SEDIMENTARY RECORD OF THE TOMBOLO-HEPTASTADIUM OF ALEXANDRIA, EGYPT

Alexandria was founded in 331 BC by Alexander the Great on the western margin of the Nile delta. The city lies leeward of Pharos, a rocky island, and has been built upon a sandstone ridge of Pleistocene age. Early on, this island was linked to the city by means of the Heptastadion, a 1300 m long causeway. With time, sediments accumulated on either side of the Heptastadion and by the fifteenth to sixteenth centuries AD the tombolo accommodated the Ottoman city. In light of the dense urban fabric, the spit has never undergone significant archaeological or geomorphological investigations. The aim of our geoarchaeological research was to establish whether or not the tombolo existed before the arrival of Alexander the Great. Was the artificial causeway built upon a shallow proto-tombolo or artificial infill deposited by the Greeks to raise the level of the marine bottom?

The morphogenesis of Alexandria’s tombolo is dated 7800 yrs BP (6400 to 6150 cal. BC). The tombolo’s sedimentary base was deposited on a topographical high between the island and the continent. The main sedimentary corpus of the tombolo comprises a Cladocora biodeposition that accreted until 4200 yrs BP (2440 to 2170 BC). These coral branches were reworked from coralline colonies. At this time, the tombolo lay near the water surface, later facilitating construction of the Heptastadion during the Hellenistic period. Construction of the causeway entrained a depositional hiatus between the Greek and the mid-Roman periods.

A return to sediment deposition (units B and C) is dated 2085 ± 45 yrs BP (3rd to 4^th c. AD). These units corroborate the accretion of the tombolo and/or a change in the configuration of the eastern harbour. Plastic clays and the rapid rates of sedimentation (10 mm/yr) recorded in unit D are consistent with a harbour facies which ends around 1720 ± 45 yrs BP (6th to 7th c. AD). Unit E, dated 1635 ± 35 yrs BP, comprises pebble beach deposits indicative of the progradation of the tombolo’s eastern flank. This mid-littoral beach was subsequently fossilised by subtidal marine sands around 1530 ± 35 yrs BP (10th to 11th c. AD). The rapid influx of marine ostracods coupled with the fossilisation of the pebble beach deposits attest rapid tectonic movement having affected the eastern bay.