204 ELEMENTARY GEOLOGY 
From these rock walls, disintegrated material fre- 
quently drops upon the back of the glacier, either as 
single pieces or in the form of great avalanches. So 
the sides of the valley glaciers are littered with rock 
debris, which is slowly moving down with the ice. It 
Fic. 108. 
A Swiss glacier, showing snow field, ice stream, medial, lateral, and terminal 
moraines. 
is a moving talus, and is called the lateral moraine 
(Fig. 108). 
Where two glaciers unite, two of these moraines 
may meet in the centre of the resulting glacier, form- 
ing a medial moraine (Figs. 103, 108, and 109). Some 
of the rock fragments of the surface, fall to the bot- 
tom of the ice through the crevasses, and these join 
