126 THE FAIRY-LAND OF SCIENCE. 
last it becomes a great gaping chasm, or crevasse as 
it is called, so that you can look down it right to the 
bottom of the glacier. Into these crevasses large 
blocks of rock fall, and when the chasm is closed 
again as the ice presses on, these masses are frozen 
firmly into the bottom of the glacier, much in the 
same way as a steel cutter is fixed in the bottom of a 
plane. And they do just the same kind of work; for 
as the glacier slides down the valley, they scratch and 
grind the rocks underneath them, rubbing themselves 
away, it is true, but also scraping away the ground 
over which they move. In this way the glacier be- 
comes a cutting instrument, and carves out the valleys 
deeper and deeper as it passes through them. 
You may always know where a glacier has been, 
even if no trace of ice remains ; for you will see rocks 
with scratches along them which have been cut by 
these stones; and even where the rocks have not been 
ground away, you will find them rounded like those in 
the left-hand of the Frontispiece, showing that the 
glacier-plane has been over them. These rounded 
rocks are called " roches moutonnees," because at the 
distance they look like sheep lying down. 
You have only to look at the stream flowing from 
the mouth of a glacier to see what a quantity of soil 
it has ground off from the bottom of the valley ; for 
the water is thick, and coloured a deep yellow by the 
mud it carries. This mud soon reaches the rivers into 
which the streams run; and such rivers as the Rhone 
and the Rhine are thick with matter brought down 
from the Alps. The Rhone leaves this mud in the 
Lake of Geneva, flowing out at the other end quite 
