GEOARCHAEOLOGY OF THE PORTUS : THE ANCIENT HARBOUR OF ROME, TIBER DELTA, ITALY

Neglected by scholars, ancient harbors yield TWO key elements: the container (harborworks, structures) and the contents (sedimentary sequence accumulated on the bottom through time). The goal here is to clarify the geo-history of former harbors of Roma: the Claudian one, from the 1st cent. AD, and that of Trajan, built in the 2nd cent. AD.

METHODOLOGICAL CLUES: In a multidisciplinary approach (archaeology, history, geology, geomorphology, biology, geochemistry), three phases are necessary to acquire data with geoarchaeological techniques: 1— Preliminary geomorphological and geophysical mapping of a site is required to select the coring spots. 2— Coring equipment facilitates extraction of deposits and elucidates coastal stratigraphy. This option avoids financial and logistic problems of urban, archaeological prospective excavations; it overcomes difficulties linked to the water table (rise of relative sea-level). 3— High-resolution laboratory investigation of such deposits must include correlated, statistical analyses.

AMONG THE SOLVED ISSUES: Debates were stranded, longing for chronological information about the foundations, durations and reasons of abandonments of both harbors.

1— Why had they been sunk 3 km north of Ostia? Observing fluvial sediments below marine-harbor ones has led to deduce the Tiber paleo-mouth location up to the 9th century BC (defluviation); and induce the presence of a naturally remaining, favourable depression. While streamlining earthworks, the Roman civil engineers just traced the original, straight river course seawards.

2—Whether the over-sized Claudian harbor opened westwards or northwards was pondered for some 150 years. The first theory predicted two offshore jetties, perpendicular to the shore. The later one envisioned a bay protected from marine influence by a sand spit parallel to the coast. Though falsifying the idea of a natural bay opening northwards, the new observations still corroborates both says with a shallow, northern passage and a deep roadstead accessible from the West.

3— Modelling the evolution of these Roman endeavours combines a deepened basin dug inland, connected by one channel to the sea and another to the Tiber. This fits the ideal-type of a mixed –maritime and fluvial— harbor, achieved under Trajan if not before: the famous Cothon.