EROSION (Lat. crosio, from crodere, to gnaw, from e, out + rodere. to gnaw; connected with radcre, to scrape, Skt. rade, tooth), or DENUDATION. The process by which the surface forms of the earth are sculptured and worn down. The present features of the earth's surface, while they have the appearance of great stability, in reality represent a single stage of development that has been determined by the cooperation of various geological agencies working through long periods of time. Some of these agencies contrib ute to the erosion or denudation of the land, carving out valleys in plateaus, wearing down mountains, dissecting plains. and generally low ering the level to that of the sea. Rivers are most active in this process. The surface waters supplied by rain and by melting of snow wash the soil and disintegrated rock materials down the slopes of the land into the valleys. where the detritus is carried along by the streams and deposited in their channels or borne to the sea. The solid particles suspended in water exert an abrasive action on the sides and floor of river channels, thus tending to widen and deepen them. A large amount of material is also held in solu tion and transported in this manner to the sea. The rate at which rivers carry on the destruc tive work varies in particular regions with the climate, slope of land, and character of rocks. Rainfall, sunshine, and frost, and the chemical action of the atmosphere by means of its car bonic-acid gas, ammonia, and nitrous gases, greatly facilitate the breaking down of rocks, which is a preliminary step to their erosion and transport. Estimates of the sediment carried by the Mississippi each year into the Gulf of :Mexico reach a total of 7,500,000,000 cubic feet. an amount equivalent to the lowering of the whole Mississippi basin one inch in 500 years; while if the material removed in solution also he taken into count the reduction of level would be still greater. The Po is said to remove one foot of rock from its basin in 730 years. This wasting or destruction work of rivers. when continued through long periods of time. must produce great changes. Glaciers, like rivers, are denuding and transporting agents. The weight of the thick masses of ice gives them great erosive power. which is further increased by the rocks carried along the bottom of their beds. At present the occurrence of glaciers in the warmer zones is lint ited to regions of high elevation, but in past ages it is known tnat they occupied great continental areas. The Rocky Mountains, the Sierra Ne
vadas, a large part of the Northern United States. and nearly the whole of Canada were once the seat of ice sheets which have profoundly modified the surface features. Another important denud ing force is the sea, particularly in the upper por tior.s, where the water is kept in motion by waves. tides, and currents. Wave-action breaks down cliffs, and gives to the coast-lines of continents a constantly changing form. Tides carry seaward the sediment brought down by rivers to their mouths.
The immediate effect of erosion is to produce a variety of contour on land surfaces. The forms or types cf scenery exhibited in any one locality depend upon the combination of factors at work and the material exposed to their action. A level land area comptised of rocks unequally resistant to abrasion must in time be carved into a series of hills and valleys, the position of which will depend upon the relative disposition of the harder and softer materials. In the process of land sculpturing it is also necessary to consider the predominant erosive agencies, which will vary in different regions and in different climates. Arid districts, like the Bad Lands of South Dakota and the plateaus of Arizona, have peculiar types of scenery that cannot be found in countries hay ing a heavier rainfall. The general tendency of ero sion is to reduce the level of continents to that of the sea (base-level ). This destructive process is off set in a measure by movements of the earth's crust which repair what has been lost by superficial waste. The amount of material removed from the land is represented by an equivalent accumulation beneath the sea, and by upheaval this nceumula tion may be raised above water-level. The ac tivity of the two processes, antagonistic in their effect, is illustrated by the areas of stratified rocks, such as sandstones, shales, limestones, which form by far the larger portion of the sur face of continents. Consult: Text-book of Geology (London, 1693) ; Davis, Physical Geog raphy (Boston, 1900) : Gilbert, "Geology of the I I en ry Mountains," United Stutrs Geological Sur rey Reports (WaAlington, 1377). See PII YSIOG RA Pli ; MOUNTAIN; SHORE; CONTI NENT, etc.