WELL; WELLS; WATHR SUPPLY.
Any natural outlet for ground water is a spring. As a rule springs are most abundant in lowlands and valleys where the ground sur face is nearest the water table. At any point, however, an impervious layer beneath a porous bed may prevent the downward seepage of water and deflect it laterally to outlets or springs. Most small springs are mere seep age from porous rocks or soils. Larger ones usually come from fissures, and some of the largest known like those in the Ozarks, flow from caves in limestone regions, and are power ful enough to furnish considerable water power.
Warm or hot springs are known in many regions. In some cases it is believed the heat is due to the great depth from which the water ascends along large fissures or faults. In other regions of recent volcanic activity the heat is probably derived from still uncooled masses of rock buried beneath the surface. Such are the hot springs of Yellowstone National Park. The temperature of the water in springs varies from the normal ground temperature to the boiling point. See SPRING; VOLCANOES; etc.
Geysers are a special variety of hot springs from which the water and steam are violently erupted at intervals more or less periodic. They are invariably found in regions of recent volcanic activity, and the two most noted areas are in Yellowstone National Park in the United States, and in Iceland. See GEYSERS ; also VOLCANOES.
Work of Running Water. (Rivers).— Running water is the most important geologic agent in the shaping of, land forms, though its efficacy was long unrecognized. In early times it was scarcely realized that valleys are carved the rivers that occupy them and that most land forms are strongly modified by running water. This work can be divided into erosion (denudation or degradation) and deposition (or aggradation). Erosion includes the mak ing of rock material into fine particles (weath ering) and its removal (transportation).
There is a constant struggle between the internal and external forces of the earth. The former no sooner bodily raise a region out of the sea (see section on Diastrophism) than the latter tend to tear it down and carry it back. If given sufficient time any region, no matter high it may have been elevated, will be reduced again to a featureless plain, at or near sea level. The level below which rivers can not reduce a region is known as base-level and a region so reduced is said to be base-levelled or peneplained (made almost a plain). The
complete history through which a region passes from the beginnings of the first drainage es tablished on it after its emergence back to a peneplain only slightly above sea-level is said to constitute an erosion cycle. In the earliest stages of the cycle a- few very crooked streams without valleys and with few tributaries ac commodate the run-off from rainfall. Large areas are almost without drainage. Each stream, however, using the sand or gravel of its bed as tools, starts wearing its channel deeper and deeper until deep but still narrow alternate with broad flat undissected divides. Undrained depressions are still occu pied by lakes and the streams usually have falls and rapids because they cut more rapidly on soft than hard rocks, making their gradients very uneven. This stage is known as youth. As the valleys deepen, gullies are cut in their slopes and these enlarge into tributaries which work back until the entire area is dissected. There are' no longer flat-topped divides but an intricate network of valleys, each carved by its own streamlet. The main streams have cut as low as they can and yet have sufficient gradient to flow to the sea. Even the harder rocks of their beds have been planed down and falls and rapids are almost worn away. Lakes are filled up with sediment or drained by the wearing down of the outlet. The region is now in maturity. Since the streams are now as low as they can erode, their chief work is to carry away the material which weathering and minor tributaries bring to them from the valley sides. The valley, though no longer deepening, is rapidly widening. Divides become more and more worn away until they are re duced to mere gentle swells. Each stream be comes sluggish, is easily deflected by obstruc tions, and begins to wind (meander) in great curves over a broad flat bottom known as a flood plain. In times of floods the curves are cut through and loop-shaped portions of the channel are abandoned and constitute oxbow lakes. The region, now in old age, is said to be base-levelled or peneplained. Youth, matur ity and old age are only comparative terms to express how far a region has progressed in the erosion cycle. It cannot well be expressed in terms of years since a region on, soft rock might progress to old age before an adjacent region on harder rock had well reached maturity.