International congress : People/environment relationships from the Mesolithic to the Middle Ages: recent geo-archaeological findings in Southern Italy.
Salerno (Italy) 4-7 September 2007
Recent geo-archeological findings of Portus, the ancient Harbor of Rome
GOIRAN J.Ph.1, OGNARD C. 2, TRONCHERE H. 1, CANTEROT X. 3, CLUZE J.A2
1
2
UMR 5133, Maison de l’Orient et de la Méditerranée
Université Lyon 2 Lumière, Laboratoire Rhodanien de Géomorphologie
3
Université Paris IV Sorbonne
The town of Ostia was established at the mouth of the Tiber in the middle of the fourth century BC in
order to serve as an advance port for Rome. But this fluvial port was soon found to be impractical because of
strong currents and the shifting sand banks of the river mouth, which prevented the flow of supplies to the
Roman capital. In the first century AD the emperor Claudius had a new port for Rome constructed in the Tiber
delta, three kilometres north of Ostia. In the second century AD Trajan completed the plan by building a closed
hexagonal basin.
Our new geoarchaeological work looks to answer three important questions: (1) Why? Why did the two
emperors build the harbors at this location on the Tiber delta? (2) What was the palaeogeography of the Tiber in
antiquity? (3) Where did ships enter the port of Claudius and what sort of foundations were the breakwaters built
upon?
Scholars have attempted to answer these questions using ancient texts allied with intermittent
archaeological excavations since the Renaissance. In light of the pronounced deltaic progradation, presence of
the water table and the logistic difficulties posed by Rome’s international airport, it was decided that
stratigraphic studies by coring would be the most appropriate means by which to investigate the harbour.
(1) Delta and coastal stratigraphy (pre-harbour environment): The stratigraphic study shows three distinct
sedimentary sequences: fluvial, marine and seaport sediments. The lowest sedimentary unit constitutes Tiber
fluvial sediments. The abrupt fluvio-marine transition, positioned at –8m below MSL, is dated to 3100 BP. The
presence of such recent marine sediments (9th c. cal BC) at this depth implies a defluviation of the Tiber and a
concomitant marine ingression. The overlying unit is typical of a silty harbour environment. These results
confirm that Claudius and especially Trajan used the palaeo-mouth of the Tiber to facilitate the construction of
their basins.
(2) Fluvial geomorphology: Beneath the ancient harbour lie geological traces of the palaeo- river mouth that
flowed into the sea from east to west, much straighter than today. In the 9th century BC the river was subject to a
defluviation which caused it to take a southerly direction, where five centuries later Ostia was established on the
left bank of a meander.
(3) Harbour configuration: For the position of Claudius’ entrance, the first hypothesis, put forth in the 16th
century, implies a full-west opening flanked by two encircling breakwaters partly constructed offshore. The
second hypothesis, proposed in the 20th century, is based on the presence, to the west, of a sand spit formed by
the sediments of the Tiber. This natural obstacle suggests a port configuration with an opening to the north.
Fig.1: Western opening hypothesis, dating back to XVIth century
Fig.2: Northern opening hypothesis, from O. Testaguzza
Boreholes were drilled along Claudius’ northern breakwater. In its western section, the stratigraphy
does not indicate a sandy spit upon which the breakwater rested. A core taken from the breakwater shows that
the foundations, composed of rough components and pozzolana (hydraulic cement), descend to a depth of 7 m
below MSL and rests on the marine bottom. In this area, breakwaters were thus constructed offshore.
In contrast, the cores taken in the north - eastern sector of the Claudian breakwater show an
accumulation of 2 m of marine sediments. The base of these harbor sediments was dated to the 2nd century AD.
There was a northern communication with the sea but it was in shallow water. It was probably a second entry of
later date, intended for ships of low draught (additional cores currently being studied in order to confirm depth
and chronology). We therefore conclude that the port of Claudius possessed a double entry and the two initial
hypotheses are not contradictory. This twin entry would have generated a flushing current in the basin, limiting
silt accumulation.
Fig.3: New configuration deducted from the sediments analysis