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]