214 
SCENERY OF SWITZERLAND. 
upper waters; Secondly, later earth movements in 
many cases changed the course of the rivers; 
Thirdly, they have in many cases been diverted by 
masses of glacial deposits; and Fourthly, the summit 
ridge of the Alps is slowly retreating northwards, 
which affects the river system of all the upper dis- 
tricts. 
In the great Swiss plain the country slopes on 
the whole northwards from the Alps, so that the 
lowest part is that along the foot of the Jura. 
Hence (Fig. 42) the main drainage runs along the 
line from Yverdun to Neuchatel, down the Zihl to 
Soleure, and then along the Aar to Waldshut. The 
Upper Aar, the Emmen, the Wigger, the Suhr, the 
Wynen, the lower Reuss, the Sihl, and the Limmat, 
besides several smaller streams, running approximately 
parallel to one another — N.N.W., and at a right angle 
with the main axis of elevation, all join the Aar from 
the south, while on the north it does not receive a 
single tributary of any importance. 
On the south side of the Alps again, and for a 
corresponding reason , all the great affluents of the 
Po — the Dora Baltea, the Sesia, the Ticino, the 
Olonna, the Adda, the Adige, etc., come from the 
north, and run S.S.E. from the axis of elevation to 
the Po. 
