Rain waters, that from the spurs of the Gennargentu concentrate on the stream Flumineddu, and going through the Supramonte, have eroded, smoothed and dissolved the calcareous rocks as far as to form the Gorropu gorge.
Smoothed and karstified carbonatic rocks contain a rich content of fossils (shells of various shapes, sea urchins and gastropods) that reveals their undersea genesis that took place between 190 and 60 million years ago.
During the Mesozoic Era, specially during the Jurassic and Cretaceous period, strong thickness of sediments were deposited on the bed of a shallow basin, then turned into limestones (limestone, calcareous dolomite, dolostone or dolomite rock, calcareous marlstone), that we find in the Supramonte.
The water , that flows in large quantities and with a deafening noise during flash floods (in Sardinian you say “cando falat sa còdula”, literally “when flash flood occurs”), flows under the riverbed of the Flumineddu for the rest of the year. Part of the water infiltrates following the huge underground tunnels of the Supramonte. Another part of this water reappears from the resurgences present downstream of the canyon, pushed by the impermeable layer of Paleozoic schists.