After a big flood, if you know the stream well, you will find many changes. Here a bank of gravel has been carried away; there a new one has gathered. At every bend the bank shows undermining, and pieces have been swept away. The fine stuff makes mud: part of this is laid down on the meadow, but much of it is carried right out to sea. That running water will wash away sand, gravel, and mud is not new to you. You have often seen it on the roads and in the roadside ditches, in the little runnels around the farmhouses, or in the ploughed fields. The burn is always doing the same thing, according to its powers. In dry weather it does little, for its current is weak; in floods it works rapidly. For perhaps two dozen days in a year every burn is in great strength, and is a powerful agent in changing the form of the land. This leads you to grasp the fact that the stream has dug out its own channel, and that it carries rock material to lower levels, and at last to the sea. If you know some of our burns well, and study and watch them closely, you will find a world of interest in them. Every feature of their channels is due to the work the flowing water is doing, and shows the manner in which it is done. But what of the wide valley in which the burn flows? Other agencies have been at work here besides the water: ice has left its mark on every part of our valleys. But the burn has done most. On either side it is joined by branches. Each of these is cutting its own channel, and thus gradually deepening the valley. Each branch has its lesser branches; together they cover the whole valley with an intricate system of water channels. Between these channels, heather and grass are growing in the stony soil. The soil, as you have learned already, is due to the decay of the rocks. Frost and rain begin the work, and the growth of plants hastens and helps it. Over the whole of the sloping valley sides the rocks are being broken up into finer and finer particles. When heavy rain comes it washes away the smaller particles, and little runnels appear which carry away the surface water. Every year a portion of the soil is swept away to the meadows, or to the mud sheets which floor the shallow sea below. None of this ever comes back; it is sheer loss—a little at a time, but if the time be long enough it amounts to a very great quantity. Every day since that burn began to flow it has carried downwards a greater or smaller burden of soil.