Fig. 22. A glacial soil, containing numerous transported pebbles and boulders, resting on the bed rock.
Not only is soil made up of bits of powdered rock, but it everywhere rests upon rock ([Fig. 25]). Some consider soil to be only the surface layers in which plants grow; but really this is, in most places, essentially the same as the layers below, down even to the very rock, so that we might call it all soil; though, since a special name, regolith (meaning stone blanket), has been proposed for all the soft, soil-like rock-cover, we may speak of it as regolith and reserve the word soil for the surface layers only.
In some places there is no soil on the bare rocks; elsewhere the soil-cover is a foot or two in depth; but there are places where the regolith is several hundred feet deep. In such places, even the wells do not reach the bed rock; nor do the streams cut down to it; but even there, if one should dig deep enough, he would reach the solid rock beneath.
How has the hard rock been changed to loose soil? One of the ways, of which there are several, may be easily studied whenever a rock has been exposed to the air. Let us go to a stone wall or among the pebbles in a field, for instance, and, chipping off the surface, notice how different the inside is from the outside. The outer crust is rusted and possibly quite soft, while the interior is harder and fresher. Many excellent examples of this may be found in any stony field or stone wall.
Fig. 23. The bed of a stream at low water, revealing the rounded pebbles that have been worn and smoothed by being rolled about, thus grinding off tiny bits which later are built into the flood-plains.
As hard iron rusts and crumbles to powder when exposed to the weather; so will the minerals and the rocks decay and fall to bits; but rocks require a very much greater time for this than does iron. It happens that the soil of New York has not been produced by the decay of rock; and, therefore, although the soils in many parts of the world have been formed in this way, we will not delay longer in studying this subject now, nor in considering the exact way in which rocks are enabled to crumble.
Another way in which rocks may be powdered may be seen in most parts of New York. The rains wash soil from the hillsides causing the streams to become muddy. In the streams there are also many pebbles, possibly the larger fragments that have fallen into the stream after having been broken from the ledges. The current carries these all along down the stream, and, as they go, one piece striking against another, or being dragged over the rocks in the stream bed, the pebbles are ground down and smoothed ([Fig. 23]), which means, of course, that more mud is supplied to the stream, as mud is furnished from a grindstone when a knife or scythe is being sharpened on it. On the pebbly beaches of the sea or lakeshore much the same thing may be seen; and here also the constant grinding of the rocks wears off the edges until the pebbles become smooth and round.
