Fossatum Africae

Fossatum Africae ("African ditch") is one or more linear defensive structures (sometimes called "limes") claimed to extend over or more in northern Africa constructed during the Roman Empire to defend and control the southern borders of the Empire in Africa. It is considered to be part of the greater frontier system in Roman Africa.
It is considered to have many similarities of construction to Hadrian's Wall, one of the northern borders of the Empire in Britain.
History.
The Fossa regia was the first frontier line to be built in Roman Africa, used to initially divide the Berber kingdom of Numidia from the territory of Carthage that was conquered by the Romans in the second century, but this is considered to be independent of the Fossatum Africae. 
There is only a single mention of the Fossatum (as such) in historical literature prior to the 20th century. This occurs in a letter written by the co-emperors Honorius and Theodosius II to Gaudentius, the "vicarius Africae", in 409, and preserved in the "Codex Theodosianus". Noting that the "fossatum" had been established by the "ancients", the emperors warned the Roman citizens of Africa that if they did not maintain the "limes" and "fossatum" then the job (with associated land rights and other advantages) would be given to friendly barbarian tribes. 
Consequently, it is not known with certainty when the Fossatum was constructed. Of course, a structure of this size would be the work of centuries, and the archaeological excavation of the many forts and towns along its route has yielded many dates from the reign of Hadrian in the 2nd century to Constantine in the 4th century. Current opinion has not advanced since the discussion by Baradez in 1949, who concluded that construction probably began after the first visit of Hadrian to Africa in 122 (and before or after his second visit in 128). This conclusion is based on the similarities with Hadrian's Wall in Britain and with what is known about Hadrian's concern to protect the Empire. Baradez also postulated a pulse of construction during the reign of Gordian III in the 3rd century, and finally abandonment of the Fossatum in 430–440 after the Vandal invasion.
Having been built in an arid region of strong winds and blowing sand, the Fossatum quickly eroded and only traces remain. During the Middle Ages, Arab nomads of the Banu Hilal occupied much of the area and 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 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:
There may also be a further segment north of Tobna.
Generally the Fossatum consists of a ditch and earth embankments on either side using the material from the ditch. Sometimes the embankments are supplemented by dry stone walls on one or both sides; rarely, there are stone walls without a ditch. The width of the Fossatum is generally but in exceptional cases may be as much as . Wherever possible, it or its highest wall is constructed on the counterscarp.
Excavations near Gemellae showed the depth there to be , with a width of at the bottom widening to at the top.
The Fossatum is accompanied by many small watchtowers and numerous forts, often built within sight of one another.
There are similar, but shorter, "fossata" in other parts of North Africa. Between the Matmata and Tabaga ranges in modern Tunisia there is a "fossatum" which was duplicated during World War II. There also appears to be a "fossatum" at Bou Regreg in Morocco although this would not have been within the scope of the proclamation of the "Codex Theodosianus" because at that time the province was not in Africa, administratively speaking.