Fig. 24. Near view of a cut in glacial soil, gullied by the rains, and with numerous transported pebbles embedded in the rock flour.

Supplied with bits of rock from the soil, or from the grinding up of pebbles and rocks along its course, the stream carries its load onward, perhaps to a lake, which it commences to fill, forming a broad delta of level and fertile land, near where the stream enters the lake. Or, possibly, the stream enters the sea and builds a delta there, as the Mississippi river has done.

Fig. 25. A scratched limestone pebble taken from a glacial soil.

But much of the mud does not reach the sea. The greatest supply comes when the streams are so flooded by heavy rains or melting snows that the river channel is no longer able to hold the water, which then rises above the banks, overflowing the surrounding country. Then, since its current is checked where it is so shallow, the water drops some of its load of rock bits on the flood-plain, much as the muddy water in a gutter drops sand or mud on the sidewalk when, in time of heavy rains, it overflows the walk.

Many of the most fertile lands of the world are flood-plains of this kind, where sediment, gathered by the streams farther up their courses, is dropped upon the flood-plains, enriching them by new layers of fertile soil. One does not need to go to the Nile, the Yellow, or the Mississippi for illustrations of this; they abound on every hand, and many thousands of illustrations, great and small, may be found in the State of New York. Doubtless you can find one.

Fig. 26. The grooved bed rock scratched by the movement of the ice sheet over it.

There are other ways in which soils may be formed; but only one more will be considered, and that is the way in which most of the soils of New York have been made. To study this let us go to a cut in the earth, such as a well or a stream bank ([Figs. 22] and [24]). Scattered through the soil numerous pebbles and boulders will doubtless be found; and if these are compared with the bed rock of the country, which underlies the soil ([Fig. 22]), some of them will be found to be quite different from it. For instance, where the bed rock is shale or limestone, some of the pebbles will no doubt be granite, sandstone, etc. If you could explore far enough, you would find just such rocks to the north of you, perhaps one or two hundred miles away in Canada; or, if your home is south of the Adirondacks, you might trace the pebbles to those mountains.

On some of these pebbles, especially the softer ones, such as limestone, you will find scratches, as if they had been ground forcibly together ([Fig. 25]). Looking now at the bed rock in some place from which the soil has been recently removed, you will find it also scratched and grooved ([Fig. 26]); and if you take the direction of these scratches with the compass, you will find that they extend in a general north and south direction, pointing, in fact, in the same direction from which the pebbles have come.