This geological study aimed to (a) clarify the mechanical characteristics of the terrain on which the buildings in the southern part of Herculaneum stand, (b) to reconstruct the pre-eruption geomorphology and© to re-interpret the beach sediment present on the SW edge of the city. Data relating to paleo-topography, outcrop stratigraphy, excavated trenches and new core samples were used in a fruitful collaboration with the archaeologists of the HCP. The results obtained confirmed that Herculaneum stood on a coastal terrace bordered to the SW by a small cliff and was literally delimited by two gullies formed by torrents. The SE Valley formed a small “ria” gulf which was probably the site of the port. The NW Valley, which can be placed between the Villa of the Papyri and the remains of a sea-front villa found beneath the stables of Palazzo Reale at Portici, must have provided a panoramic position for the belvederes of both these villas. The SW Scarp, to date interpreted as an ancient marine cliff, was modified and perhaps created by the quarrying of “Tufo Rossicio” (Auct.), a material used for construction during the Samnite period. This activity, which continued until the beginning of the 1st century A.D., is attested by the furrows and geometric cuts visible on the Coastal Platform in front of the Suburban Buildings. For the period when the quarry was active the sea level can be placed where today the quote is -6.5 m (due to later land slippage). This was followed by a phase of subsidence, between the end of the 1st century B.C. and beginning of the 1st century A.D., which, through the deposition of sand at the base of the SW Scarp and marine corrosion of the walls of the south wing of the House of the Relief of Telephus, took the coastal level to its present height -2.5/-3m. Between the third and sixth decades of the 1st century A.D. an upward movement of the terrain forced the sea back again and this encouraged widespread building along the Coastal Scarp. In the 60s A.D. subsidence raised the sea level to -2.5 m and deposited sand up against the sea front buildings. (Aldo Cinque, Giolinda Irollo)