The Gauls could only quit the town by this defile, and by the western slope of the mountain. It became, consequently, interesting to ascertain if the Romans made a countervallation along the Tourmente, on the slopes of the heights of the castle of Termes and of Montbuisson. Unfortunately, the railway from Perigueux to Capdenac, which passes over the very site where the countervallation might have been made, has destroyed all traces of the Roman works: the excavations made above this line have produced no result. The most interesting discovery was that of the subterranean gallery.[554] Until the moment when the excavations were commenced, a part of the rain-water absorbed by the plateau of the Puy-d’Issolu issued near the village of Loulié by two springs, _A_ and _A’_. (_See Plate 32._) The spring _A’_ flows from a ravine, and corresponds to the _thalweg_ of the slope; as to the source _A_, it is easily seen by the appearance of the ground that it has been turned from its natural course. The excavations, in fact, have proved that it is produced by the waters which run in the Roman gallery. This gallery has been opened over an extent of forty mètres. It was dug in a solid mass of tufa nearly ten mètres thick, which had been formed in the centuries anterior to Cæsar. Its form is that of a semicircular vault, supported by two perpendicular sides; its average dimensions are 1·80m. in height, by a width of 1·50 mètres. The mud carried along by the waters, and accumulated since the time of the siege of Uxellodunum, had almost filled the gallery, leaving only at the top of the _intrados_ an empty space in the form of a segment of a circle, with 0·50m. chord by 0·15m. for the absciss of its curve. Through this empty space the water ran at the time of the excavations. Before reaching the tufa, the first subterranean works of the Romans had been made in the pure soil, which had to be propped: fragments of blindage have been found, some of them fixed in silicious mud, corroded or reduced to the state of ligneous paste, others petrified by their long contact with the waters charged with calcareous sediments. A considerable number of these petrified blocks, and remains of wood, collected in the interior of the gallery, are deposited in the museum of Saint-Germain.