The history of this ravine will tell us a great deal about the carving of water.  Once it was nothing more than a little furrow in the hillside down which the rain found its way in a thin thread-like stream.  But by and by, as the stream carried down some of the earth, and the furrow grew deeper and wider, the sides began to crumble when the sun dried up the rain which had soaked in.  Then in winter, when the sides of the hill were moist with the autumn rains, frost came and turned the water to ice, and so made the cracks still larger, and the swollen steam rushing down, caught the loose pieces of rock and washed them down into its bed.  Here they were rolled over and over, and grated against each 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, and keep the fall upright instead of its being sloped away at the top, and becoming a mere steam.  Secondly, you may often see curious cup-shaped holes, called “pot-holes,” in the rocks on the sides of a waterfall, and these also are concerned in its formation.  In these holes you will generally find two or three small pebbles, and you have here a beautiful example of how water uses stones to grind away the face of the earth.  These holes are made entirely by the falling water eddying round and round in a small hollow of the rock, and grinding the pebbles which it has brought down, against the bottom and sides of this hollow, just as you grind round a pestle in a mortar.  By degrees the hole grows deeper and deeper and though the first pebbles are probably ground down to powder, others fall in, and so in time there is a great hole perforated right through, helping to make the rock break and fall away.