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.