Denudation

Denudation (Latin denudo, I strip) is the geological term for the disintegration and more especially the removal after disintegration of the surface of the ground, laying bare fresh surfaces. These processes are generally classified as marine and sub-aerial. The former, partly chemical, partly the result of waves, of shingle dashed against cliffs by waves, and of air compressed in crannies of rocks by the waves, tends to wear down everything into a plain cf marine denudation, thus cutting horizontally; but, acting, as it does, only along the coast-line, it is estimated as doing less than a twentieth part of the work done by sub-aerial agencies. These latter include the "weathering" action of the air, the rusting produced by moisture, the solvent action of rain, the pulverisation due to the freezing of interstitial water, the flaking of rocks alternately exposed to heat and cold, the sculpturing by blown sand, the underground action of carbonated water and the "rotting" of granite, the mechanical erosion of their channels by rivers and their transport of sediment carried into them, the slower steady grnding of glaciers and the transport of material by icebergs, the loosening of rocks by roots and the burrowing of earthworms, moles, rabbits, and other animals, together with various other related agencies. Their general effect is the vertical sculpturing of the earth into hills of denudation and valleys, escarpments, ravines, and caves, owing to the unequal resistance of various rocks. As they act practically over the whole surface of the earth, the amount of matter removed by them to be deposited elsewhere is far greater than in marine denudation. Striking results of denudation are: the Grand Canon (q.v.) of the Colorado, the gorge below the Falls of the Niagara (q.v.), and the removal of the chalk dome that once extended over the valley of the Weald (q.v.) between the North and South Downs.