Itius Portus or Portus Itius, an ancient Roman name for a port in Nord-Pas-de-Calais, of unknown location. The main candidates have been Wissant and Boulogne, more usually called Gesoriacum, and later, Bononia, but a silted-up lagoon on the Flanders shore behind Calais now seems most likely.
It was certainly near the uplands round Cap Gris Nez (Promunturium Itium), but the exact site has been violently disputed ever since the Renaissance. Many critics have assumed that Caesar used the same port for his first expedition, but the name does not appear at all in that connection. This fact, coupled with other considerations, makes it probable that the two expeditions started from different places.
It used to be generally agreed that he first embarked at Boulogne. The same view was widely held about the second, but T. Rice Holmes in an article in the Classical Review (May 1909) gave strong reasons for preferring Wissant, 4 miles east of Gris Nez. The chief reason is that Caesar, having found he could not set sail from the small harbour of Boulogne with even eighty ships simultaneously, decided that he must take another point for the sailing of the more than 800 ships of the second expedition. Holmes argues that, allowing for change in the foreshore since Caesar's time, 800 specially built ships could have been hauled above the highest spring-tide level, and afterwards launched simultaneously at Wissant, which would therefore have been commodissimus or opposed to brevissimus traiectus.
In fact the logical place to assemble 800 or so ships was in the large lagoon (where now there is farmland and a maze of drainage ditches) behind a coastal fringe of islands (where the modern ports of Calais and Dunkirk now sit) because in Caesar's day an estuary stretched all the way inland to the site of modern St Omer. Albert Grenier explained this (in French)  in 1944 soon after D-day when the practicalities of large invasions across the Channel were fresh in mind. Careful analysis of Caesar's text versus modern understanding of wind, weather, tides, and siltation makes it almost certain that this was Caesar's departure port. Boulogne, Wissant, and Quentovic later became important as ports for small numbers of ships. The English side of the Channel also has many places that were Roman-era ports but are now far inland.
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Boulogne is presumed to have been the point of departure for the conquest of Britain of 43 under Aulus Plautius, although the only surviving account of the invasion, that of Cassius Dio, does not mention it. The emperor Claudius followed later with reinforcements, and Suetonius tells us he sailed from Gesoriacum.
- Julius Caesar, Commentarii de Bello Gallico 4.21-23
- Julius Caesar, Commentarii de Bello Gallico 5.2
- Julius Caesar, Commentarii de Bello Gallico 4.21
- William Smith, , A Dictionary of Greek and Roman Antiquities, John Murray, London, 1875.
- Cassius Dio, Roman History 60:19
- Suetonius, Claudius 17