It is not in the bottoms of the rivers, then, that the material worn away from the surface of the land can find any lasting rest. And yet the rivers do get rid of a good deal of this material as they roll along. You have, perhaps, noticed that a river is often bordered with a strip of flat plain, the surface of which is only a few feet above the level of the water. Most of our rivers have such margins, and, indeed, seem each to wind to and fro through a long, level, meadow-like plain. Now this plain is really made up from the finer particles of decomposed rocks which the river has carried along. During floods, the river, swollen and muddy, rises above its banks, and spreads over the low ground on either side. Whenever this takes place, the overflowing water moves more slowly over the flats; and, as its current is thus checked, it can not hold so much mud and sand, but allows some of these materials to settle down to the bottom. In this way the overflowed tracts get a coating of soil laid over them by the river, and when the waters retire this coating adds a little to the height of the plain. The same thing takes place year after year, until by degrees the plain gets so far raised that the river, which all this while is also busy deepening its channel, can not overflow it even at the highest floods. In course of time the river, as it winds from side to side, cuts away slices of the plain and forms a newer one at a lower level. And thus a series of terraces is gradually made, rising step by step above the river. Still the laying down of its sand and mud by a river to form one or more such river-terraces is, after all, only a temporary disposal of these materials. They are still liable to be carried away, and in truth they are carried off continually as the river eats away its banks. When the current of a river is checked as it enters the sea or a lake, the feebler flow of the water allows the sand and mud to sink to the bottom. By degrees some portions of the bottom come in this way to be filled up to the surface of the river, and wide flat marshy spaces are formed on either side of the main stream. During floods these spaces are overflowed with muddy water, in the same way as in the case of the valley plains just described, and a coating of mud or sand is laid down on them until they slowly rise above the ordinary level of the river, which winds about among them in endless branching streams. Vegetation springs up on these flat swampy lands; animals, too, find food and shelter there; and thus a new territory is made by the work of the river. These flat river-formed tracts are called deltas, because the one which was best known to the ancients, that of the Nile, had the shape of the Greek letter Δ (_delta_). This is the general form which is taken by accumulations at the mouths of rivers; the flat delta gets narrow toward the inland, and broader toward the sea. Some of them are of enormous size; the delta of the Mississippi, for example.