From the higher parts of the picnic area of Curlo (Arenzano) you can see the various morphological aspects of the surrounding mountains. The sharp drop of the Tyrrhenian slop is due to tectonic movements occurred in the past (Plio-Quaternary, approximately 2 million years ago). Tension phenomena (horizontal arrows) generated faults (with the red line representing the fault line) along which the southern blocks dropped.
Looking towards the village of Arenzano, in front of you, and Sciarborasca to the west, you can see that they are located in a valley that extends almost east-west in direction, parallel to the coast. This is a broad “tectonic valley”, called a graben, with the higher ridge towards the sea called a horst. The different types of rock in the zone of Arenzano are an example of the high geodiversity of the Beigua Park. Indeed, the rocks are of a completely different genre to those we have previously encountered. They are “crystalline” rocks, formed by minerals that are richer in silica. This is a very ancient rock (dating to 280 million years ago) which made up an ancient continent. The Promontory of Arenzano has a crudely truncated- pyramidal form, crowned by a vast sea terracing with pebbles and sand. The relief is bordered by faults, with the northerly one acting as the edge of the Pliocene graben mentioned above.
The area of Arenzano presents two types of rock: the metamorphic rock that made up the continental crust dating to a time before the Mesozoic era and the slightly metamorphic sedimentary rock of the continental platform. In the Pliocene epoch there was a rigid tectonic tension force which affected the entire Tyrrhenian edge and led to the sinking of the Ligurian Gulf. The collapse occurred with the formation of eastern horst and graben structures roughly parallel to the coast. The sea spread inwards to the inlets and longitudinal valleys of the Tyrrhenian edge where marl, clay, and sand sedimented. The Pliocene outcrops of the zone of Terralba and Arenzano are in contact due to a fault with both the ophiolitic units to the north and the crystalline ones of the Promontory to the south. Other systems of tension faultiness from the Plio-Quaternary era then intersected the previous structures, thus generating phenomena of river capture and deviation, as in the case of the Lerone torrent.