Aspetti archeologici e condizioni geologico-morfologiche degli antichi acquedotti pugliesi. L'esempio dell'Acquedotto del Triglio nell'area tarantina.

The Apulia region, in southern Italy, hosts several evidence of ancient aqueducts dating back to the roman and medieval age. The karst territory of Apulia, where, due to the geological and morphological features of the region, the presence of water at the ground is extremely rare forced man to look for water supplies and to collect water since the establishment of the very first human settlements in the area. This explains the presence of many Apulian aqueducts, covering the entire region, from the Dauno Apennine (Bovino, Torremaggiore, Lucera), to the Bari province (Canosa di Puglia, Gravina in Puglia), down to Salento Peninsula (Brindisi, Gallipoli). Within this complex framework, the present article examines the geological and morphological setting in the Taranto area, where two remarkable aqueducts are present: Saturo and Triglio. In particular, the attention will be here focused on the Triglio aqueduct, due to the following reasons: i) continuity in the use of the structure; ii) good state of preservation of the aqueduct; iii) availability of many historical documents. The Triglio aqueduct takes origin from the southern slope of the Murge Plateau north of Taranto. It is formed by six branches, that follow the course of the typical fluvial-karst valleys of the area (locally called gravine). The branches join near the S. Michele Church to form the main tunnel. The overall aqueduct is about 18 km-long, and has a mostly subterranean course. The final part, 3 km-long, just before entering the town of Taranto, has, on the other hand, an external course, marked by more than 200 arches. Geologically, the local setting is made of a limestone bedrock of Cretaceous age, that is overlaid by Pliocene-Pleistocene calcarenites and marly clays. Two aquifers characterize the hydrogeology of the Taranto area: a shallow aquifer, contained within the calcarenites, and resting over the underlying impervious clays; and a deep aquifer in the Cretaceous bedrock. Development of the Triglio aqueduct has to be related to the shallow aquifer, and to outflow of water at the contact between calcarenites and clays. Starting from these observations, in ancient times the work to realize the subterranean aqueduct began. In addition to the above considerations, a detailed analysis of the historical documents, and of the archaeological researches in the area as well, is presented. The final part of the article provides some preliminary considerations about dating of the hydraulic work, based upon the previously considered evidence.