Petit Lac Research Articles

Etudes sédimentologique et structurale de la partie occidentale du lac Léman par la méthode sismique à réflexion continue

350 km of seismic profiles have been realized on the lake Geneva and have made possible the geological and structural study of the "Petit Lac". This prospection, performed with the help of a "boomer" disposal, 1,000 joules, has been made in order to admit the importance of glacial and postglacial formations which cover the molass and to mark the limits of homogenous wholes of those moraines and lake deposits. Furthermore, this study supplies interesting informations on the structure, as well as of the origin, of the "Petit Lac". The continuation of the Ivoire anticline could be followed beneath the lake and informations on the thickness and distribution of various formations have been obtained.

The Formation of Lake Geneva

On studying a topographical map of Lake Geneva, two characteristic features strike one at once. There is, first of all, the division of the basin into two parts of different orientation—the “Grand Lac” set almost E.W., and the “Petit Lac” deviated by about 60° towards the S.S.W. The inhabitants of the region have always distinguished between the “Grand Lac”, also called “Haut Lac”, and the “Petit Lac”. The former has a depth of 300 metres and consists of a single basin, the floor of which is remarkably flat whilst the second, separated from the other by a submerged ridge 66·3 metres below the surface, begins by a first fosse76·2 metres deep, followed by three others 70·1, 70·4, and 50·1 metres deep respectively. IfLake Geneva is considered as having a single origin, and if the theory of glacial overdeepening is adopted, it is very difficult to explain the ridge separating the two basins. On the other hand, the “Haut Lac”, taken alone, presents nothing abnormal, compared with other basins in whose formation glaciers have taken a large part.