Magnetic detection of ship ballast deposits and anchorage sites in King Herod's Roman harbour, Caesarea Maritima, Israel

Ballast stone deposits are a common feature of sediments in ancient harbour basins but are often overlooked as a potential source of archaeological information. Recent geophysical investigations at Caesarea Maritima in Israel have discovered a thick, laterally extensive ballast layer in the area seaward of the 1st c. BC Roman harbour. The ballast deposits were identified by low-relief mounds on the seabed with elevated magnetic intensities. Jet probing and excavation of magnetic anomalies at several locations revealed a 20–60 cm thick rubble layer containing large quantities of Late Roman and Byzantine pottery, local sedimentary boulders (kurkar sandstone, limestone cobbles) and foreign igneous and metamorphic boulders (granite, schist, volcanics; ca. 50%). The foreign boulders and pottery identify the rubble layer as ballast and ships refuse jettisoned by merchant ships outside the harbour. The strong magnetic contrast between the ballast deposits and the natural seabed sediments is attributed to the high magnetic susceptibility (>10−3 SI) of crystalline boulders and pottery materials within the ballast rubble.AMS 14C and OSL dates and pottery evidence indicate the ballast accumulation began in the Late Roman period (ca. 200–330 AD) and continued well into the Late Byzantine (ca. 7th c. AD). The main phase of ballast deposition (ca. 4th–5th c. AD) coincided with a decline in the state of Caesarea's harbour following a destructive tsunami event in the early 2nd c. AD and records a shift to a more intensive use of the area seaward of the harbour as an anchorage and ballast lightening area.