noticed southwest of Biskra a ditch which they called a saqiya (irrigation canal) and attributed it to a legendary Arab queen Bint al-Khass (or al-Krass), who was supposed to have built it to supply pilgrims to Mecca with water. Elsewhere the remains of a wall associated with the Fossatum was attributed to al-Fara'un (Pharaoh). Historians and archaeologists in the 19th century continued to believe that it was an irrigation canal, until at the beginning of the 20th century Gsell correctly identified it with the fossatum of the Codex Theodosianus. However, the full extent of the Fossatum was not known until after World War II, when the use of aerial photography to locate archaeological sites was pursued by Col. Jean Baradez. He followed up the aerial work with traverses on the ground and excavations at many sites along its route. His resultant book, with many aerial and ground photographs, remains the standard work of reference. Ideas on the purpose of the Fossatum have evolved since Baradez' time. Whereas Baradez was a military man, and World War II just having finished with the military use of ditches very much in mind, the military aspect of the Fossatum was emphasized. In the more peaceful modern era, the use of the Fossatum as a customs and migration control has been brought to the fore, suggested by inscriptions at Zaraï giving long lists of products and tariffs. Construction The Fossatum as proposed by Baradez consisted of at least four segments: Hodna or Bou Taleb section: begins near the modern town of Ain Oulmene on the north-east slopes of the Hodna Mountains, heads south following the foothills then east towards Zaraï, then doubles back westward to enclose the eastern end of the Hodna mountains, standing between them and the Roman settlements of Cellas and Macri. The length of this segment is about . It probably criss-crosses the border between Numidia and Mauretania Sitifensis. Tobna section: begins near Tubunae (modern Tobna), heads south-south-east to the gorge where the Oued Ksour emerges from the Awras Mountains (south of modern town of al-Kantara), south to the Roman town of Mesarfelta, then a short section west to enclose a northeast branch of the Zab Mountains. Length of this section is about . The fossatum is associated with the Roman administrative border district known as the limes Tubunensis, but as it is up to away from the known border it cannot