SNOW AND ICE. 
123 
and striated. The importance of the ground moraine 
was first pointed out by Martins* The pressure of 
the glacier on its bed must be very great. On the 
Aletsch glacier it has been calculated to be as much 
as 4 tons to the square decimeter; under the Arctic 
glaciers it must be much greater. In the winter 
of 1844 some poles of timber were dropped under 
the edge of the Aar glacier, in the following year 
they were found to be crushed to small fragments. 
Blocks of stone are gradually ground down and re- 
duced to glacial mud. This is so fine that it re- 
mains a long time in suspension in water, and gives 
their milky colour to glacial streams. The ground 
moraine is no doubt formed in some measure from 
surface blocks which have found their way through 
crevasses, and have to a great extent been crushed 
and reduced to powder; but as ground moraines 
occur under ice-sheets, such as that of Greenland, 
when there are scarcely any surface blocks, it is 
clear that the material is partly derived from the 
underlying bed. 
At the lower end of the glacier a terminal moraine 
gradually accumulates, which may reach a height of 
50, 100, or even 500 metres. They are more or 
less curved, encircling the lower end of the glacier. 
* Revue des Deux Mondes, 1847 . 
