The Weathering Of Rocks

Many people who till the soil, either as a business or as a recreation, look upon it merely as dirt — cold, inert, lifeless, changeless. I have met farmers in New England who took it for granted that the land they till to-day is about the same as it was two hundred years ago, when their forefathers cleared it, except for being less fertile. They had not noticed, or at least had not interpreted, the soil-building and soil-changing agencies at work all about them — wearing away the uplands, enriching the meadows, reducing the rocks, filling the swamps ; changing from year to year the contour of their farms and their agricultural value. Every farm soil is a complex material and has an interesting history. Most soils are a mixture of ground rock, decayed plants and the remains of insects and animals. Some soils, as the sands, are almost entirely particles of rocks; others, as peat and muck land, are made almost entirely of decayed plants. Neither of these extremes makes a a good farm soil, as a rule. The majority of the soils in which plants are cultivated are made mostly of ground rock, with the addition of a greater or less amount of decayed plants. Rock has been, and is still being, ground by weathering — the action of air, rain, snow, frost, heat and ice. Ever since the surface of the earth cooled, making a crust of rock, these agencies have been constantly at work, breaking up this crust, wearing away fragments of rock and carrying them to lower levels. They are Nature's plows. All mountains and hills are slowly wearing away.

We can no longer regard them as "firm and everlasting." "Whole mountain chains of geologic yesterday have disappeared from view," says Merrill, "and we read their history only in their ruins." The Appalachian Mountains have already lost by weathering and erosion as much material as now remains. Even within the memory of one man, a hill may become noticeably lower. The whole earth is being levelled — very slowly, yet quite perceptibly. The face of every rock is roughened and chipped by the elements. Drops of rain wear away particles of it; water freezes in the crevices, expands, and chips off fragments. The air searches these crevices and corrodes them, as it does iron. Everywhere cliffs are lower, rocks are smaller and soils are finer than they used to be. The big rock that the farmer has plowed around for thirty years is smaller now than it was when he first "rode horse" for his father. The stones on the gravelly knoll pass between the cultivator teeth easier than they used to. All about us, in the wild and on the farm, are indisputable evidences that soil is being made by the weathering of rocks. Most farm soils are still incomplete — they contain rocks and stones that are slowly being made into soil. A few, as the clays and alluvial soils, are changing less; but even the finest clay soil is affected by weathering to some extent.

The reducing and fining process is universal and ceaseless. An interesting example of soil formation by weathering is the heaving of stones to the surface, especially in the clay soils of northern states. A vivid recollection of my boyhood is the thankless task of picking up stones from rocky New England fields. This had to be done every fall and every spring. Though we might pick up and cart off in the fall every stone to be seen, there would always be many on top of the ground by the time for spring plowing. These stones were heaved up. The clay soil in which they were embedded became wet, froze and expanded, throwing the stones upon the surface, there being the least resistance in that direction. So many of our small fields of a few acres had immense piles of stones in each of the four corners, the accumulation of many years. When these stones are not picked up they lie upon the surface and are slowly reduced to soil. Soil Becoming Rock. — The reverse process, of changing soil into rocks, is also taking place. Many of the common rocks and stones that we may pick up in our fields were once soil. Sandstone, which is now sought for trimming buildings, is sand that has been hardened into stone. "Pudding" stones, or conglomerates, are made of gravel. Sometimes these rocks may be broken up, by weathering or erosion, and the soil in them again become available for plant growth. Thus the materials of the earth's surface may be worked over and over during countless cycles of time. The soil that nourishes plants to-day may be the building stones of a future generation. The soil of every farm has an antiquity of no ordinary character.

Extreme Changes in Temperature Crack Rocks. — Weathering from changes in temperature is as effective, though often not as noticeable, as weathering from other causes. The changes of temperature from summer to winter, and even from the heat of mid-day to evening, are sufficient to tear rocks to pieces. Rocks are made of several or many different minerals, each of which expands and contracts differently when subjected to heat or cold. The result is that the rocks are cracked and split from being pulled many ways. There are few parts of the world where surface temperatures are uniform for any length of time; hence nearly all surface rocks, even the smallest stones, and especially those in the North, are being slowly pushed and pulled to pieces by alternate expansion and contraction. According to Shaler, a change of temperature of 150° F., which is common in the North between the extremes of summer and winter, makes a granite rock 100 feet in diameter expand one inch. In regions having great extremes of temperature daily, particularly in Texas, Montana, Arizona, and other parts of the West where rocks are sparsely protected by vegetation, the splitting of rocks is quite noticeable and is sometimes attended with gun-like reports and cracking sounds loud enough to be heard many rods.

Livingstone states that in South Africa blocks of stone weighing 200 pounds are frequently split off during the night by the contraction due to the rapid fall of temperature. Many people have noticed how pieces are chipped off from the foundation stones of a building that has burned. In most parts of eastern United States, where the rocks are more or less protected by vegetation, the cracking of rocks from this cause is less noticeable; but it is certain that all rocks everywhere are being affected more or less. The simile — "immovable as a rock," is not perfect. Even the rock, our common symbol of stability, is subject to the universal law of change; it is broken down, re-created and broken down again, over and over, while it fills its place in the working out of the Great Design.