THE CENTOAL PLAIN, 
35 
At the close of the Mollasse period the great 
Swiss plain must have been nearly horizontal, but its 
elevation above the Sea was probably not entirely 
Uniform, and some inequalities were thus produced. 
The present hills, however, are mainly due to un- 
equal erosion; the hard Nagelflue especially has been 
able to resist the destructive action of time and 
weather. 
Towards the south boundary the Mollasse is 
thrown into two well-marked arches, separated by a 
Synclinal line, marked blue on the Swiss geological map. 
The anticlinal line runs south-westwards from 
Bavaria to the valley of the Rhine, a little south of 
Bregenz, on the Lake of Constance, then to the Lake 
of Zurich, passing along the Obersee to Uznach, 
then across the Lake of Zug at Oberwyl, so to 
I-ucerne, bends sharply south near Schangnau, crosses 
the Aar near Kirch dorf, by Guggisberg on the Sense 
to Lausanne, and finally in a more accentuated form 
gives rise to the Mont SaRwe (Fig. 78), south of 
*jeneva, where the arch is broken and the Jurassic 
und Cretaceous strata become visible. This great 
3 -nticlinal has a length of not less than 370 km.* 
^t its southern limit the Nagelflue appears, though 
this is not yet absolutely proved, to be in some 
* Favre, Reck. Ge'ol., vol. 1. 
3 
