The apparatus in Fig. 54. The under surface, of the lips of the beakers should be vaselined to prevent the water trickling down the sides.
It is not uncommon to find cliffs or crags in inland places, but they usually show one very striking difference from seaside cliffs. The seaside cliffs may be nearly vertical, but the inland cliffs are not, excepting for a little way at the top; lower down a heap of stones and soil lies piled against the face of the cliff and makes a slope up which you can climb. If you look at the cliff you can find loose fragments of it split off either by the action of freezing water (p. 83) or by other causes ready to roll down if sufficiently disturbed. So long has this been going on that a pile has by now accumulated, and has been covered with plants growing on the soil of the heap. Our interest centres in this soil; no one has carried it there; it must have been made from the rock fragments. When you get an opportunity of studying such a heap, do so carefully; you can then see how, starting from a solid rock, soil has been formed. This breaking down of the rock is called weathering.
Fig. 52. Cliffs at the seaside, Manorbier, Pembrokeshire
The same change has gone on at the top of the cliff. Fragments have split off and the rock has broken down into soil which stops where it is unless the rain can wash it away. If there are no cliffs where you live you can see the same kind of action in the banks of the lanes, in a disused quarry, gravel pit or clay pit. Wherever a vertical cutting has been made this downward rolling begins and a heap quickly forms, making the vertical cut into a slope. Plants soon begin to grow, and before long it is clear that soil has been made out of the fragments that have rolled down. This process is known as soil formation, but there is another always going on that we must now study. The heap does not invariably lie at the foot of the cliff. If there is a stream, river, or sea at the foot the fragments may be carried away as fast as they roll down: the differences shown in Figs. 52 and 53 between a cliff at the seaside and a cliff inland arise simply in this way. In inland districts great valleys are in course of time carved out, and at the seaside large areas of land have been washed away.
What becomes of the fragments thus carried away by the water? The best way of answering the question would be to explore one of these mountain streams and follow it to the sea, but we can learn a good deal by a few experiments that can be made in the classroom. We want to make a model stream and see what happens to little fragments of soil that fall into it.
