Under certain circumstances caves may give rise to striking features of another sort. If for any reason the roof is destroyed at the two ends of a cave, remaining intact over the middle, the latter part constitutes a natural bridge. Natural bridges also originate in other ways (pp. 151, 153).

Creep, slumps, and landslides.—When the soil and subsoil on a slope become charged with water they tend to move downward. When the movement is too slow to be sensible it is called creep. The common downward inclination of trees growing in such situations, the result of the more rapid creep of the surface as compared with the deeper part of the soil, is both an expression of the movement and of its slowness. Other factors besides ground-water are involved in creep (see p. 112).

When the movement is rapid enough to be sensible the material is said to slump or slide. This may happen when the slope on which water-charged mantle rock lies is steep (Fig. 207). Great landslides of this sort have been recorded, and some of them have done great damage. Where a stream’s banks are high, and of unindurated material, such as clay, considerable masses sometimes slump from the bank or bluff into the river, or settle away slowly from their former positions. This is a common phenomenon along streams which have cut valleys in drift, and along shores on which waves are encroaching. The same phenomenon is common on a larger scale on the slopes of steep mountains.[107] Considerable terraces are sometimes developed on their slopes in this way, but they are usually irregular and discontinuous (Figs. 208, and 209). The loose débris on steep slopes sometimes assumes a sort of flowing motion and descends the slope with some such form and at some such rate as a glacier. Such bodies of débris are sometimes called “talus glaciers” (Fig. 210). In many such cases, snow and ice have had some part in their development.

In creep and in landslides gravity is the force involved, and the ground-water only a condition which makes gravity effective. Gravity alone accomplishes similar results, as illustrated by Fig. 211.

Summary.

All in all, ground-water is to be looked upon as a most important geological agent. When it is remembered that a very large part of all the water which falls on the surface of the earth, either in the form of rain or snow, sinks beneath the surface; that much of it sinks to a great depth; that much of it has a long underground course before it reappears at the surface; that it is everywhere and always active, either in subtracting from the rock through which it passes, in adding to it, in effecting the substitution of one mineral substance for another, or in bringing about new chemical combinations; and when it is remembered that this process has been going on for untold millions of years, it will be seen that the total result accomplished must be stupendous. The rock formations of the earth to the depths to which ground-water penetrates are to be looked upon as a sort of chemical laboratory through which waters are circulating in all directions, charged with all sorts of mineral substances. Some of the substances in solution are deposited beneath the surface, and some are brought to the surface where the waters issue. Much of the material brought to the surface in solution is carried to the sea and utilized by marine organisms in the making of shells. Without the mineral matter brought to the sea by springs and rivers, many shell-bearing animals of great importance, geologically, would perish. Biologically, therefore, as well as geologically, ground-water is of great importance.

SPRINGS AND FLOWING WELLS.

The term spring is applied to any water which issues from beneath the surface with sufficient volume to cause a distinct current. If the water issues so slowly as to merely keep the surface moist, it is not called a spring, but seepage. The spring from which water issues with a strong current, especially if it be upward, is comparable to a flowing well, while the spring from which water issues with little force, and without upward movement, is comparable to the flow of water into a common well.

Springs often issue from the sides of valleys (Fig. 212), the bottoms of which are below ground-water level. They are especially likely to issue at the surface of relatively impervious layers, and where the valley slopes cut joints, porous beds, or other structures which allow free flow of ground-water.

Springs are classified in various ways, and these several classifications suggest characteristics worthy of note. They are sometimes said to be deep and shallow. The “deep” spring, as the term is ordinarily used, is one which issues with great force, and with something of upward movement, and the “shallow” spring, one which issues with little force, and without upward movement; but the spring which issues with force is not necessarily deep, nor is the one which issues with little force necessarily shallow. The idea involved in this grouping would be better expressed by strong and feeble. Springs are also classified as cold and thermal, the latter term meaning simply that the temperature is such as to make the springs seem warm or hot. The temperature of thermal springs ranges up to the boiling-point of water. Between deep springs and shallow ones, and between cold springs and thermal, respectively, there is no sharp line of demarkation. Again, some springs are continuous in their flow, while others are intermittent. Most intermittent springs flow after periods of precipitation, but dry up during droughts (see p. 202). Springs are also classified as mineral and common. Mineral springs, in the popular sense of the term, are of two types: (1) Those which contain an unusual amount of mineral matter, and (2) those which contain some unusual mineral. Springs are especially likely to be called mineral if the substances which they contain, have, or are supposed to have, some medicinal property. All springs which are not “mineral” are “common.” This classification is not altogether rational, for all springs contain more or less mineral matter, and many springs which are “common,” contain more mineral matter than some springs that are “mineral.” Mineral springs are themselves classified according to the kind and amount of mineral matter they contain. Thus saline springs contain salt; sulphur springs contain compounds (especially gaseous) of sulphur; chalybeate springs contain iron compounds, especially the sulphate; calcareous springs contain abundant lime carbonate, etc. These various mineral substances are extracted from the rock, sometimes by simple solution, and sometimes by solution resulting from other chemical change. The salt of saline springs is usually extracted from beds of salt beneath the surface. Lime carbonate, one of the commonest substances in solution in ground-water, is dissolved from limestone, or derived by chemical change from rocks containing other calcium compounds. Thus lime feldspars, by carbonation, give rise to lime carbonate. The chalybeate waters often arise from the oxidation of iron sulphide, a mineral which is common in many sedimentary rocks. The iron sulphate is itself subject to change in the presence of the ubiquitous lime carbonate. From this change iron carbonate results, and this is usually quickly altered to iron oxide, which, being relatively insoluble, is precipitated. About chalybeate springs, therefore, iron oxide is frequently being deposited. Medicinal springs are those which contain some substance or substances which have, or are supposed to have, curative properties.

Mineral matter in solution.—The number and variety of mineral substances in spring water is very great, and the amount of solid matter in solution varies widely. Some of the hot springs of the Yellowstone Park contain nearly three grams (2.8733) of mineral matter per kilogram.[108]

The composition of various spring and well waters is shown in the accompanying table, which gives some idea of the range of mineral substances commonly in solution in ground-water.

Geysers.—Geysers are intermittently eruptive hot springs. They occur only in volcanic regions (past or present) and in but few of them. Active geysers are virtually confined to the Yellowstone Park and Iceland, though they formerly existed at other places. Those of New Zealand have but recently become extinct. The great geyser region of the world is the Yellowstone Park, where there are said to be more than sixty active geysers.

The cause of the eruption is steam. The surface-water sinks down until, at some unknown depth, it comes into contact with rock sufficiently hot to boil it. The source of the heat is not open to inspection, but it is believed to be the uncooled part of an extrusive lava flow, or of an intrusive lava mass. From what was said on pp. 216 and 217 it is clear that geysers do not have their origin in water which sinks down to the zone of great heat, where the increment of heat is normal.

The water of a geyser issues through a tube of unknown length. Whether the tube is open down to the source of the heat is not determinable, but water from such a source finds its way to the tube. Water may enter the tube from all sides and at various levels from top to bottom. The heating may precede or follow its entrance into the tube, or both. So far as the water is heated after it enters the tube, the point of most rapid heating may be at the bottom of the tube or at some point above. If the temperature of the source of heat were high enough to convert the descending water into steam as fast as it enters the tube, the steam would escape continuously, though there would be no geyser; but if the rock is only hot enough to bring the water to the boiling-point after some lapse of time, and after some water has accumulated, an eruption is possible.

The exact sequence of events which leads up to an eruption is not known, but a definite conception of the principles involved may perhaps be secured by a definite case. Suppose a geyser-tube filled with water, and heated at its lower end. As the water is heated below, convection tends to distribute the heat throughout the column of water above. If convection were free, and the tube short, the result would be a boiling spring; but if the tube is long, and especially if convection is impeded, the water at some level below the surface may be brought to the boiling-point earlier than that at the top. Under these circumstances if even a little water in the lower part of the tube is converted into steam, the steam will raise the column of water above, and it will overflow. The overflow relieves the pressure on all parts of the column of water below the surface. If before the overflow there was any considerable volume of water essentially ready to boil, the relief of pressure following the overflow might allow it to be converted into steam suddenly, and the sudden conversion of any considerable quantity of water into steam would cause the eruption of all the water above it (Figs. 213 and 214). The height to which the water would be thrown would depend upon the amount of steam, the size and straightness of the tube, etc.

It is clear that everything which impedes convection in the geyser tube will hasten the period of eruption, since impeded circulation will have the effect of holding the heat down, and so of bringing the water at some level below the top more quickly to the boiling-point. It follows that anything which chokes up the tube, or which increases the viscosity of the water, or its surface tension, would hasten an eruption.[119]

Geysers often build up crater-like basins or cones (Figs. 214 to 217) about themselves, the cone being of material deposited from solution. In the Yellowstone Park the precipitation of the matter in solution (chiefly silica) is partly due to cooling and partly to the algæ which abound even in the boiling water, and the brilliant colors of the deposits about the springs are attributable to these plants. When the water from any geyser or hot spring ceases to flow the plants die and the colors disappear. The details of the surface of the deposits about geysers and hot springs are often complicated, and frequently very beautiful (Fig. 218).

The heating of geyser and hot-spring water must cool the lava or other source of heat below. As this takes place, the time between eruptions becomes longer and longer. In the course of time, therefore, the geyser must cease to be eruptive, and when this change is brought about the geyser becomes a hot spring. Within historic times several geysers have ceased to erupt and new ones have been developed. In the Yellowstone Park, where there are said to be something like 3000 vents of all sorts, hot springs which are not eruptive greatly outnumber the geysers. From many of the vents but little steam issues, and from some, little else.

A few geysers have somewhat definite periods of eruption. Of such “Old Faithful” is the type; but even this geyser, which formerly erupted at regular intervals of about an hour, is losing the reputation on which its name is based. Not only is its period of eruption lengthening, but it is becoming irregular, and the irregularity appears to be increasing. In the short time during which this geyser has been under observation its period has changed from a regular one of sixty minutes, or a little less, to an irregular one of seventy to ninety minutes. In the case of some geysers years elapse between eruptions, and in some the date of the last eruption is so distant that it is uncertain whether the vent should be looked upon as a geyser or merely a hot spring.

In the Yellowstone Park[120] the geysers are mainly in the bottoms of valleys (Fig. 219), but the deposits characteristic of geysers are found in not a few places well above the present bottoms. These deposits record the fact that in earlier times the geysers were at higher levels than now. It is probable that they have been, at all stages in their history, near the bottoms of the valleys, and that, as the valleys have been deepened the ground-water has found lower and lower points of issue. In this respect the geysers have probably had the same history as other springs.

Unless new intrusions of lava occur, or unless heat is otherwise renewed at the proper points, it is probable that all existing geysers will become extinct within a time which is, geologically, short. New geyser regions may, however, develop as old ones disappear.

Artesian wells.—Originally the terms artesian wells and flowing wells were synonymous; but at the present time any notably deep well is called artesian, especially if it descends to considerable depths below the mantle rock. The artesian well which does not flow, does not differ from common wells in principle; but being deeper, the water which it affords is often more thoroughly filtered and frequently more highly mineralized than that of other wells. The flowing well is really a gushing spring, the opening of which was made by man.

Flowing wells[121] depend upon certain relations of rock structure, water supply, and elevation. Generally speaking a flowing well is possible in any place underlain by any considerable bed of porous rock, if such rock outcrops at a sufficiently higher level in a region of adequate rainfall, and is covered by a layer or bed of impervious, or relatively impervious rock. This statement involves four conditions, all of which are illustrated by Fig. 199, where a is the bed of porous rock. It is not necessary that the beds of rock form a structural basin, nor is it usually necessary to take account of the character of the rock beneath the porous bed which contains the water.

The bed of porous rock is the “reservoir” of the flowing well. Formations of sand or sandstone, and of gravel or conglomerate, most commonly serve as the reservoirs. In order that it may contain abundant water it must have some thickness, and its outcropping edge must be so situated that the water may enter freely and be replenished, chiefly by rain, as the water flows out at the well.

A relatively impervious layer of rock above the reservoir (b, Fig. 199) is most important; otherwise the water in the reservoir will leak out, and there will be little or no “head” at the well site. Thus if the rock overlying stratum a (Fig. 199) were badly broken, the fractures extending up to the surface, the conditions would be unfavorable for flowing wells. Under such conditions, wells in the positions of those shown in Fig. 199 might get abundant water, but they would not be likely to flow. If the stratum next below the reservoir is not impervious, some lower one probably is. No layer of rock is more impervious than one which is full of water, and the substructure of any bed which might serve as a reservoir is usually full of water, even if the rock be porous.

If the outcrop of the reservoir be notably above the site of the well, and if it be kept full by frequent rains, the “head” will be strong, though the water at the well will not rise to the level of the outcrop of the reservoir. Experience has shown that an allowance of about one foot per mile of subterranean flow should be made. Thus if the site of the well be 100 miles from the outcrop of the water-bearing stratum, and 200 feet below it, the water will rise something like 100 feet above the surface at the well. This rule is, however, not applicable everywhere. The failure of the water to rise to the level of its head is due to the adhesion and the friction of flow through the rock. The more porous the rock the less the reduction of head by friction. The height of the flow is also influenced by the number of wells drawing on the same reservoir, on the degree of imperviousness of the confining bed above, etc.

Flowing wells, often relatively shallow, are frequently obtained from unconsolidated drift. Some such relations as suggested by Fig. 220 would afford the conditions for flowing wells in such a formation.