The Fosse of Twenty Feet.—This fosse has been discovered in its whole extent: it barred the plain of Laumes, following a direction perpendicular to the course of the Ose and the Oserain, and did not go round Mont Auxois. Plate 28 represents two of its most remarkable sections. It was not exactly twenty feet in width, as stated in the “Commentaries;” neither was it everywhere 400 paces distant from the countervallation. This measurement is only exact towards the extremities of the fosse, near the two rivers.
Countervallation.—Vercingetorix, having retired to the plateau of Alesia, could only have escaped by the plain of Laumes, and, at the worst, by the valley of the Rabutin; for the spurs situated to the south, the east, and the north of Mont Auxois are surmounted by a belt of perpendicular rocks, which form insurmountable barriers, and the valleys of the Oserain and the Ose, which divide them, constitute veritable defiles. It became important, therefore, to bar the plain of Laumes with impregnable works. Hence Cæsar accumulated there the means of defence; but he simplified them everywhere else, as the excavations have shown.
These are the works, peculiar to the plain of Laumes, which Cæsar describes in chapters 72 and 73. The traces of the two fosses exist over the whole extent of the plain, from one river to the other. They had not the same form: the one nearest to Mont Auxois is square-bottomed; the other is triangular. (See Plates 27 and 28.) The width of the first is fifteen feet, as stated in the text; that of the triangular fosse is fifteen feet at certain points, but more frequently a little less. The two fosses have the same depth; but it does not reach fifteen feet, as the translators have wrongly understood it. To dig a fosse of fifteen feet deep is so considerable a work, on account of the two stages of workmen which it requires, that it has, perhaps, never been executed for a temporary fortification. Moreover, the result of the excavations leaves no doubt on this subject: the two fosses of the countervallation have both only a depth of from eight to nine feet.
The fosse which is nearest Mont Auxois was filled with water. The Romans had naturally introduced the water into that of the two fosses which, owing to its square bottom, could contain the most considerable volume. A careful level made in the plain of Laumes has proved that this water was derived from the Oserain. During the excavations, the gravel has been found which the water of this river had carried with it, at the time of the investment of Alesia, almost to the middle of the length of the fosse.
To the left of the Oserain, the countervallation cut the first slopes of the hill of Flavigny for a length of 800 mètres; thence it continued, having but one fosse, the various sections of which are indicated on Plate 28. It ran at first along the left bank of the river, at a mean distance of fifty mètres, as far as the mill of Chantrier, then cut the western extremity of Mont Penneville, between the Oserain and the Ose, followed the right bank of the latter river, along the slopes of the mountain of Bussy, and, after having crossed the small plain of Grésigny, joined the camp established at the foot of Mont Réa. Nearly everywhere the Romans had the advantage of a commanding position to defend the countervallation. The excavations have proved that in the plain of Grésigny the fosse of the countervallation had been filled with water from the Rabutin. They have led to the discovery in the ancient bed of this stream (see Plate 25), at the very point where the fosse joined it, of a wall of unhewn stones, which barred the waters so as to conduct them into this fosse.[538]
Circumvallation.—Over the extent of the plain of Laumes, and on the slopes of the mountain of Flavigny, the circumvallation was parallel to the countervallation, at a mean distance of 200 mètres. It had only one single fosse, which in the plain was square-bottomed, so as to allow more soil to be dug out; everywhere else its form was triangular. (See Plate 28.) The circumvallation of the mountain of Flavigny ceased towards the escarpments, where the defences became useless; then, again, it continued on the plateau, where it formed the connection between the camps. After this, it descended towards the Oserain, cut the point of Mont Penneville, re-ascended the slopes of the mountain of Bussy, where it similarly united the camps, descended into the plain of Grésigny, which it crossed in a direction parallel to the countervallation, and ended at camp D. On the heights it was made to follow the undulations of the ground, so that its defenders should occupy as much as possible a commanding position with respect to that of the assailants. Moreover, the works of the circumvallation were not everywhere the same. Thus, near the escarpments and ravines which cut this line, the Romans had made no fosse with epaulment, but only accessory defences, such as abatis and wolf-pits, which even alternated on divers points.
Above the castellum 21, between Grésigny and Mont Réa, the excavations have brought to light a fosse of great dimensions, the bottom of which was full of bones of animals of divers kinds. Its position, near a small ravine in which runs a brook, may lead us to suppose that here was the abattoir of the Roman army. In considering this fosse, and those which have been discovered on the top and on the slopes of Mont Réa, as forming part of the circumvallation, there will be found for the development of this line about twenty kilomètres, which represents with sufficient accuracy the fourteen miles of the text of the “Commentaries.”[539]
Wolf-pits.—In the plain of Laumes, at the top of the circumvallation, and close to the exterior bank of the fosse, there have been counted more than fifty wolf-pits, in five rows. Others have been cleared out on the heights—nine between the camp A and the escarpments, twenty-seven on the mountain of Bussy, near the castellum 15; they are dug in the rock, and in such a perfect state of preservation that they appear as though they had been made but yesterday. At the bottom of some of these last, fifteen arrow-heads were picked up. All these wolf-pits are three feet deep, two feet in diameter at the top, and a little less than one foot at the bottom.
Gaulish Camp.—During the first days of the investment, the besieged encamped on the slopes of Mont Auxois, towards the eastern part of the hill. They were protected by a fosse and a wall of unhewn stones six feet high. We have traced the site of this camp at P Q R S on Plate 25. The excavations have brought to light, in the direction of Q R on the slopes which shelve towards the Oserain, traces of fosses and remains of walls. On the plateau of Mont Auxois it might be interesting to attempt to discover the ancient Gaulish wall. It has been uncovered in pieces here and there over the whole space of the declivities; hence it may be concluded that the town occupied the whole of the plateau.
A remarkable specimen of this wall is visible at a point of Mont Auxois, near the spot where recently the statue of Vercingetorix has been erected.