Erosion

“ All the processes of erosion are affected directly by the amount of rainfall, and by its distribution through the year. All are accelerated by its increase and retarded by its diminution. When it is concentrated in one part of the year at the expense of the remainder, transportation and corrasion are accelerated, and weathering is retarded. ”

“ The preceding discussion, and the illustrations which accompany it, give some idea of the topography which characterizes an area in various stages of its erosion history. Whether the valleys are deep or shallow, and the intervening ridges high or low, depends on the original height of the land and its distance from the sea. The higher the land, and the nearer it is to the sea, the greater the relief developed by erosion. A plateau near the sea may become mountainous in the mature stage of its erosion history, while a plain in the same situation would only become hilly. ”

“ But the mechanical erosion effected by running water is what impresses us most with the importance of rivers as geological agencies. This erosive action is due to the gravel, sand, and mud carried along by the water. These ingredients act as files in the hand of a workman, and grind, polish, and reduce the rocks against which they are borne. The beds of some streams that flow over solid rock are often pitted with circular holes, at the bottom of which one invariably [Pg 52] finds a few rounded stones. ”

“ And this newer configuration is the direct result of erosion, guided by the mineralogical composition and structural peculiarities of the rocks.
It is difficult, or even impossible, for one who is ignorant of geological structure to realise that the apparently insignificant agents of erosion have played so important a rôle in the evolution of notable earth-features. It may be well, therefore, to illustrate the matter by reference to one or two regions where the geological structure is too simple to be misunderstood. The first examples I shall give are from tracts of horizontal strata. ”

“ 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. ”

“ Subaërial erosion then wears down the undulations and the crests of the closely folded strata, often planing them off until a long section, crossing mountain ranges, passes from older to newer beds, and onward from newer to older, in several alternations, having throughout the whole a nearly constant steep dip. ”

“ The eroding instruments have been of a simpler, quieter kind, tireless but gentler, continuing their delicate labour for centuries, almost with tenderness. Alone their results are so stupendous. For the erosion, which I shall presently describe, is due to meteorological conditions⁠—rain, dew, temperature, especially changes of temperature, unequal expansion of various rock constituents, the varying hardness of the different strata, and⁠—the ceaseless friction of the wind-borne sand. ”

“ Whenever the force with which the water moves along is stronger than the cohesion of the soil, erosion occurs. This erosion will continue, the channel of the stream becoming ever deeper, until a soil is encountered whose resistance is equal to the erosive action of the water. As the alluvion is carried on, the slope of the stream will become steeper and steeper the higher one goes. ”

“ Falls, rapids, narrows, and other peculiar features, due primarily to inequalities in the hardness of the rock affected by erosion, will be considered later.
Fig. 84.—A surface illustrating the struggle for existence among gullies. Most of the smaller gullies shown on the slope can have but little growth before being absorbed by their larger neighbors. A type of erosion surface common in the Bad Lands. ”

“ Changes in level between land and sea which take place during the erosion of a region may also disturb the normal cycle of erosion. For example, a region in old age may be considerably upraised so that the streams have their velocities notably increased. ”

“ Weathering prepares the material for transportation and transportation leads to corrasion. Since the goal of all material transported by [Pg 37] running water is the sea, subaerial erosion means degradation of the surface.
Erosion without valleys.—In the work of degradation the valley becomes the site of greatest activity, and in the following pages especial attention is given to the development of valleys and to the phases of topography to which their development leads. ”

“ The present conception of general eolic erosion, a sculpturing power in every way comparable to erosion by river and by ocean, appears destined to take its place among the first half-dozen great and new thoughts which shall especially distinguish geologic science of the twentieth century. ”

“ The above normal order of events may be disturbed at any stage, especially after maturity, by renewed uplift when the streams are revived in activity and increased ruggedness results. Even after the whole range as a relief feature has been planed away, the site of the range may be uplifted and a new cycle of erosion started. ”

“ Movements of elevation—including uplifts by folding and fracture of the crust to be noticed later— are the necessary conditions for erosion by whatever agent. They determine the various agencies which are to be chiefly concerned m the wear of any land,—whether streams or glaciers, weathering or the wind,—and the degree of their efficiency. The lands must be uplifted before they can be eroded, and since they must be eroded before their waste can be deposited, movements of elevation are a prerequisite condition for sedimentation also. ”

“ Theoretically every land surface elevated above sea-level should pass through what has been called a cycle of erosion. There should be a period when the active forces are working upon a surface as yet but little modified; this is the period called by analogy youth. At a later stage the drainage has been well established, and the rivers run in broad valleys, from which lakes and waterfalls have largely disappeared. ”

“ Declivity originates in upheaval, or in the displacements of the earth’s crust by which mountains and continents are formed; but it receives its distribution in detail in accordance with the laws of erosion. Wherever by reason of change in any of the conditions the erosive agents come to have locally exceptional power, that power is 124steadily diminished by the reaction of rate of erosion upon declivity. Every slope is a member of a series, receiving the water and the waste of the slope above it, and discharging its own water and waste upon the slope below. ”

“ But the carving of the canons and mountains is insignificant, when compared with the denudation of the whole area, as evidenced in the cliffs of erosion. Beds hundreds of feet in thickness and hundreds of thousands of square miles in extent, beds of granite and beds of schist, beds of marble and beds of sandstone, crumbling shales and adamantine lavas, have slowly yielded to the silent and unseen powers of the air, and crumbled into dust and been washed away by the rains and carried into the sea by the rivers. ”

“ Rate of Erosion and Declivity. In general erosion is most rapid where the slope is steepest; but weathering, transportation, and corrasion are affected in different ways and in different degrees.
With increase of slope goes increase in the velocity of running water, and with that goes increase in its power to transport undissolved detritus.
The ability of a stream to corrade by solution is not notably enhanced by great velocity; but its ability to corrade by mechanical wear keeps pace with its ability to transport, or may even increase more rapidly. ”

“ The effects of river erosion, together with the weathering of rocks are seen in Fig. 31, which is a sketch of a section of Wilmington Pass, in the Adirondacks. The precipitous sides of rock, are shown with evergreens growing wherever they can find a foothold in the soil made by the disintegration of the rocks above. Boulders and trees have been brought down by the loosening of masses of rock, through the action of frost, heat, and melted snow, causing obstructions in the stream, over and between which the waters tumble and roll. ”

“ Man's "improvement" of the soil increased the erosion from its surface; his arrangements for confining the lateral spread of the water in floods compel the rivers to transport to their mouths the earth derived from that erosion even in their upper course; and, consequently, the sediment they deposit at their outlets is not only much larger in quantity, but composed of heavier materials, which sink more readily to the bottom of the sea and are less easily removed by marine currents. ”

“ At a later stage in the stream’s history, when active erosion at the head has ceased because of the reduction of the surface, less material will be carried from the upper part of the valley, and the stream on the flood-plain below, formerly loaded with material from up the valley, is now free to take up and carry away material temporarily left on the flood-plain. ”

“ More than this, when erosion has been continued through very long periods, it tends toward the ultimate result of removing the upward curved or anticlinal portions of the great folds and sparing the originally lower downward curved or synclinal portions, until valleys take the places which were originally occupied by the highest upheavals, while the original troughs, where the rocks were most compacted by pressure, remain now as the principal mountain ridges. ”

“ The cycle of erosion. The successive stages through which a land mass passes while it is being leveled to the sea constitute together a cycle of erosion. Each stage of the cycle from infancy to old age leaves, as we have seen, its characteristic records in the forms sculptured on the land, such as the shapes of valleys and the contours of hills and plains. The geologist is thus able to determine by the land forms of any region the stage in the erosion cycle to which it now belongs, and knowing what are the earlier stages of the cycle, to read something of the geological history of the region. ”

“ In this latter case the erosion, or the wearing away, caused by trickling water, frost and snow, sharpens the edge of the rock, as a grindstone does the edge of an ax, and traveling along one of these ridges presents almost the same difficulties that travel along the edge of an upturned ax would do to a microscopic man. ”

“ Sapping is directly favored by great declivity.
In brief, a steep declivity favors transportation and thereby favors corrasion. The rapid, but partial, transportation of weathered rock accelerates weathering; but the complete removal of its products retards weathering.
Rate of Erosion and Rock Texture. Other things being equal, erosion is most rapid when the eroded rock offers least resistance; but the rocks which are most favorable to one portion of the process of erosion do not necessarily stand in the same relation to the others. ”

“ When any considerable thickness of ice at the bottom of the glacier is full of débris, this loaded basal portion may approach stagnancy, and the lower limit of considerable movement may lie between the loaded ice below and the relatively clean ice above. A moderate, but not an excessive load of débris is, therefore, favorable285 for great erosion. Something depends, too, on the character of the load. Coarse, hard, and angular débris is a more effective instrument of erosion than fine, soft, or rounded material. ”

“ The master streams and their main tributaries now wander with sluggish currents over the broad valley floors which they have planed away; while under the erosion of their innumerable branches and the wear of the weather the divides everywhere are lowered and subdued to more and more gentle slopes. Mountains and high plateaus are thus reduced to rolling hills, and at last to plains, surmounted only by such hills as may still be unreduced to the common level, because of the harder rocks of which they are composed or because of their distance from the main erosion channels. ”

“ This soil resource destruction from erosion leads to the destruction of other valuable resources. We appear to be upon the eve of an epoch of waterway construction and experiment. The greatest injury to waterways is channel filling by down-washed mud. ”

“ Geologically considered, the results which they accomplish, may be summed up under the two heads of deposition and erosion—the formation of new rocks in the sea, and the destruction of old rocks on the land. In the rôle of rock-makers they produce the very important original structures of the stratified rocks; while as agents of erosion they develop the most salient of the subsequent structures of the earth’s crust—the infinitely varied relief of its surface. ”

“ The arrest of stream erosion by the more resistant rocks.—The capacity of a river to erode and carry away the rock material that lies along its course is dependent not only upon the velocity of the current, but also upon the hardness, the firmness of texture, and the solubility of the material. Particularly in arid and semiarid regions, where no mantle of vegetation is at hand to mask the surfaces of the firmer rock masses, differences of this kind are stamped deeply upon the landscape. ”