external appearance of glaciers. 803 
always in the lower part of the glacier a broad 
depression between the ice and the rocky walls, 
while, as this ettect is not felt in the centre of 
the glacier, it there retains a higher level. The 
natural result of this is a convex surface, arching 
upward toward the middle, sinking toward the 
sides. It is in these broad, marginal depressions 
that the lateral moraines accumulate ; masses of 
rock, stones, pebbles, dust, all the fragments, in 
short, which become loosened from the rocky 
walls above, fall into them, and it is a part of 
the materials so accumulated which gradually 
work their way downward between the ice and 
the walls, till the whole side of the glacier be- 
comes studded with them. It is evident, that, 
when the glacier runs in a northerly or southerly 
direction, both the walls will be affected by the 
sun, one in the morning, the other in the after- 
noon, and in such a case the sides will be uni- 
form, or nearly so. But when the trend of the 
valley is from east to west, or from west to east, 
the northern side only will feel the full force of 
the sun ; and in such a case, only one side of 
the glacier will be convex in outline, while the 
other will remain nearly on a level with the mid- 
dle. The large masses of loose materials which 
accumulate between the glacier and its rocky 
walls and upon its margins form the lateral mo- 
raines. These move most slowly, as the marginal 
