After Nature finally gets an original waste of barren rock all nicely set with grass and flowers and trees and things, the raindrops help to make soil in still another way. Soaking through the decaying leaves, they pick up acids which are just the thing for eating into rock and crumbling it into soil. To be sure, the water soaking into the soil and coming out of springs carries some plant food away with it; but it takes it to lands farther down the river valleys, and more than makes up for what it carries away by the new soil made by its acids from the rocks, as it soaks into their pores and runs among the cracks.
[HOW RAINDROPS MANAGE TO GRIND UP THE ROCKS]
Moreover, raindrops actually grind up rocks. In order to do this a lot of raindrops have to get together, to be sure, and become rivers; but after all it's the raindrops that do it. There'd never be any rivers if it weren't for the rains and, of course, the snows.
Well, anyhow, the rivers, besides running other people's mills, have mills of their own; and millstones. Most of these stones originally came from mountains and were brought into the milling business by mountain streams, with the help of Jack Frost. For the frost not only pries stones from the mountains and so sends them tumbling down the slopes, but it keeps edging them along and edging them along, farther down, after they have fallen. You'd hardly think that, would you? Yet it's simple enough. The water in the pores of the rock expands when it freezes and that makes the whole rock expand, for the time being. Then when the frozen water in the rock pores thaws out, the rock contracts, and this spreading out and pulling together, small as it is, causes the rock to keep hitching along down the incline; oh, say a fraction of an inch a year. But still, in the course of the ages, these inches foot up, and after a while this tortoise-like gait lands the stone—lands tens of thousands of such stones—in the beds of the mountain torrents that run along at the bottom of these inclines. There they get ground together and so grind out more soil material, particularly when the floods are on, with the melting of the snows in spring and the falling of the heavy and frequent rains.
AN OLD RIVER MILL
It used to do a lot of business—this old river mill. Its grist was ground-up rock that helped make fine farming land in the bottoms along the river's course. Such mills, called "pot holes," are found in the rocky floors of rapid streams, where the eddying current or the water of a waterfall wears depressions in the bed. Into these depressions stones are washed, and then by the whirl of the flowing water kept going round and round, grinding themselves away and grinding out the sides and bottom of the mill.
Another curious thing is how the river mills help themselves to new millstones when they need them. If a river hasn't enough for its work, it has a way of drawing on its banks for more. Whenever the stones in its bed get scarce, so that it can make comparatively little new soil—having so few stones to grind together—it proceeds to dig its own bed deeper, since this bed is no longer protected by a rock pavement in the bottom. This, of course, deepens its channel, and so adds to the steepness of the slope of its banks. Then, owing to this increase in the incline of the slope, more rocks tumble in, and the "milling business" picks up again.