo Valley. In the back, the swamp area of La

Marana.

1 Excavations by Pierre Comiti [4].

IV. THE STRATIGRAPHIC EVIDENCE Sound and more extensive archaeological evidence is today available from the area immediately south of La Canonica church. This is the archaeological area where excavations took place in the 1950s and 1960s, and where the recent large scale digs have confirmed and refined the structural evidence of an early Imperial commercial zone, with rows of units (tabernae) on both sides of a colonnaded street. The street, until recently the only tangible evidence of the possible town grid, is approximately oriented E-W (declination 13º WNW-ESE [3]), and is also flanked by housing complexes, that were partly reused in the 6th century to build a Christian basilica with baptistery [8] (fig. 3, n. 3). Most of the dating evidence for the well organized Roman period structures seem to belong to the period between the beginning of our era (Augustus) and the 3rd century.

V. THE GPR SURVEY To test the validity of the hypothesis that this colonnaded street indicated the main orientation of the whole street network of Mariana, we decided to start a geophysical survey, in the large parcel 81 north of the Canonica church, now in use as pasture. A GPR survey was carried out in September 2008 over an area of approximately 5000 m² [9] (fig. 4). A Sensors & Software pulseEKKO PRO GPR was used, with a 500 MHz antenna. The in-line sampling interval was 0.05 m, the line spacing was 0.25 m. Relatively standard processing was applied (dewow, first arrival alignment, uniform gain applied to all data, bandpass filtering 100MHz-1GHz, background removal). Strong reflections caused by metal objects in the topsoil had to be removed. Migration tests with a range of velocities resulted in a high velocity (0.11 m/ns) for the very dry topsoil, decreasing to around 0.075 m/ns at a depth of 1.5 m. This velocity profile was used for the conversion from time to depth. As the area is relatively flat without abrupt changes in the topography, a static shift was applied without tilt correction. Finally, 5 cm thick depth slices were created, taking the square root of the average of the squared amplitudes in the 5 cm depth range. Since the complex results could not easily be interpreted on the basis of profiles or depth-slices alone, a three-dimensional interpretation was made (fig. 5). The interpretation was carried out manually, on the basis of the depth-slices. Since this process is time-consuming, for each feature one slice was selected for defining the horizontal extent: the slice where the feature had the largest extent and where the contrast with the surrounding soil was the sharpest. Furthermore, two depth slices were selected, one being the most shallow slice where the feature started to appear (representing its top) and one where the feature had almost completely disappeared (representing its bottom). Using the depth of these slices, the feature was given its vertical extent. As most features in the dataset were walls, this simple box model with six faces can be assumed to represent the real situation relatively adequately.

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Figure 3. Map of the area of the town: 1. Parcel 81, 2. trial trench (“sector 12”), 3. excavated area south of the church La Canonica, 4. town wall, 5. hypothetical town wall, 6. road axis (partially excavated south of La Canonica), 7-9. necropleis, 10. Golo river

Particular attention was given to possible discontinuities, such as a doorway in a wall or depth variations within a single feature. The orthogonal layout of the town is immediately noticeable, with two streets dividing the prospected area in four insulae, as well as the correspondence in orientation with the excavated colonnaded street south of the Canonica (fig. 4). On the sides of these two streets there are buildings, whose perimeter walls and internal partitions respect the orientation of the big axes. Most of them can be interpreted as private dwellings (especially in the eastern part of the survey area, e.g. fig. 5, nn. 1-3), whereas a big part of the northwestern insula is occupied by what appears to be a large building structure, maybe with bathing facilities (fig. 5, n. 4). Additional information was extracted from a fluxgate magnetometer survey conducted in 2007 with a Geoscan Research FM256. For example, strong magnetic anomalies in building 5 (fig. 5), in the southwestern insula, suggest a hypocaust or, more probably, an oven or metallurgic activity. Other magnetic anomalies indicating industrial activity were found more to the south. From the GPR survey, parts of the water supply and

drainage system of the town were also detected, e.g. within the large complex with baths. The combination of GPR and fluxgate gradiometer results, as well as the mapping of a few wall sections still visible above ground allowed locating one of the trenches from the 1930s. The old excavation plan showing a large apsidal room and one or two basins, has been fitted into the overall plan, so that it can now be inferred that the large complex with baths extend over an area of at least 3000 m². In many parts of the town, the GPR survey provides indications of at least two occupation phases. Often, the distance between walls is very small so that they cannot be contemporary (see e.g. fig. 5, n. 6). Some open spaces had a well-defined function in the oldest phase, like the courtyards in the mentioned complex, but became built up in later phase(s). Moreover, there seems to be a difference in the construction quality of the walls. The walls with the deepest foundations are very straight, are always built exactly according to the main orientation of the town layout and provoke strong GPR-reflections. On the contrary, the shallow

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Figure 4. Results of the GPR survey (depth-slice 0.80-0.85 m, before topographical correction). 1. Parcel 81, 2. “sector 11”, 3. “sector 12”.

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Figure 5. Three-dimensional interpretation of the GPR results. Height has been exaggerated twice. 1-3. houses, 4. building with baths, 5. oven/metallurgy, 6. different building phases of a house.

ones often slightly deviate from this orientation and their construction seems to be less sturdy.

VI. THE GROUNDTRUTHING To confirm this reconstruction and to further investigate the differences in construction quality observed, the GPR results were compared with data from a small excavation trench at the northern limit of parcel 81 (fig. 4, n. 3). Here we discovered, at some 70 cm depth below walking level, two parallel brick walls with opus caementicium core, again with an orientation deviating some 12/13º from NS [1] and matching two linear GPR reflections north and south of the trench (fig. 4, n. 3). The maximum height of the preserved walls is about 35 cm, while also part of their foundations was found. According to the building technique the walls can be dated in Early Imperial times (1st-2nd century AD), but the absence of datable finds from their foundation trenches prevents us to propose a more precise chronology for the construction of this building. However, considering even some uncertainties due to the scarcity and the poor quality of the finds from the occupation phases, we can fix the abandonment of these rooms at the end of the 3rd century AD [10]. At a later date, these walls were intentionally cut and walls with a somewhat different orientation made with large river cobbles were built above them. This trial excavation offers the

chronological markers for a general dating of the urban development of this central quarter of the city: the first and more organic layout, perfectly matching with the orientation of the commercial district, dates to Early Imperial times.

VII. CONCLUSIONS

The case-study of application of GPR survey in large parts of the abandoned urban site of Mariana shows the potentials of fine-resolution ground-based radar as contribution to Roman urbanization. Compared to other non-destructive approaches (aerial photography, GIS based cartographic analysis, magnetic survey...) to this site, GPR survey was by far the most efficient for mapping multi-period underground structures in the intramural town surface. A particular contribution of GPR included the repositioning and interpretation of old and badly located excavation data. Furthermore, the radar survey allowed to propose functional interpretations of the newly discovered town structures. In this new map of the layout of the townscape lays the most important historical aquisition of this project: in an island always considered as “resistant” to Romanization [10], Mariana can now be reconstructed as a typical colonial town with regular street network and functionalization of districts.

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*Research done in the framework of the GDRE «Le monde insulaire en Méditerranée: approche archéologique diachronique des espaces et des sociétés». Our thanks go to prof. Philippe Pergola, Coordinator of the GDRE and Director of the Projet Collectif de Recherche, and to Daniel Istria for their friendly support and their indefatigable activity to promote the scientific research in Mariana.

REFERENCES [1] F. Carboni, C. Corsi, C.P. Venditti, L. Verdonck, and F. Vermeulen,

“Urbanistica delle città romane di Corsica: l'esempio di Mariana”, Proceedings of the 17th International Congress of Classical Archaeology, (Rome, 22nd -26th September 2008), forthcoming.

[2] F. Nucci, "Les campagnes de fuilles à Mariana (Corse) en 1936 et 1937 par Louis Leschi et Albert Chauvel", Cahier Corsica, 195, 2001, pp. 1-20.

[3] C. Corsi, "Le paysage urbain de Mariana: observations préliminaires sur l’urbanisme des périodes tardo-républicaine et imperial", Ph. Pergola ed., Actes du Colloque International “Mariana et la basse Vallée du Golo”. Bastia – Lucciana, France, September 10-16 2004, in press.

[4] P. Comiti, "Les campagnes de fouilles du secteur 11 de 2005 à 2007", Ph. Pergola (ed.), Rapport de fouilles programmées 2006-2007 du site de Mariana & Rapport du Project Collectif de Recherche «Mariana et la basse vallée du Golo de l’Age du Fer à la fin du Moyen Age», Ajaccio 2008, pp. 42-58.

[5] C. Corsi, and F. Vermeulen, "Digital vs. historical cartography for studying archaeological sites: a case study from Corsica", Proceedings of the 11° Workshop «Archäologie und Computer. Kulturelles Erbe und Neue Technologien», (Vienna 18-20 Oktober 2006), Wien, 2007, pp. 1-19.

[6] C. Corsi, A. Roblin-Jouve, and F. Vermeulen, "Géoarchéologie autour de la ville romaine de Mariana et de la basse vallée du Golo (Haute-Corse, France)", in M. De Dapper, F. Vermeulen, S. Deprez, and D. Taelman (eds.), Ol'Man River. Geoarchaeology of Rivers and River Plains, Proceedings of the International Colloquium “Ol’Man River – Geoarchaeological aspects of rivers and river plains” (Ghent 22nd-24th September 2006), (Archaeological Reports Ghent, 5), Gent, 2009, pp. 267-282.

[7] Ph. Pergola, Articles "Alalia", "Corse", "Mariana et Sardaigne au Bas-Empire", J. Leclant ed., Dictionnaire de l’Antiquité, P.U.F., Paris, 2005, pp. 59-60, 580-582, 1339, 1958.

[8] Ph. Pergola, "Mariana, capitale de la première Corse chrétienne", M. Fixot ed., Paul-Albert Février de l’Antiquité au Moyen Âge, Aix-en-Provence, 2004, pp. 238-257.

[9] L. Verdonck, and F. Vermeulen, “GPR survey at the Roman town of Mariana (Corsica)”, C. Benech, D. Fabre, A. Schmidt, and A. Tabbagh (eds.) Mémoire du sol, espace des hommes [ArchéoSciences. Revue d’archéométrie 33 (suppl.)], pp. 241-243, 2009.

[10] C. Corsi, and C.P. Venditti, "The role of Roman towns in the Romanization process in Corsica. The case-study of Mariana", in C. Corsi, and F. Vermeulen eds. Proceedings of the International Colloquium "Changing Landscapes. The impact of Roman towns in the Western Mediterranean", in press.

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