Monte Casoli di Bomarzo Nature Reserve

Geology

The territory of the Monte Casoli Nature Reserve and its surroundings include diverse geological formations: marine sedimentary rocks, volcanic and continental rocks.
The most ancient formations are those of the regressive marine, Pliocene-Pleistocene sedimentary cycle, which cover the greater part of the territory and emerge in particular in the central-southern portion of the Park, near the beds of the Vezza torrent and its tributaries.
The most ancient volcanic strata, quartz-latitic ignimbrite, coming from the Cimino emission centre (about 1.3 million years ago), known locally as 'Peperino tipico' from its frequent black biotite crystals, similar to pepper grains, are to be found on the southern side of the park, to the south of the Monte Cassoli trench, along the northern border of the volcanic upland which looks out over the trench itself.
The volcanic formation produced by the Vulsino volcano some 0.8 million years ago, the Vulsine basal tufa, consisting of layers of variously coloured tufa, emitted during several eruption phases, is superimposed on the marine sedimentary terrain, in the north western portion of the Park, along the left hand side of the Santa Maria trench and on the left bank of the Vezza torrent.
The more recent red tufa with black scoriae typical of the area, produced by the Vicano volcano about 0.15 million years ago, forms the hilly ridge extending between Vezza torrent to the north and the Monte Casoli trench to the south, along the crest of Monte Casoli.
To the north of the Vezza torrent, and to lesser degree to the south, we find a very extensive outcrop of travertine, of hydro-thermal origin, dating from the last phases of volcanic activity in the area and marked by the presence of numerous faults along the bed of the Vezza torrent, which have largely shaped the area's watershed layout.
The travertine is formed by deposits of limestone concretions from thermal waters rich in carbonates; it is recognisable by its typical grey-beige colour and often by its spongy aspect. The travertine plate appears near the surface in the north western sector of the park, on the plateau known as 'Piano della Colonna', which at its highest point is 230 m asl. The whole plate has a slight north-easterly slope, in the direction of the Tiber valley.
The strata of volcanic material have been worn away by the erosive action of the waters - above all in post-glacial periods - to such a depth that this erosion has brought to the surface the underlying Pliocene clays. And this has produced a formation typical to the territory of Tuscia, the 'forre' (trenches), that is deep valleys winding between high vertical rock cliffs with numerous streams running along the valley bottoms, of which we should mention the Fossi di Santa Maria di Monte Casoli, Serraglio, and the Vezza, a tributary of the Tiber.