<p>165</p>
The retrenchments which Cæsar calls murus fossaque could not be a wall, in the usual acceptation of the word: first, because a wall would have been but a weak obstacle; further, because the materials were not found on the spot; and lastly, because if so great a quantity of stones had been collected on the bank of the Rhone, we should still find traces of them. I have therefore sought another explanation, and thought that murus might be understood of a natural escarpment rendered steeper by a slight work. Penetrated with this idea, I sought Baron Stoffel, the commandant of artillery, to inspect the localities, and the result of his researches has fully confirmed my suppositions. The following is a summary of his report: —
Considered in its ensemble, from Geneva to the Pas-de-l’Ecluse, the Rhone presents the appearance of an immense fosse from 100 to 120 mètres broad, with abrupt and very elevated scarp and counter-scarp. The parts where it does not present this character are few, and of relatively small extent. They are the only ones where operations for passing the river could be attempted – the only ones, consequently, which Cæsar would have need to fortify on the left bank.
1. From Geneva to the confluence of the Arve and the Rhone, an extent of 1½ kilomètres. Breadth of the river, 90 to 100 mètres. – The left bank is flat in the whole of this extent. The right bank has escarpments almost vertical, the height of which varies from 15 to 35 mètres. (See Plate 3, mean profile between Geneva and the Arve.) No attempt at passage could have taken place, neither at Geneva, nor between the town and the Arve.
2. From the Arve to the plateau of Aire-la-Ville, extent 12½ kilomètres. – After leaving the confluence of the Arve, the heights of the right bank of the Rhone increase in elevation; the escarpments become formidable. – The left bank is bordered with similar escarpments, and the river runs thus between high and abrupt banks, everywhere impassable. It preserves this character to a kilomètre above the ravine of Avril, near Peney. The profiles a a and b b give an idea of the escarpments of the banks from the Avre to the ravine of Avril. (See Plate 3.) – The heights which, on the right bank of the Rhone, extend from Vernier to Peney, sink gradually from one of these villages towards the other, and they form to the east of the ravine of Avril a plateau, the mean elevation of which above the bed of the river is only 20 mètres. Opposite, on the left bank, extends the plateau of Aire-la-Ville. Length 1,700 mètres; breadth, 700 mètres; mean elevation above the bed of the Rhone, 20 to 25 mètres. The heights of the Peney are well disposed for the establishment of an army, and the plateau of Aire-la-Ville would permit an army, the Rhone once passed, to deploy easily. But, in spite of these advantages, it is certain that the Helvetii attempted no operation on this side, for the Rhone flows at the foot of a slope of the height of from 14 to 16 mètres and an inclination of at least 45 degrees.
3. From the plateau of Aire-la-Ville to the point of Epeisses, extent 6 kilomètres. – Down the river from the escarpments of Peney, the heights of the right bank (heights of Russin) form with those of the left bank an immense amphitheatre, nearly circular, the arena of which would be the ground represented green on Plate 3 (diameter, 1½ kilomètres). From the heights of Russin we can descend into the plain to the water of the river. The Rhone, in this part, has never been deep or rapid. The left bank is little elevated, entirely flat opposite the mill of Vert, and the slope of the heights which command it is far from impracticable.
Thus, it was here possible for the Helvetii to effect the passage of the river, and climb the heights of the left bank, if they had not been fortified or guarded. This operation presented least difficulty in the part t t o. And we can hardly doubt that the Romans fortified it to add to the natural obstacles, which were insufficient in this extent. (See the profile c c.)
An attentive examination of the locality, the discovery of certain irregularities of ground, which we may be allowed to consider as vestiges, lead us to explain in the following manner the expression murum fossamque perducit.
Cæsar took advantage of the mean heights at the foot of which the Rhone flows, to cause to be made, on the slope towards the river, and beginning with the crest, a longitudinal trench, of such a depth that the main wall had an elevation of 16 feet. The earth arising from the excavation was thrown down the side of the slope, and the crest was furnished with palisades. (See the profile of the retrenchment.) It was, properly speaking, a fosse, the scarp of which was higher than the counter-scarp.
The hills on the left bank, which rise opposite Russin, are accessible, especially in an extent of 900 mètres, reckoning from the point where the ravine which descends to Aire-la-Ville opens upon the river. They form there, among other peculiarities of the ground, a terrace 8 mètres in breadth, rising from 13 to 14 mètres above the plain, and descending to this by a tolerably uniform talus of 45 degrees.
The Romans would be able to prevent the access by means of the trench just described. They, no doubt, continued it to the point o, where the terrace ceases, and the heights become impracticable. It would then have been from 800 to 900 mètres long.
If we continue to descend the Rhone, we meet, on the left bank, first with the perpendicular escarpments of Cartigny, which are 70 or 80 mètres in height, and then abrupt beaches to near Avully. Below Cartigny, the Rhone surrounds a little plain, very slightly inclined towards the river, and presenting a projection of land (v r) from 5 to 6 mètres high, with a talus of less than 45 degrees. The bank being of small elevation, the Helvetii might have landed there. To prevent this, the Romans opened, in the talus which fronted the Rhone, a trench similar to the preceding; it was 250 mètres long.
The heights of Avully and Epeisses leave between them and the river a tolerably vast space, composed of two distinct parts. The first is formed of gentle slopes from Avully to a projection of land, q p; the other part is a plain comprised between this projection of land and the left bank of the river. On the right bank a torrent-like river, the London, debouches into flat ground named La Plaine. The Helvetii might have made their preparations for passing the Rhone there, and directed their efforts towards the western point of La Plaine, in face of the low and flat land comprised between the left bank and the escarpment q p. In this part the left bank is only from 1½ to 2 mètres high. Moreover, the slopes of Avully are not difficult to climb, and therefore the Romans must have sought to bar the passage in this direction. (See the broken profile d e f.) The escarpment q p, from its position and height, is easy to fortify. Its length is 700 mètres; its mean elevation above the plain, 18. It presents to the river a talus of less than 45 degrees. The Romans made in this talus, along the crest, a trench, forming wall and fosse. Its length was 700 mètres.
4. From the point of Epeisses to the escarpments of Etournel, extent 6 kilomètres. – From Epeisses to Chancy the Rhone flows in a straight line, and presents the appearance of a vast fosse, 100 mètres wide, the walls of which have an inclination of more than 45 degrees. (See the profile g g.)
At 200 mètres above Chancy, at k, the character of the banks changes suddenly. The heights on the right sink towards the river in tolerably gentle slopes, through an extent of 2,300 mètres, reckoning from k to the escarpments of Etournel. Opposite, on the left bank, extends the plateau of Chancy. It presents to the Rhone, from k to z, in a length of 1,400 mètres, an irregular crest, distant from 50 to 60 mètres from the river, and commanding it by about 20 mètres. The side towards the Rhone, from k to z, presents slopes which are very practicable. (See the profile h h.)
The position of Chancy was certainly the theatre of the most serious attempts on the part of the Helvetii. Encamped on the heights of the right bank, they could easily descend to the Rhone, and there make their preparations for passing, on an extent of 1,500 mètres. The river once crossed, they had only before them, from k to z, slopes which were practicable to debouch on the plateau of Chancy.
The Romans had then to bar the gap k z by joining the impassable escarpments which terminate in k with those which commence at z, and which are also inaccessible. To effect this, they opened from one of these points to the other, in the upper part of the slope at the foot of which the Rhone flows, a longitudinal trench k z, similar to that already spoken of. It was 1,400 mètres in length.
5. From the escarpments of Etournel to the Pas-de-l’Ecluse, an extent of 6 kilomètres. – At the escarpments of Etournel, the Rhone removes from the heights on the right, and only returns to them towards the hamlet of the Isles, 2 kilomètres farther down. These heights form a vast semi-elliptical amphitheatre, embracing a plain slightly inclined towards the river. It is marked by a green tint on Plate 3. People can descend from all sides and approach the Rhone, the bank of which is flat. Opposite, the left bank presents insurmountable obstacles until below Cologny, at s. But below this point, from s to y, the bank is flat, and the heights situated behind are accessible on an extent of 2 kilomètres.
The Helvetii, established on the heights of Pougny and Colonges, could descend to the Rhone, and cross it between Etournel and the hamlet of Les Isles. The Romans had thus to unite the escarpments which terminate at Cologny with the impracticable slopes of the mountain of Le Vuache. Here again we shall see that they took advantage of the peculiarities of the ground.
At the village of Cologny, the heights form a triangular plateau, s u x, of which the point s advances like a promontory towards the Rhone, which it commands perpendicularly by at least 20 mètres. A projection of land, s u, bounds it in front, and separates it from a plain which extends to the river. The escarpment produced by this projection of land presents to the Rhone a slope of about 45 degrees. It rises over the plain about 14 mètres towards its extremity s, but diminishes gradually in height, until it is only 2 to 3 mètres in height near the point u. (See the profile n n.) The Romans hollowed, on the slope of the escarpment from s to u, a length of 800 mètres, a trench forming wall and fosse. The plateau of Cologny, situated in the rear, offered a favorable position for the defence of this retrenchment. (See the profile p p.) They prolonged their works towards the west as far as y; beyond that, the heights presented sufficient natural obstacles. We may thus estimate that, from Cologny to the mountain of Le Vuache, the Romans executed from 1,600 to 1,700 mètres of retrenchments.
To sum up: the works executed on five principal points, between Geneva and the Jura, represent a total length of about 5,000 mètres, that is, less than the sixth part of the development of the course of the Rhone.
Admitting that Cæsar had at his disposal 10,000 men, we may suppose that he distributed them in the following manner: – 3,000 men on the heights of Avully, his head-quarters; 2,500 at Geneva; 1,000 on the plateau of Aire-la-Ville; 2,000 at Chancy; and 1,500 on the plateau of Cologny. These 10,000 men might be concentrated: in two hours, on the heights between Aire-la-Ville and Cartigny; in three hours, on the heights of Avully; in three hours and a half, on the plateau of Chancy; in three hours and a half, these troops, with the exception of those encamped at Geneva, might be brought together between Cologny and the fort of L’Ecluse. It would require five hours to carry the detachment from Geneva thither.
The detachments mentioned above, with the exception of that of Geneva, were established in what Cæsar calls the castella. These were constructed on the heights, in the proximity of the retrenchments which had to be defended – namely, at Aire-la-Ville, Avully, Chancy, and Cologny. They consisted probably of earthen redoubts, capable of containing a certain number of troops. They are represented by squares in Plate 3.
Cæsar could reconnoitre every instant the march and designs of the Helvetii, the heights of the left bank of the Rhone presenting a great number of positions where it was easy to place advantageously posts of observation. Commandant Stoffel has pointed out six, which are marked on Plate 3. As it will be observed, the Helvetii, in crossing the Rhone, could not be disturbed by darts thrown from the top of the retrenchments, for these darts would not carry to the left bank of the river. Now there exists at present, between this bank and the foot of the heights in which these trenches were cut, flat ground of more or less extent. Admitting, then, that the Rhone flowed nineteen centuries ago in the same bed as at the present day, we may ask if the Romans did not construct, in these low parts near the bank, ordinary retrenchments, composed of a fosse and rampart. The excavations undertaken by the Commandant Stoffel have revealed everywhere, in these plains, the existence of ground formed by alluvium, which would lead us to believe that the Rhone once covered them. However, even if at that epoch these little plains had been already uncovered, either wholly or in part, we can hardly suppose that Cæsar would have raised works there, since the heights situated in the rear permitted him, with less labour, to create a more redoubtable defence – that of the trenches opened along the crests. As we see, the obstacle presented to the assailants began only with these trenches, at the top of the slopes.
As to the vestiges which still appear to exist, they may be described as follows. The slopes which the Romans fortified at Chancy, from k to z, and at Cologny, from s to y, present, in the upper parts, in some places, undulations of ground, the form of which denotes the work of man. On the slope of Chancy, for instance, the ground presents a projection, i i (see the profile h h), very distinctly marked, and having the remarkable peculiarity that it is about 11 feet high and 8 to 9 feet broad. Now, is it not evident that, if one of the fosses which have been described should get filled up, either naturally, by the action of time, or by the processes of agriculture, it would take absolutely the form i i, with the dimensions just indicated? It would not, therefore, be rash to consider these peculiarities of the ground, such as i i, as traces of the Roman trenches.
We must further mention the projection of land v r, situated below Cartigny. Its form is so regular, and so sharply defined, from the crest to the foot of the talus, that it is difficult not to see in it the vestiges of a work made by men’s hands.
It is easy to estimate approximately the time which it would have taken Cæsar’s troops to construct the 5,000 mètres of trenches which extended, at separate intervals, from Geneva to the Jura.
Let us consider, to fix our ideas, a ground A D V, inclined at 45 degrees, in which is to be made the trench A B C D. The great wall A B C had 16 Roman feet in elevation: we will suppose that A B was inclined at 5 on 1, and that the small wall D C was 6 feet high.
The amount of rubbish removed would be as follows: – Section A B C D = 64 square feet, or, reducing it into square mètres, A B C D = 5 square mètres 60 centimètres.
The mètre in length of the earth thrown out would give thus 5·60 cubic mètres.
If we consider the facility of labour in the trench, since the earth has only to be thrown down the slope, we shall see that two men can dig three mètres in length of this trench in two days. Therefore, admitting that the 10,000 men at Cæsar’s disposal had only been employed a quarter of the time, from two to three days would have been sufficient for the execution of the complete work.
