(Speleogenetic theory of the Gruppo Grotte Giara Modon di Valstagna, that emerged along with discoveries in the speleological field in 1980-81)
The real karst phenomenon existing in the Asiago plateau was at its peak at the end of and during the last glacial period when meltwater from the snow and ice of the enormous plateau ice sheet was highly abundant. Karst activity was occurring also before the last great glacial period but it was certainly not that developed. The peak was caused by the chemical action of glacial meltwater in this period. It is therefore not an old system, but a very young one and this has been proven by the absence of secondary rivers or fossils at high altitudes. The current active passages are still the same ones. There are no primary passages at higher levels, just secondary, smaller passages. As regards the two higher caves (Covol dee Soree and Covol dei Assassini), they are not ancient rivers of a higher system, but ancient rivers of the current karst system. There has therefore only been a variation in the level of the outflows.
Testing conducted shows that both Covol dee Soree and Covol dei Assassini are connected to Covol dei Siori with internal branches that were once active. This indicates that beyond all shadow of doubt the two upper caves are the ancient outflows of the waters that we now see flowing out lower down, from Covol dei Siori. It has thus been proven, because of the presence of diaclases and very weak layer boundaries that the water abandoned the ancient outflows to exit lower down, where we see it today, excluding the main tributary passage, the one that feeds the internal lake.
In the first phase the water flowed out of Covol dei Assassini more or less at the same level of the Brenta river. In the second phase, the lowering of the Brenta river level allowed the water create a new outflow, which establishes itself once again more or less at the level of the river. The old passage was blocked up by material deposited when the rock face of Covol dei Assassini collapsed, subsequently stabilised by calcareous deposits of dripping water. The communicating passage between the two caves was thus cut off.
Covol dei Veci developed in a much simpler way in comparison. Karst activity started there in the same period as the other caves and there is clear evidence of this because the upper section of each cave is roughly at the same level. The level of the outflow subsequently lowered until it reached the present level, excluding also here the main tributary conduit. What confirms these hypotheses are existing morphological features on the cave walls and rock face of Covol dei Siori. These phenomena are called “mixing corrosion cupolas” and can only occur during the phreatic phase, as in when the cave is completely flooded with water. After the caves had formed, concretion took place within them, which is the series of the rich variety of calcareous forms that are now one of the main attractions for visitors.
In Covol dei Siori, stalactites, stalagmites, flowstones, festoons and straws are present at every small diaclase or leptoclase, where water droplets are able to descend. Worthy of note is the amazing terminal calcareous flowstone that covers the entire wall upstream in the final chamber and is 14 metres high. The age of these concretions has never been calculated precisely, but we can approximately say that they are no more than 25-30,000 years old. This is because prior to that period the cave would have been full of water, and no calcareous deposit could have possibly formed in a submerged environment.