THE CENTRAL PLAIN. 
43 
one another by this Quaternary gravel, which indeed 
has so much raised the bed of the whole valley of 
the Aa, that the bottom of the Baldegger See is at 
the same level as Lenzburg many miles down the 
valley.* 
Round the Lake of Geneva, especially on the 
south side, but also elsewhere, as for instance round 
Aubonne, the glacial deposits attain a great thick- 
ness. The ravines, though deep, are excavated al- 
together in them, and rarely penetrate to the Mollasse. 
They are moreover very impenetrable, as shown by 
the great detour which they compel the Versoix to 
make on its way to the lake. 
To these glacial deposits the lowlands of Switzer- 
land owe much of their great fertility. 
Kaufmann** and Gremaud*** attribute much im- 
portance to these deposits in determining the present 
courses of the rivers. They point out that while some 
comparatively large streams, such as the Sihl, run 
in narrow valleys, others, such as the Suhr, occupy 
much wider ones, out of all proportion to the water 
* Beitr, z. GeoL A' d. Schw., L. XT. 
** JMd. 
*■** “Etudes sur les vall6es de Fribourg,” Bull. Soc. Sc. 
Nat., 1888. 
