252 INTERNAL STRUCTURE AND 
annual growth and progress, and registers also 
the inequality in the rate of advance between 
the axis and tlio sides. 
But these are only surface phenomena. Let 
us see what will be the effect upon the internal 
structure. We must not forget, in considering the 
changes taking place within glaciers, the shape of 
the valleys which contain them. A glacier lies in 
a deep trough, and the tendency of the mass will 
be to sink towards its deeper part, and to fold in- 
ward and downward, if subjected to a strong lat- 
eral pressure, — that is, to dip toward the centre 
and slope upward along the sides, following the 
scoop of the trough. If, now, we examine the face 
of a transverse cut in the glacier, we find it trav- 
ersed by a number of lines, vertical in some places, 
more or less oblique in others, and frequently 
these lines are joined together at the lower ends, 
forming loops, some of which are close and ver- 
tical, while others are quite open. These lines 
are due to the folding of the strata in consequence 
of the lateral pressure they are subjected to when 
crowded into the lower course of the valleys, and 
the difference in their dip is due to the greater or 
less force of that pressure. The wood-cut on the 
next page represents a transverse cut across the 
Lauter-Aar and the Finster-Aar, the two princi- 
pal tributaries to the great Aar glacier, and in- 
cludes also a number of small lateral glaciers 
