In this area of Sardinia the outcrops of rocks are the oldest of the island and among the oldest in Europe. It is clearly established that the origin for deposition of sediments was in a predominantly marine environment, in a period between the Cambrian (about 570 million years) and the Lower Carboniferous (about 280 million years), but perhaps the beginning of sedimentation dates back even the upper Precambrian (670 million years). The names that identify the local outcrops, recall the toponym of the areas in which they have been studied and described for the first time: the formation of Bithia (named as the Phoenician and Carthaginian coastal settlement near to Chia), the formation of Gonnesa and Nebida, the formation of Cabitza, the formation of Monte Orri, the unit of San Leone and unity of the Arburese.
On the bottom of the ancient sea they were initially deposited (Bithia Formation, Lower Cambrian) sandstone and clay sediments with calcareous layers, then (Formation of Nebida, Lower Cambrian) calcareous sandstone, limestone and dolomite rocks. Subsequently the sea level underwent a lowering and formed a platform on which deposited carbonate sludge (Formation of Gonnesa, Lower Cambrian). But a new deepening phase of the sea produced a new sedimentation of sand and clay (Formation of Cabitza, Middle Cambrian – Late Ordovician). Between the end of the Cambrian and early Ordovician started the Caledonian orogeny, with the related tectonic movements such as the surfacing and folding of the sedimentary rocks of the seabed. It formed a large mountain range, of which these rocks were part.
During the late Ordovician period of the Paleozoic era it took place a gradual reduction of tectonic stress and started dismantling of the high mountain range rise during the Caledonian orogeny. At the base of these mountains new sedimentary rocks began to form . Initially it was a coastal flood plain, consisting of conglomerates and sandstones containing large blocks of limestone and Cambrian dolomite rocks. From this period there are the well-known purple puddingstone, visible on the cliffs of Masua but in this part of the mountain range there aren't any similar outcrops. Subsequently it started again, in the shallow sea, the sedimentation of sandstones, siltstones and clays, also rich in fossil remains.
Just in front of you, in the Mine of San Leone they emerge metasandstones, metaquarzites and metasiltstones, and it is no rare to find fossils that indicate a former marine sedimentary environment, firstly coastal and progressively deeper. There are also: metaquarzites, dark grey metasiltstones and metalimestones strongly silicified associated with mineralization such as skarns with magnetite, andradite, hedenbergite, wollastonite and epidote, which until mid-century ago were intensely mined. In 1861 the Societè Anonyme des Auts Fourneaux, Forge set Accieries, Petit Gaudet et C. bought 360 hectares of land including the iron ores fields and started the exploitation of the vein in 1863 under the direction of engineer L. Gouin.
Looking North-East, in the area of Monte Arcosu, between Capoterra (Saint Lucia) and Villamassargia, above the Ordovician and Devonian sediments, stratified rocks crop out, sandstone and clay belonging to a sedimentary complex attributed to the Lower Carboniferous, called Hercynian Flysch (typical of a submarine sedimentation). They were the first effects of a new orogeny. The compressional tectonic movements, much more energetic than those of previous Caledonian orogeny, led to the formation of the European Hercynian chain and caused additional and huge deformation in earlier sedimentary rocks. These initially emerged from the sea, and then they were bent and repeatedly broken, turning from sedimentary rocks into metamorphic rocks.
The tectonic forces permitted that huge rock masses, very extended, were bent and moved several miles, going to cover the older rocks. The strong erosion triggered by the unstable environment, generated sedimentation of powerful layers of terrigenous deposits at the base of the mountain range during lifting phase. The rocks, so modified, may in many cases be considered as neogenesis, because of high temperatures and strong pressures to which they were subjected. The silica was separated, going to concentrate in the folds of the packages of clay layers, getting schist. The white silica, during the Quaternary, became invaluable to our ancestors, who made spearheads and daggers, but above all it was utilized to ignite the fires (called in Sardinian perd'arba).
This area is known about the widespread presence of characterizing metamorphic stones, but a big part of the reliefs in front of us is made up of rocks of magmatic origin intrusive or plutonic. This is the Batholith of southern Sulcis, whose age has been assessed at around 289 million years (Upper Carboniferous). It is a result Hercynian orogeny. When magmas climbed up coming from the mantle and began to consolidate under the mountain range provoking also the phenomena of neogenesis for thermo-metamorphism by contact. They initially remained concealed, but from the Mesozoic to the present, erosion has gradually removed the Hercynian metamorphic rocks, bringing them to light, forming, in two areas separated by a ridge of metamorphic rocks, the backbone of the current findings.
Granites are frequently associated with other lithologies related to magmatic Hercynian cycle, generated by fluids in different composition from granite, which were consolidated along the fractures in the rock. It is quartz-porphyry (differentiated acids with leucogranites composition) which are found precisely in hypabyssal rocks; or hydrothermal white quartz, which constitutes the majority of events hypabyssal rock present within the basin and of which are found lodes long several hundreds of meters. Because of the increased meteorological resistance than other rocks close to (differential erosion),some of these lodes, today, emerge on the crest of the mountains and the hills in front of us (as Poggio dei Pini).