Weathering

“ Such rounded cores, known as bowlders of weathering, are often left to strew the surface.
Differential weathering. This term covers all cases in which a rock mass weathers differently in different portions. Any weaker spots or layers are etched out on the surface, leaving the more resistant in relief. Thus massive limestones become pitted where the weather drills out the weaker portions. In these pits, when once they are formed, moisture gathers, a little soil collects, vegetation takes root, and thus they are further enlarged until the limestone may be deeply honeycombed. ”

“ The amount of decay depends partly on conditions of climate, especially the range of temperature, the abundance of moisture, height above the sea and exposure to prevalent winds. Many rocks liable to be saturated with rain and rapidly dried under a warm sun are apt to disintegrate at the surface with comparative rapidity. The nature and progress of the weathering are mainly governed by the composition and texture of the rocks exposed to it. ”

“ In order to make still clearer some of the above principles of weathering and also to give the reader some understanding of the most common types of residual soils, we shall consider what happens to a few rather definite types of ordinary rocks when they are subjected to weathering. ”

“ Solution and frost, the chief agents of rock decay, are both retarded by the excessive accumulation of disintegrated rock. Frost action ceases altogether at a few feet below the surface, and solution gradually decreases as the zone of its activity descends and the circulation on which it depends becomes more sluggish. Hence the rapid removal of the products of weathering stimulates its action, and especially that portion of its action which depends upon frost. ”

“ As a rule, these processes of weathering of rocks to produce soil, of erosion of this soil, and of deposition of this transported soil through rivers into some nearby sea or ocean, takes place so slowly, as time is generally measured, that we can only see through detailed and scientific observation the results within our own lifetime. ”

“ The prompt removal of weathered materials, by exposing fresh surfaces of rock, accelerates weathering. The total amount of weathering may therefore not be diminished by the increase of slope, even though certain of its processes are hindered.
The effect of high declivity on transportation, the second element of erosion, is too patent to need explanation. ”

“ Differential Weathering. If, after covering a given region, the ice retreated, the drift which it left in the area which it previously covered would be subject to oxidation, leaching and disintegration. The depth to which this oxidation, leaching « 72 » and disintegration would extend, would be dependent upon the length of time during which the drift was exposed, and upon the climate which affected the region during its exposure. The longer the exposure and the warmer the climate, the deeper would the weathering extend. ”

“ High temperature favors chemical action, and the weathering of rock by decomposition is at its best in the presence of abundant moisture in regions where the temperature is uniformly high. Furthermore, a warm moist climate favors the growth of vegetation, the decay of which supplies the water with organic acids which greatly increase its solvent power. The climatic conditions favoring mechanical weathering are therefore different from those favoring chemical weathering. ”

“ During the short span of the ordinary human life weathering effects are generally of very little consequence, but during the long ages of geologic time the various processes of weathering have been slowly and ceaselessly at work upon the outer crust of the earth, and such tremendous quantities of rock material have been broken up that the lands of the earth have everywhere been profoundly affected. ”

“ Weathering is favored by abundance of moisture. Frost accomplishes most when the rocks are saturated; and solution when there is the freest subterranean circulation. But when the annual rainfall is concentrated into a limited season, a larger share of the water fails to penetrate, and the gain from temporary flooding does not compensate for the checking of all solution by a long dry season. ”

“ The tendency of weathering is to produce a mantle of residuary earth over55 solid rock. Weathering by mechanical means tends to produce material which, though in a finer state of division, is still like the original rock in chemical composition. Weathering by chemical means tends to produce a mantle made up chiefly of the less soluble parts of the rock from which it was derived. All processes of weathering prepare material for transportation by wind and water. ”

“ The origin of soils. Rocks form soils in consequence of weathering, under the influence of physical and biotic factors. Weathering consists of two processes, disintegration, by which the rock is broken into component particles of various sizes, and decomposition, in which the rock or its fragments are resolved into minute particles in consequence of the chemical disaggregation of its minerals, or of some other chemical change. ”

“ Every storm that falls upon them wastes them, cutting gaps, disintegrating boulders, and carrying away their decaying material into new formations, until at length they are no longer recognizable by any save students, who trace their transitional forms down from the fresh moraines still in process of formation, through those that are more and more ancient, and more and more obscured by vegetation and all kinds of post-glacial weathering. ”

“ Against these, there are always working the forces of degradation—the slow rotting of weathering caused by the direct chemical action of the moist atmosphere or the alternation of hot and cold which crumbles rocks far above the line where rain never falls. ”

“ In some localities the soil may accumulate to a considerable depth, allowing the processes of weathering to go to an extreme; in others the processes may be interrupted by erosion, which sweeps off the weathered products at intermediate stages of decomposition and may leave a very thin and little decomposed soil. ”

“ The result has been that wherever, during the gradual denudation to which the country since its elevation has been subjected, these soft deposits have become exposed on the surface, weathering has proceeded at a greatly increased rate, and eventually produced deep and broad depressions separated by high limestone tablelands. ”

“ Rain, when it falls to the ground, carries with it some carbonic acid gas which it has absorbed from the atmosphere. Armed with this solvent, it attacks certain rocks, more especially limestones and chalk, a certain proportion of which it licks up and delivers over to brooks and streams. Under its influence, also, the finer particles of the soil are ever slowly making their way from higher to lower levels. Rocks which are being gradually disintegrated by weathering have their finer grains and particles, thus loosened, carried away by rain. ”

“ It is of so absorbent a nature that it is only in the extraordinary event of the ground being hard frozen at the time of a heavy fall of rain, or of a rapid thaw of snow; or of some inches of rain falling in the course of a few hours, that the soil is unable to absorb the water as fast as it is delivered upon it. The moisture when once in the soil is either carried off again by evaporation and vegetation, or descends to a point at which the chalk is saturated with water, which is, however, constantly being drained off by springs along the valleys. ”

“ The water in freezing expands, and in so doing pushes asunder the component particles of soil or stone, or widens the space between the walls of joints or crevices. When the ice melts the loosened grains remain apart ready to be washed away by rain or blown off by wind, while by the widening of joints large blocks of rock are detached from the faces of cliffs. Where rivers or lakes are frozen over the ice exerts a marked pressure on their banks; and when it breaks up large sheets of it are driven ashore, pushing up quantities of gravel and stones above the level of the water. ”

“ The uprooting of trees on steep slopes often causes the descent of considerable quantities of loose rock and soil. Again, organisms of various sorts (certain types of seeds, germs, etc.) , as well as dust and sand, are extensively transported by the wind. While this is important biologically its geological effects are remote.
Fig. 23.—Figure showing details of wind-carving on rock surface (rhyolite) . Mono Valley, California. Fig. 24.—Wind-worn stones (Dreikanter) . Fig. 25.—Shows the disturbance of surface earth and rocks by upturning of trees. ”

“ This action goes on everywhere; every drop of rain helps it. It is aided by frost, or even by the changes of expansion and contraction which occur in the rocks from variations of heat. The result is that, except where the slopes are steep, the surface is quickly covered with a layer of fragments, all of which are in the process of decay, and ready to afford some food to plants. Even where the rock appears bare, it is generally covered with lichens, which, adhering to it, obtain a share of nutriment from the decayed material which they help to hold on the slope. ”

“ The “beating” of rains quickly compacts the surface to a certain extent, varying with the nature of the soil, its condition of more or less perfect tilth, and the degree of violence with which the rain strikes the surface. ”

“ Another cause, which greatly tends to promote the purity of the air, is the lightness, friability, and dryness of the soil, which, distributed for the most part in thin layers over ledges of rocks, permits no stagnation of moisture, but enables the rain that falls to trickle through, collect in rills and brooks, and find its way rapidly to the sea. The plains and irregular valleys, which form an exception to this rule, are not numerous enough, or of sufficient magnitude to affect the general proposition. ”

“ Frost plays havoc with walls built of flints and with old bricks that are beginning to wear. If there are several frosts, with falls of rain or snow and thaws coining in between, the soil is moved about a good deal by the freezing and melting water. ”

“ When the wind blows, which it is apt to do, dust fills the air. Much of the soil seems to be as fine as flour and very light when it is very dry. We rode for hours without seeing a drop of water, in creek or lake. ”

“ The few streams which creep in winter over this miserable waste shrink away and disappear before the scorching rays of the summer sun, which penetrate the soil to a great depth, and pump up every particle of moisture as far as they reach. ”

“ The proportion borne by the available to the total rainfall varies very much, being affected by the rapidity of the rainfall and the compactness or porosity of the soil, the steepness or flatness of the ground, the nature and quantity of the vegetation upon it, the temperature and moisture of the air, which will affect the rate of evaporation, the existence of artificial drains, and other circumstances. ”

“ Short and heavy rains in a warm country will produce grass, which, having its roots near to the surface, springs up in a few days, and withers when the moisture is exhausted; but transitory rains, however heavy, will not nourish trees; because, after the surface is saturated, the remainder of the water runs off, and the moisture lodged in the soil neither sinks deep enough, nor is in sufficient quantity, to furnish the giants of the forests with the necessary sustenance. ”

“ Rain will percolate through chalk or any open soil until it meets an impervious stratum, or to that place which is in a state of saturation, when the water must either flow away or the level of saturation must gradually rise. ”

“ When the ground is cleared and brought under cultivation, the leaves are ploughed into the soil and decomposed, and the snow, especially upon knolls and eminences, is blown off, or perhaps half thawed, several times during the winter. The water from the melting snow runs into the depressions, and when, after a day or two of warm sunshine or tepid rain, the cold returns, it is consolidated to ice, and the bared ridges and swells of earth are deeply frozen. ”

“ Much of what little rain does fall is absorbed by the trees and scrub, and much is taken by the sun's heat, so that a very small proportion can sink below the surface soil, and only when there is some underground basin in the rock beneath will water be found by sinking, except immediately after rain. ”

“ In a country well supplied with rains, so that there is an abundance of vegetation, the water slowly penetrates the loose soil, and gradually disintegrates the underlying solid rock, quite as fast as, or even faster than, it is carried away by the wash of the rains, and the indurated rock has no greater endurance than the more friable shales and sandstones ”

“ Raindrops, born a long way up in the hills, united to run through fissures in the earth, through soil-drains, through runnels in the moss of woods, through channels in the clay of sodden fields, each drop bearing infinitesimal grains of what, towards the sea at the end, becomes alluvial soil—each drop quarried, each drop carried, each drop endured for its moment, and then went hence and was no more seen. ”