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Trastevere west

The western part of Ostia's "trastevere" is the Sacred Island, the Isola Sacra, a name documented for the first time in late antiquity. Perhaps it was prompted by the presence of the Basilica of Saint Hippolytus near Portus.


Two shipwrecks

In 2011 two shipwrecks were discovered at a depth of 2.5 m during construction work of a new bridge on the Tiber (Ponte della Scafa) and its connected road system. The wrecks were found behind the coast and even to the east of the north-south running Roman road on the Isola Sacra. At present it is still unclear in what ancient environment they are. The two ships sank before the first half of the 3rd century AD, perhaps due to a flood of the Tiber. It is also conceivable that, when the ships were end-of-life after many repairs, they were reused, for example to strengthen the bank of the canal (cf. B. Ford, "The Reuse of Vessels as Harbor Structures: A Cross-Cultural Comparison", Journal of Maritime Archaeology 8 (2013), 197-219).

The excavation of the first shipwreck, named Isola Sacra 1, started in June 2011 and has been completed in the beginning of October. It measures 12 x 4.88 m., with a hull depth of about 1 m. It is characterized by a flat, vertical bow (transom bow). The vessel has been identified as a horeia-type vessel, a type of craft used primarily for transferring goods from larger boats to the docks or for fishing within the harbour. Examples of such ships have also been found in Toulon, France and Naples, Italy.

The second shipwreck (Isola Sacra 2) lays perpendicularly to Isola Sacra 1 and it is partially covered by it. Only the southern side of Isola Sacra 2 has been investigated and documented.



The wreck of Isola Sacra 1. Photo: Boetto 2017, fig. 3.


Geophysical research

Extensive geophysical work has been carried out on the Isola Sacra by the University of Southampton. The surveys from 2007 to 2012 identified a large number of different monuments, sites and features across the Isola Sacra.



Click on the image to enlarge. Map of the southern part of the Isola Sacra with recent discoveries.
Germoni et al. 2018, fig. 14.


Canal

The dominant feature located by the geophysical surveys is a wide canal, running from north to south, from the Fossa Traiana in the north towards the mouth of the Tiber to the south. The canal measures some 90m across close to the Fossa Traiana, and narrows quite significantly as it heads southwards, to 40m. Possibly the canal is a continuation of the Canale Romano to the north of the Fossa Traiana, to the south of the hexagonal basin at Portus. The line of the canal is traversed by a series of positive features, one of which is still visible as an opus caementicium pier belonging to a bridge that spanned the canal.



Map of the Isola Sacra with the canal. Keay et al. 2020, fig. 5.1.

Several phases have been recognized. In a first, most likely Trajanic, phase there was a straight canal running between the Fossa Traiana and the mouth of the Tiber near Ostia. In a second phase the canal took a new route, while the earlier course had been filled with sediments. This new stretch of the canal was then filled in with fine sediment and probably disconnected upstream from the Fossa Traiana. Absolute dates have not yet been assigned.

The southern end of the canal is very complex to understand and its sea or river connection is rather difficult to identify. This could be due to the constantly moving coastlines and riverbanks right at the river mouth. Was it connected to the Tiber opposite Ostia or directly to the sea?

At the north end the depth of the canal was between 2 and 3.5 meters below the Roman sea level, which means that it could not be used by the largest vessels. At the south end the depth was 5 meters, suited for large cargo ships. Because the canal does not lead immediately to the sea, it was most likely not a drainage canal. There do not seem to have been warehouses along the canal. Therefore goods may have been unloaded from cargo ships and loaded on tow-boats for which storage in horrea was not practical or not deemed necessary. We may think for example of marble, and of the wood that was brought to Rome as fuel for the baths. Wooden roofs would offer enough protection. Furthermore, tow-boats may have moored here in periods when the water table of the Tiber did not allow the journey to Rome. Wharves for repairing and building ships might also have been here. Only excavation will answer these questions.


Buildings

A substantial area of settlement measuring at least 600 m by 200 m was located in the southernmost part of the Isola Sacra. Here a series of warehouses and other large buildings was recognized, that line the bank of the Tiber, many with a form that is reminiscent of warehouses in Ostia to the south of the river Tiber. Elements of these latter structures had already been discovered in the 1960s.

Quoting Germoni et al. (2018, 23-24): "There is evidence for five principal buildings, some only fragmentary. From the west, the first and most completely understood building is a courtyard warehouse c. 175 m wide and more than 175 m long (Building 1). It appears to comprise a range of store rooms facing onto a portico that surrounded a courtyard; the plan of its southern part is uncertain, but there is possibly a second courtyard towards the river frontage. The form of this building bears similarities to the layout of the Grandi Horrea and the Piccolo Mercato at Ostia, supporting its identification as an horreum, although the latter is smaller in size. Adjoining this building to the east, and sharing a common boundary, is a further courtyard building, with storerooms similarly opening onto a portico (Building 2). The western range extends for at least 75 m, whilst the northern that lies at an obtuse angle can be traced for about 25 m. This is almost certainly a further rectangular horreum. There is a gap in the survey data to the east of this building, with enough space to contain a further horreum of similar size. The next building to the east (Building 3) is represented by a building section of similar form, with a north-south range of store rooms facing a portico to the west, which can be reasonably interpreted as the eastern range of this. The row of storerooms excavated in the 1960s that lie to the south of it almost certainly formed part of the same building."



Plan of buildings 1-5 and the defensive wall. Germoni et al. 2018, fig. 13.

"The north-east corner of another courtyard building has been located further to the north-east, but this is set back considerably further from the river. It is also slightly different in plan, with rooms only visible in its eastern range, and these are of different proportions and flanked by a corridor on both sides (Building 4). This arguably suggests a different function, with no clear parallels amongst the structures excavated at Ostia. Finally, closest to the Tiber at the eastern edge of the Isola there is a further large building on the same alignment and again set back from the river to the south, although very different in form (Building 5). It is about 50 m wide and at least 50 m deep, divided into two by a north-south wall. The space to the west contains three or perhaps four rows of regularly spaced columns c. 8 m apart. The function of the building is uncertain, although the differences between its layout and those of the other buildings to the west would seem to argue against it being a warehouse. The character of the rooms to the east is less clear."


Defensive wall

A defensive wall can be traced clearly from the present bank of the Tiber in the east, running in a line for c. 345 m to the west south-west. The wall is about 3-5 m thick and has three rectangular external towers (6 m x 8 m) placed about 80 m apart. A probable fourth tower lies partly beneath the trackway a further 74 m to the west. To the east, the wall has clearly been cut by the river when it changed course in the 16th century. To the west the wall changed course, turning to run north-south. Given the character of this wall and especially the provision of external towers, there can be no doubt that this is a town wall delimiting the settlement on this side of the Tiber.

There are significant differences between the Isola Sacra wall and the late-republican city-wall of Ostia. We could then think of a late-antique date, but it should be noted that Procopius, writing about events in c. AD 570, notes that Ostia was "without walls".


[jthb - 8-Dec-2020]