EROSION, or DENUDATION, the proc ess of slow removal in air or water of the products of rock decay whereby the surface features of the earth are obliterated. It in cludes the destructive work of winds, of streams, of glaciers and of the ocean. Its various aspects may be considered under two heads : (1) subaerial; (2) marine.
1. Under subaerial erosion comes the action of air and water on all land surfaces above sea-level, first in malcing rock material fine, weathering, and second, in its removal, trans portation. Chemical processes, due to moisture and CO. in the air, break up the rock-minerals. Changes of temperature crack off flakes from rock-ledges and reduce them to smaller flakes. Water, freezing in cracks, forces apart large blocks of rock. In a climate with dry seasons the dust may be swept away by the winds (de flation), leaving the larger pebbles. Stony des erts have thus been formed in Arizona and other parts of the world. The dust-charged winds can carve and wear down rock surfaces. Instances are common in the arid regions of the West and in the desert of Sahara. Glaciers scour out their valleys powerfully and carry away much material.
The erosive action of water begins with the raindrop. If a piece of soft ground with small stones lying about be examined closely after a shower, it will be found that soil has been beaten down and washed away from the areas not protected by stones. This action sometimes takes place on a larger scale in semi-arid cli mates when rock-decay is slow and curious pillars of earth or soft rock capped by pro tecting boulders are formed. Examples may be found in the Garden of the Gods, Colorado. When the raindrops unite to form tiny rivulets the process of river-erosion has begun. In fact, a patch of soft ground on a hillside during a shower shows many of the phenomena of stream-formation, as likewise does even a dust covered street. The work of surface water is continuous. Some rock-constituents are dis solved and borne away in solution ; other particles are carried away in suspension and, by abrasion on rocks below, help the stream carve its channel deeper. When the current slackens
some of the waste from the higher lands may be deposited, the coarser materials first. Thus mountains are worn down and plains formed. The higher the mountains, the deeper the valleys can be carved; but even the highest mountains are ultimately deeply dissected, and finally worn down so that the current of the river may not be strong enough to transport the detritus. The carving of the Grand Canyon of the Colorado, a stupendous piece of work as it is, is but a Be ning in the complete leveling of the region. A country thus worn down is said to have reached a base-level of erosion, and its nearly level surface forms a peneplain. If such a region be uplifted the streams will start work again vigor ously, and a new cycle of erosion will begin. A region reduced to its base-level, if neither ele vated nor depressed, can remain unchanged through millions of years.
The amount of waste brought down by some rivers is enormous. Thus the Po in flood car ries one part sediment to every 300 parts of water; the Ganges one part sediment to 835 parts of water. These may seem small ratios of sediment, but the total amount of material discharged annually by the Ganges is calculated to be 378,100,000 tons, while the Nile annually brings down 150,000,000 tons. The Mississippi annually brings to the Gulf of Mexico 406,250, 000 tons of material in suspension, enough to lower its whole basin one inch in 300 years. It is calculated that the average amount of material removed as sediment annually by streams over the whole land surface is 600 tons per square mile.
2. Marine erosion is continually wearing away the continents. Waves undermine cliffs, grind up sand and boulders, and working with the undertow, drag the materials out and deposit them in the ocean. Some geologists even hold that marine planation is more important than river work in the destruction of continents.