Denudation, in Geology, means the laying bare of underlying rocks by the removal of superficial matter, and also the process by which the earth's surface is broken up and the loose material carried away. The more important agents of denudation are wind, rain, running water (springs, underground streams, brooks, and rivers), frost, snow, ice (glaciers), the sea (waves, breakers, currents), plants, and animals. The action of wind is seen in the erosion of rocks produced by the sand and grit which in certain dry regions is swept by the winds against projecting rocks, an action often resulting in the undermining of cliffs and the downfall of rock-masses. The sand produced by the superficial disintegration of rocks is carried forward and heaped up in the form of dunes or sandhills (see DRIFT). Rain is also a potent agent of denudation, its action being both chemical and mechanical. Rocks are more or less altered and decayed, and the decomposed materials carried off in solution by rain-water. The more soluble rocks, such as rock-salt, gypsum, and limestone, readily succumb, but there are very few rocks indeed which are not more or less acted upon chemically by rain. So that in many places the rocks are thus 'weathered' to considerable depths, the decomposed crusts varying in thickness from a mere line up to many feet or even yards. The mechanical action of rain consists chiefly in the sweeping away of this disintegrated material, which often accumulates in hollows, forming what is called rain-wash. Running water acts also chemically and mechanically. Thus, the rain that sinks underground and rises to the surface again in springs, brings about many changes in rocks. Immense quantities of mineral matter are brought up in solution, and thus, in time, underground cavities are formed, especially in the more soluble rocks. In regions of calcareous rocks, the whole drainage is sometimes conducted underground, the ingulfed streams and rivers acting both chemically and mechanically, and giving rise to a large series of subterranean tunnels (see CAVES). The action of underground water often brings about local subsidences, falls of rock, and great landslips. The denuding action of superficial terrestrial waters is seen in the excavation of gullies, ravines, and river-valleys, one of the most remarkable examples of river erosion being the Grand Cañon of the Colorado (q.v.), which is a chasm nearly 400 miles long, with approximately vertical walls rising to a height of 4000 to 7000 feet above the bed of the river.
Frost acts with great intensity at high levels and in high latitudes, but even in temperate regions its action is very marked and productive of great disin- tegration of rocks. Indeed, in the production of the weathered crusts of rocks, frost is hardly less active than rain. It is in arctic and mountainous countries, however, that its action is most conspicuous. The rocks under its influence are ruptured and shattered to such a degree that frequently the parent rock-masses become buried under shivered heaps of their own debris. Glaciers are likewise powerful denuding agents. They are not only instrumental in transporting the rock-rubbish which is showered down upon them from overhanging cliffs, &c. (see BOULDERS), but by means of the blocks and debris which they drag forward on their beds, they grind, furrow, and smooth the rocks over which they flow. The peculiarly muddy character of the water that escapes from the terminal front of a glacier shows how powerful this erosive action must be, for the mud carried in suspension is simply the fine flour of the rocks which has resulted from the grinding action. The sea, again, acts like a great horizontal saw, which is continually rasping away the rocks along the coast. Cliffs are in this way undermined, rock-falls take place, and the tumbled rock-masses are by-and-by pounded down into shingle, gravel, and sand, which are hurled by the waves against the cliffs, and thus ere long the latter is again undermined, and further rock-falls take place.
The chief denuding agents have now been mentioned, but the destructive action of plants and animals cannot be ignored. Thus plants aid in the demolition of rocks by sending their roots into rock-crevices and wedging the masses asunder, and hence they aid the freer percolation of water, and prepare the way for the better action of frost. Vegetation also, by attracting rain, tends to increase the flow and erosive action of streams and rivers, while its decomposition yields to rain those organic acids which so greatly increase the chemical action of that agent. The destructive action of animals, again, is seen in the weakening of rocks on a sea-coast produced by the drilling and boring of Saxicava, Pholas, &c., and by annelids, echini, and sponges. Rocks so weakened fall more readily before the battering of waves and breakers. Thus, the whole surface of the land, from the summits of mountains down to the sea-coast, is subject to denudation. Little change may be perceptible in a lifetime or even during many centuries, but an examination of the rocks shows that many thousands of feet of solid strata have been gradually removed from the surface of a country. Thus, in many districts where faults occur, no inequality at the surface betrays the presence of dislocations; the whole area has been reduced by denudation to the same level, hundreds or even thousands of feet of strata having been removed from the upcast side of the faults (see DISLOCATION). Some attempts have been made to form a rough estimate of the rate at which the general surface of the land is reduced by denudation. As the sediment of gravel, sand, and mud which a river carries down to the sea represents the actual loss sustained by the surface of the area drained by it and its tributaries, it is obvious that if we could correctly estimate the amount of sediment transported to the sea by the rivers of any given area, we should at the same time ascertain the rate at which that area is denuded. Observation has shown that this denudation proceeds more rapidly in some regions than in others, and, therefore, that the work of no individual river can be taken as a standard by which to estimate the general rate of erosion all the world over. Much depends on physical and climatic conditions, and much on the geological structure of a country and the composition of its prevalent rock-masses. Thus, the Mississippi is said to remove from the general surface of its basin 1 foot in 6000 years, the Rhone 1 foot in 1528 years, the Po 1 foot in 729 years. To the matter mechanically suspended in the water or swept forward on the beds of rivers we have to add the matter carried in solution, which in many rivers is very considerable. Rivers like the Rhine, the Danube, the Elbe, and the Rhone, contain in every 6000 parts by weight one part of dissolved mineral substance. These rivers, therefore, carry seawards their own weight of dissolved matter in 6000 years. Thus, subaerial denudation progresses more rapidly than we should at first be led to suspect, and those geologists who have made a special study of this question do not hesitate to assert that the action of the subaerial agents of denudation is far more effective than that of the sea, for whereas the action of the latter is confined to a narrow belt of land, that of the former has no such limitation. The whole land surface is exposed to attack, and the loose material carried down to the sea far exceeds in amount the waste of the coast-line by waves and breakers. See Playfair's Illustrations of the Huttonian Theory, Lyell's Principles of Geology, Croll's Climate and Time, and the standard textbooks of geology.