THE FORMER EXTENSION OF GLACIERS. 
157 
metres, (secondly) that then followed a period with 
a climate somewhat milder than the present, suc- 
ceeded (thirdly) by another glacial period, during 
which the valley was again filled by ice to a depth 
of 1900 metres.* 
The first Age is represented by ground moraine, 
and by “Deckenschotter”; a diluvial gravel, curiously 
characterised by the presence of rounded hollows. 
These were formerly occupied by pebbles, which 
have been dissolved and washed away through the 
hard but permeable matrix. 
With the exception of one or two heights, as for 
instance the Napf, there is, on the whole of the 
Central Plain between the Jura and the Rhine, no 
considerable area where traces of former glacial 
action are not to be met with. They attain in places 
a great thickness, sometimes even more than 400 
metres. 
It seems at first therefore remarkable that no 
terminal moraines are known which can be referred 
to this period. But it must be remembered that the 
whole country was covered by ice, with the exception 
of the very highest parts. Hence no doubt, as is 
the case in Greenland now, the surface of the ice 
was very free from debris, and hence, perhaps, the 
* Penck, Vergletscherung der Deutschen Al^pen. 
