134 
SCENERY OF SWITZERLAND. 
The surface of a true glacial deposit is irregular, 
and presents a succession of hills and valleys, often 
more or less concentric in outline, and enclosing a 
central depression {the site of the glacier itself), so 
that it forms a sort of amphitheatre. See for in- 
stance Fig. 33. The Wettingen Feld in the valley 
of the Limmat is the cone of fluvio-glacial deposits 
from the ancient moraine of Killwangen. 
As glaciers often retreat and then advance again 
TT/WVW/j !■ 
Fig. 33. — Glacial Deposits. I), site of ancient glacier ; Z, moraine ; 
z, fluvio-glacial deposits. 
the cone of transition in many cases presents alterna- 
tions of true morainic and fluvio-glacial strata. 
When the glacier retreated, the water occupied 
the central depression between the ice and the 
moraine, forming a lake. In most cases, however, 
it cut by degrees through the moraine, and drained 
the lake. The streams then wandered over the old 
glacier bed. That the lake naturally overflowed at 
the lowest point of the moraine, explains why the 
outflow is often not in the centre of the valley, and 
occasionally at some distance from the end of the 
