Matera lies on the right bank of the Gravina river, whose canyon forms a geological boundary between the hill country of Basilicata (historic Lucania) to the southwest and the Murgia plateau of Apulia to the northeast. The city began as a complex of cave habitations excavated in the softer limestone on the gorge’s western, Lucanian face. It took advantage of two streams that flow into the ravine from a spot near the Castello Tramontano, reducing the cliff’s angle of drop and leaving a defensible narrow promontory between the streams. The central high ground, or acropolis, supporting the city’s cathedral and administrative buildings, came to be known as Civita, and the settlement districts scaling down and burrowing into the sheer rock faces as the Sassi.