In Troubadour-Land: A Ramble in Provence and Languedoc

by S. Baring-Gould

As I said when speaking of the Crau, the whole delta of the Rhone, which extended in the diluvial epoch from Cette to Fos, consists of a vast sloping plain of rolled stones from the Alps. What is now a great convexity thrust into the Mediterranean, perpetually gaining ground on the sea, was at the commencement of the present geologic epoch a great bay, and the waves of the Mediterranean broke against the cliffs of les Monts Garrigues, at Lodève, the heights of Nimes and Beaucaire, against the limestone crags of the Alpines, and swirled against that calcareous spur that now separates the lagoon of Berre from the desert of la Crau.

But, at an epoch which it is impossible to fix, which, however, is posterior to the last geologic dislocations of the soil, two formidable deluges swept from the Alps down the troughs of the Rhone and the Durance, carrying with them vast masses of stone torn from the flanks of the mountains. They were veritable avalanches of water, mud and rubble, that filled the entire bay and covered the land, wherever they poured, with the wreckage of the Alps. The stones were broken into a thousand pieces in their course, their angles rubbed down, and their surfaces polished by friction, and this vast bed of rubble measures near the mouth of the Rhone some sixty feet in depth, and extends under the blue surface of the sea to the distance of many miles.

But, when the diluvium ceased, and the rivers Rhone and Durance assumed approximately their present character, a change of procedure took place. The volume of water rolled down was by no means so great, the inclination of the fall was vastly lessened, consequently the rivers were enabled to do what they had not been able to do in the diluvial period, chew up their food of stone, and reduce it to the condition of mud. This is what the two rivers are engaged upon now, and instead of strewing their embouchures with pebbles, they distribute over them, or would do so, if permitted, a film of fertilising mud.

Through many ages the Rhone has rambled at its sweet will over the vast tract of rubble that formed its delta in the diluvial age, changing its course capriciously, and always, wherever it went, covering up the pebble bed with a deposit of fertile soil. Other streams helped in the good work—the Hérault, rich with red mud, the Ley, that flows past Montpellier, and the Vidourle from Lunel: consequently a very large portion of the rubble bed is covered with rich soil, that grows vines, mulberries, and olives. The plough and spade, however, speedily reach the boulders that lie but slightly buried beneath the surface. The canal of Craponne, that conveys the charged waters of the Durance over the Crau of Arles, is effecting artificially over that portion of the rubbly desert, the work that was done by Nature herself in past ages over the whole region from Cette to Aiguesmortes.

Now let us examine very shortly the stages through which every mountain-born river runs.

When young, sprung from eternal snows, gushing from under glaciers, it cuts its way through mountain gorges, receiving the rocks that fall from above, and carrying them along in its course, tearing its way round rocky spurs, and breaking them in its fury, and, as it travels down into the lower ground, it carries with it a vast mass of stone. Every tributary does the same. This first stage is called the zone of erosion.

But, as the river leaves the Alps, its course becomes less rapid, and the fall is not so abrupt. The bed widens, and what was a boiling torrent becomes a rapid river. As it rolls along, it carries down with it the stones that it has brought from the mountains, turning them over and over in its course, rubbing down all rough points, and becoming itself discoloured with the particles it has rubbed off the pebbles. All this matter thus produced has a tendency to fall to the bottom and form banks of gravel; but the violence of the stream is constantly altering the shape and position of these beds, carrying the gravel farther, and throwing down in their place half-triturated deposits of the same character.

This is called the zone of compensation.

Any traveller who has visited the Vallais may see the Rhone at work in its first stage. In the second he can trace the river from below Lyons, and see the thousand gravel-banks formed, swept away, and reformed, at every flood, that mark the course of the river in its second stage.

By the time the Rhone has reached Arles all its gravel has been champed up and reduced to impalpable mud. That blue crystalline flood that gushed from the Lake of Leman, unsullied by a particle of earth, is now a river of brown mud—thick as pea-soup, and as nutritious. The stones that would have killed all vegetation have been pounded into a condition so attenuated, that they form rich alluvial matter. The river now seeks to deposit all this mud. On reaching the sea, the difference in gravity between the meeting waters, and their variation in temperature, produces rapid precipitation of all the earthy matter held in suspense by the stream. This last stage in the river's course is called the zone of deposit.