In 2002 two sections in two ditches of Porto Torrão were excavated by Era Arqueologia (Valera, Filipe, 2004; Valera, 2013). In that area the ditches are just 8 meters apart. For the inner one (ditch 1), material culture and radiocarbon dating say that it was open and filled almost to the top during Late Neolithic (end of the 4th millennium BC/transition to the 3rd). The last filling deposits, though, are from Late Chalcolithic. The outside one (ditch 2) was open by the middle of the 3rd millennium BC and the filling went on until the end of the millennium (according to radiocarbon dating). That means that when the outside ditch (ditch 2) was opened the inner one (ditch 1) was visible and not completely filled, what just happened in simultaneity with the later filling of ditch 2. So, why opening a new ditch just 8 meters apart, having to excavate bedrock, when a previous ditch was just there, visible and easier to re-excavate? Well prehistoric communities do not respond to modern principles of effort-profit and this particular situation (together with many others in other ditched enclosures) should make people, at least, wondering.
Valera, António Carlos e Filipe, Iola (2004), "O povoado do Porto Torrão (Ferreira do Alentejo): novos dados e novas problemáticas no contexto da calcolitização do Sudoeste peninsular", Era Arqueologia, 6, Lisboa, ERA Arqueologia/Colibri, p.28-61.
Valera, A.C. (2013), “Cronologia absoluta dos fossos 1 e 2 do Porto Torrão e o problema da datação de estruturas negativas tipo fossos”, Apontamentos de Arqueologia e Património, 9, Lisboa, Nia-Era, p.7-11.