On the western side, the mountain of Gergovia is attached by a narrow defile of 120 mètres in width, called the _Goules_ (_see Plate 21, C_), to the heights of Risolles, an irregular mass, the plateau of which is at a mean depth of about 30 mètres beneath that of Gergovia. To the west are the detached mountains of Montrognon and Le Puy-Giroux. This latter is separated from that of Risolles by a rather deep gorge, in which the village of Opme is built. Opposite the southern slope of Gergovia, at the very foot of the mountain, rises a very steep hill, called the Roche Blanche. Its culminating point is at about 108 mètres below the plateau. Two brooks, the Auzon and the Artières,[486] tributaries of the Allier, flow, one to the south, the other to the north of Gergovia. Lastly, a low tract of ground, situated to the east, indicates the site of the ancient marsh of Sarlièves, which has been dry since the seventeenth century. Cæsar established his camp near the Auzon, on the undulations of the ground which extend to the north-west of the village of Orcet, and as far as the ancient marsh of Sarlièves. These undulations form a natural _glacis_ towards the plain, above which they rise about thirty mètres; on the side of the stream of the Auzon they terminate in almost imperceptible slopes. The camp occupied a part of the table-land and of the northern slope.[487] (_See Plate 21._) Vercingetorix had arranged the contingents of each country separately, at small intervals, on the southern slopes of the mountain of Gergovia and the mountain mass of Risolles which look towards the Auzun; they covered all the heights attached to the principal mountain, and presented, in the space which the eye could embrace, a formidable aspect.[488] His principal camps were situated between the outer wall of the _oppidum_ and a wall of large stones, six feet high, which ran along the bend of the hill. Every day, at sunrise, the chiefs who composed the council of Vercingetorix repaired to him in order to make their reports or to receive his orders. Every day, also, he tried in slight engagements[489] the courage of his cavalry mixed with archers. The Gauls occupied, as an advanced post, but by a weak garrison, La Roche-Blanche, which, scarped on three sides, presented an extremely strong position; Cæsar judged that, by taking possession of this hill, he would deprive the Gauls almost entirely of forage and water, for they could no longer descend to the Auzon, the only considerable stream in the neighbourhood. He started from the camp in the silence of the night, drove away this post before it could be succoured from the town, took the position, and placed two legions upon it. The Roche-Blanche became his smaller camp;[490] it was joined to the larger one by a double ditch of twelve feet, which allowed the troops to communicate in safety, even singly, without fear of being surprised by the enemy. (_See Plate 22._)