Their head-waters have been at one time interwoven together.

At present the waters of the Valais escape from the Lake of Geneva at
the western end, and through the remarkable defile of Fort de l'Ecluse
and Malpertius, which has a depth of 600 feet, and is at one place not
more than 14 feet across. Moreover, at various points round the Lake of
Geneva, remains of lake terraces show that the water once stood at a
level much higher than the present. One of these is rather more than 250
feet[54] above the lake.

A glance at the map will show that between Lausanne and Yverdun there is
a low tract of land, and the Venoge, which falls into the Lake of Geneva
between Lausanne and Morges, runs within about half a mile of the Nozon,
which falls into the Lake of Neuchatel at Yverdun, the two being
connected by the Canal d'Entreroches, and the height of the watershed
being only 76 metres (250 feet), corresponding with the above mentioned
lake terrace. It is evident, therefore, that when the Lake of Geneva
stood at the level of the 250 feet terrace the waters ran out, not as
now at Geneva and by Lyons to the Mediterranean, but near Lausanne by
Cissonay and Entreroches to Yverdun, and through the Lake of Neuchatel
into the Aar and the Rhine.

But this is not the whole of the curious history. At present the Aar
makes a sharp turn to the west at Waldshut, where it falls into the
Rhine, but there is reason to believe that at a former period, before
the Rhine had excavated its present bed, the Aar continued its course
eastward to the Lake of Constance, by the valley of the Klettgau, as is
indicated by the presence of gravel beds containing pebbles which have
been brought, not by the Rhine from the Grisons, but by the Aar from the
Bernese Oberland, showing that the river which occupied the valley was
not the Rhine but the Aar. It would seem also that at an early period
the Lake of Constance stood at a considerably higher level, and that the
outlet was, perhaps, from Frederichshaven to Ulm, along what are now the
valleys of the Schussen and the Ried, into the Danube.

Thus the head-waters of the Rhone appear to have originally run by
Lausanne and the Lake of Constance into the Danube, and so to the Black
Sea. Then, after the present valley was opened between Waldshut and
Basle, they flowed by Basle and the present Rhine, and after joining the
Thames, over the plain which now forms the German Sea into the Arctic
Ocean between Scotland a