Stop n. 3

"Sa trona"

We are standing in front of a formation group of extraordinary elements of beauty, consisting of a drapery with huge hanging formation, surrounded by stalactites. At the bottom of this structure, a stalagmite took shape, a different formation as it shows a flat top and a wider than usual body. This is called “The Throne”, because it resemble a noble royal seat. The flowing of water through its furrows forms the top laminated structure, also called a baldacchino Canopy in Geology.


As we said, the top part of “The throne” is flat, shaped by horizontal layers that build through deposition. The flat top is due to the source of water itself, which is not coming from a single central point as it happens usually in similar formations, but its spread equally across the complete sub-circular surface. The wide distance from the top of dripping water to the surface, the high frequency of water coming and bouncing with circular movements, together with the high volume of water coming from the drapery during raining season, gives the stalagmites this unique shape.


On the left side of the stalagmite we can see a beautiful gours, with its external surface filled by micro-gours covered by micro calcite crystals which, when properly lighted, show a spectacular array of glitters.

This type of formation is due to high evaporation as we are in a place subject to strong winds and currents. The evaporation of water, at times total, gives the stalagmite this particular shiny characteristic.

Let us see now the reason behind the change of direction. As we said before, the evolution of the cave was possible because of a series of inverted faults and over sliding, as well as the Mount Acqua quartzite and calcareous rocks. The abundant presence of quartzite rocks along the main structural alignment is linked to the intense tectonic movements of the area around San Giovanni’s cave.


One of these quartzite bodies is the left wall. Quartzite is one of the most physically durable and chemically resistant rocks in nature, making it not subject to Karst erosion, features that characterised the geologic evolution of the southern side of the cave. The river, on its course, found the hard rocky body, and literally changed is course with a sharp 150 degrees turn to skip the obstacle, to take back its original direction North- South once overcome the obstacle. In the last part of the gallery. Looking on the west, we spot several formations, making us suppose another ramification once existed, now closed by deposits.


In front of us we can see a huge stalagmite formation that is been sectioned partially by erosion and partially man made during the road construction. This body is the remnants of what once was the cave’s pavement covering the whole length of the cave. Following the increased volume of water flowing from the river, much of it is been desmanteled in its orginal central portion.

  A more careful observation of the section shows periods of higher formation followed by period of       deposition   of material or erosion and dissolution. It is possible to see a rimstone (gours) containing ten pisolites, evidence of secondary mineralization of the rocks yet to be understood.