The same change has gone on at the top of the cliff. Fragments have split off and the rock has broken {118} 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. [Illustration: Fig. 53. Inland cliff. Salisbury Crags, Arthur's Seat, Edinburgh] Fix up the apparatus shown in Fig. 54. The small beaker A is to represent the narrow mountain stream, the larger one _B_ stands for the wide river, and the glass jar _C_ for the mouth of the river or the sea. Run water through them; notice that it runs quickly through _A_, slowly through _B_, and still more slowly through _C_: we want it to do this, because the stream flows quickly and the river slowly. {120} Now put some soil into _A_. At once the soil is stirred up, the water becomes muddy, and the muddy liquid flows into _B_. But very soon a change sets in, the liquid in _A_ becomes clear, and only the grit and stones are left in the bottom: all the mud--the clay and the silt--is washed into _B_. There it stops for a long time, and some of it will never wash out. The liquid flowing into _C_ is clearer than that flowing into _B_. If you keep on putting fresh portions of soil into _A_ you can keep _B_ always muddy, although _A_ is usually clear. At the end of the experiment look at the sediment in each beaker: in _A_ it is clear and gritty, in _B_ it is muddy. If you can get hold of some sea water put some of the liquid from _C_ into it: very soon this liquid clears and a deposit falls to the bottom, the sea water thus acting like the lime water on p. 20.