other, and were ground away till they became rounded pebbles, such
as lie in the foreground of the picture (Fig. 25); while the grit
which was rubbed off them was carried farther down by the stream.
And so in time this became a little valley, and as the stream cut
it deeper and deeper, there was room to clamber along the sides of
it, and ferns and mosses began to cover the naked stone, and small
trees rooted themselves along the banks, and this beautiful little
nook sprang up on the hill-side entirely by the sculpturing of
water.

Shall you not feel a fresh interest in all the little valleys,
ravines, and gorges you meet with in the country, if you can
picture them being formed in this way year by year? There are
many curious differences in them which you can study for
yourselves. Some will be smooth, broad valleys and here the
rocks have been soft and easily worn, and water trickling down
the sides of the first valley has cut other channels so as to
make smaller valleys running across it. In other places there
will be narrow ravines, and here the rocks have been hard, so
that they did not wear away gradually, but broke off and fell in
blocks, leaving high cliffs on each side. In some places you
will come to a beautiful waterfall, where the water has tumbled
over a steep cliff, and then eaten its way back, just like a saw
cutting through a piece of wood.

There are two things in particular to notice in a waterfall like
this. First, how the water and spray dash against the bottom of
the cliff down which it falls, and grind the small pebbles
against the rock. In this way the bottom of the cliff is
undermined, and so great pieces tumble down from time to time,