B) THE GEOLOGY OF THE AREA
The last glaciation (Wurm), begun approximately 120.000 years ago, ended about 18.000 years ago with a cold peak that produced a lowering of the sea level of 120 meters below the present one. This caused a strong cut of the Tiber valley: in particular in the Rome area the Tiber cut the compact substrate of the Pliocene clays (from 7 to 1.8 million years ago) up to about 50 m below the present sea level (pict. B1 ); consequently also the right and left tributaries dug deep and narrow valleys.
The following Post Pliocene raising (mainly on the right bank) and the consequent exposure to the erosion, brought about to receive the drifts produced by the next activity of Latium volcanoes.
A further raising of the ground helped the erosive action of the water streams, the Tiber and the tributary "marrane": the plain the valleys were therefore filled with Holocene drifts (10.000 years ago) and more recently with the accumulation of anthropic materials produced from the building activities in Rome.
On the right side two deep cut crossed the ridge Monte Mario-Janiculum delimiting the Vatican Hill; on the left side numerous cuts began to delineate the “hills” while one more reached the valley that would have become the marshy area of the Velabro, reaching the Tiber river near the present Tiber Island.
The Tiber flowed therefore embedded in a flat bottom valley with alluvial drifts. But after the Wurmian cold peak the sea level began to rise due to the fast increase of the temperature: this caused the progressive rise of the Tiber riverbed and its filling with alluvial Holocene drifts mainly centered in the Tiber valley (pict. B2  the represented area corresponds to the outlined one in pict. B1).