Its origin is connected with the establishment of the salt-marshes.
To the north as far as the rocky point of St Gildas, sheltering the mouth of the Loire, the shore, often occupied by salt marshes (marshes of Poitou and Brittany), is low-lying and hollowed by deep bays sheltered by large islands, those of Olron and Re lying opposite the ports of Rochefort and La Rochelle, while Noirmoutier closes the Bay of Bourgneuf.
The salt-marshes of the Mediterranean coast, especially the Etang de Berre and those of LoireInfrieure, are the principal sources of sea-salt.
The name Marica (" goddess of the salt-marshes") among the Aurunci appears also both on the coast of Picenum and among the Ligurians; and Stephanus of Byzantium identified the Osci with the Siculi, whom there is reason to suspect were kinsmen of the Ligures.
Before that time there was no basin or wet-dock, though the river Medway to some extent answered the same purpose, but a portion of the adjoining salt-marshes was then taken in, and three basins have been constructed, communicating with each other by means of large locks, so that ships can pass from the bend of the Medway at Gillingham to that at Upnor.