242 
SCENERY OF SWITZERLAND. 
8. As regards the greater Swiss lakes there has 
been much difference of opinion. 
Ramsay and Tyndall maintained that they were 
rock basins excavated by glaciers. 
Mortillet and Gastaldi* have suggested that the 
valleys were in pre-glacial times filled with alluvium, 
and that this soft material has been ploughed away 
by the glaciers. 
“That glaciers rub down rocks,” says Sir A. Geikie, 
“is demonstrated by the roches moutonnees which they 
leave behind them.” 
“Taking the case of a glacier,” says Tyndall, 
“300 metres deep (and some of the older ones 
were probably three times this depth), and allowing 
12.20 metres of ice to an atmosphere, we find that 
on every square yard of its bed such a glacier presses 
with a weight of 486,000 lbs. With a vertical pressure 
of this amount the glacier is urged down its valley 
by the pressure from behind.” ** 
Indeed, it is obvious that a glacier many hundred, 
or in some cases several thousand, feet in thickness, 
must exercise great pressure on the bed over which 
it travels. We see this from the striae and grooves 
* “Sur l’affouillement glaciaire,” Atti della Soc.Ital. 1863. 
** Tyndall, “Conformation of tlie Alps,” Phil. Mag. Oct. 
1869. 
