Gouffre Goule de Roc de Corn

The gallery that follows the entrance pit is at first wide and low, entirely cut into a bed of pink dolomite up to the Martel sump. The upper gallery is a vast paragenetic gallery, active in winter or during storms, up to the zone of shafts downstream of the Martel sump. The lower gallery has a constant direction over nearly 1500 metres. The whole upstream part up to the trout basin is cut into a bed of red dolomite with very large crystals, 4 m thick; the gallery begins at the roof of this bed and passes entirely through it. Downstream of the confluence, the gallery, of larger dimensions, continues in roughly the same direction for about 400 m. Despite their small thickness, about 4 m for each bed, the two dolomite beds contain almost all of the galleries of Roque de Cor.

The stream leaves the upper gallery immediately after the Martel sump, which explains the presence of a substantial clay fill and the speleothem deposition observed in the parts sheltered from high water, downstream of the sump. Over the first 400 metres, the stream has re-excavated the fill, as shown by the small concreted pebbles suspended on the walls; the visible layer today consists of larger deposits.

The galleries of Roque de Cor constitute the underground continuation of the Cazelle stream which, rising near Alvignac, sinks here before resurging as Padirac at Saint Georges and at Le Lombard and finally becoming a left-bank tributary of the Dordogne (dye tracing by G. De Laveur). After flowing over the basic rocks of the Horst d'Alvignac, the watercourse enters the Bajocian downstream of the confluence with the waters of the Salmière spring. The valley is then oriented S.E. - N.W. (165 Gr), superimposed on the line of a relatively important fault. Shortly after the SNCF bridge north of the Camp de Viroulou, the stream crosses the St Vincent fault, materialised here by a clear southward continuation of the strata, then branches abruptly northwards to the pit, following the direction of the dip, now oriented 20 Gr to the N. It is therefore an exception, since it is the only stream along with Autoire to cross the St Vincent fault in its aerial course. Perhaps a similarity with the Alzou can be noted, as these waters are also strongly sulphated (cf MONTPEYROUX: travaux de la faculté de science de Clermont-Ferrand, 1962, n° 18). On the other hand, the St Vincent fault does not here mark the Limargue-Causse contact and therefore no longer constitutes a preferential sinking point here. The waters then cascade into the pit through the lower Bathonian down to a bed of pink dolomite marking the beginning of the transition stage before the oolitic limestones of the Bajocian. The Roque de Cor sink therefore presents itself as a contact sink between the Bajocian and the Bathonian, differing in this from all the other sinks of the network located at the Lias-Dogger contact. The re-excavation is very pronounced shortly before the sump, probably because of local fissuring, this fissuring having allowed part of the waters to reach the lower downstream passage directly (drainage). The main course joins the underlying stage, in the Bajocian, via wide joints, where it converges with the lower passage at the trout basin. The upstream part up to the trout basin is active all year; the Cazelle stream is therefore only a temporary tributary of the lower river.

The morphology of the upper gallery downstream of the Martel sump suggests that it was the continuation of the underground course of the Cazelle stream. There would therefore have been an underground capture of the upper course in favour of the lower passage via the wide N.S. joints of the shaft zone; a study of the fills of the abandoned gallery would make it possible to clarify this phenomenon. Shortly after the former terminal sump, a fault of small throw is observed, but whose direction seems to suggest that it may have played an important role in establishing the flow directions (cf. directional diagram). The primordial role of the two dolomite beds in the excavation of the network should therefore be noted.