THE TRAVELS OF THE UNDERGROUND WATER

The descent within the unsaturated zone.—Of the moisture precipitated from the atmosphere, that portion which neither evaporates into the air nor runs off upon the surface, sinks into the ground and is described as the ground water. Here it descends by gravity through the pores and open spaces, and at a quite moderate depth arrives at a zone which is completely saturated with water. The depth of the upper surface of this saturated zone varies with the humidity of the climate, with the altitude of the earth’s surface, and with many other similarly varying factors. Within humid regions its depth may vary from a few feet to a few hundred feet, while in desert areas the surface may lie as low as a thousand feet or more.

The surface of the zone of the lithosphere that is saturated with water is called the water table, and though less accentuated it conforms in general to the relief of the country (Fig. 188). Its depth at any point is found from the levels of all perennial streams and from the levels at which water stands in wells.

During the season of small precipitation the water table is lowered, and if at such times it falls below the bed of a valley, the surface stream within the valley dries up, to be revived when, after heavier precipitation, the water table has in turn been raised. Such streams are said to be intermittent, and are especially characteristic of semiarid regions (Fig. 188).

Wherever in descending from the surface an impervious layer, such as clay, is encountered, the further downward progress of the water is arrested. Now conducted in a lateral direction it issues at the surface as a spring at the line of emergence of the upper surface of the impervious layer (Fig. 189).

The trunk channels of descending water.—While within the unconsolidated rock materials near the surface of the earth, it is clear that water can circulate in proportion as the materials are porous and so relatively pervious. As the pore spaces become minute and capillary, the difficulty of permeation through the materials becomes very great. Thus in the noncoherent rocks it is the coarse gravel and the layers of sand which serve as the underground channels, while the fine clays have the effect of an impervious wall upon the circulating waters. In coarse sand as much as a third of the volume of the material is pore space for the absorption and transmission of water. Even under these favorable conditions the movement of the water is exceedingly slow and usually less than a fifth of a mile a year.

Within the hard rocks it is the sandstones which have the largest pore spaces, but in nearly all consolidated rocks there are additional spaces along certain of the bedding planes, the joint openings (Fig. 190), and the crushed zones of displacement, so that these parting planes become the trunk channels, so to speak, of the circulating water. It is along such crevices that in the course of time the mineral matter carried in solution by the water is deposited to produce the ore veins and the associated crystallized minerals.

The caverns of limestones.—Where limestone formations have a nearly flat upper surface, a large part of the surface water enters the rock by way of the joint spaces, which it soon widens by solution into broad crevices with well-rounded shoulders. At joint intersections solution of the limestone is so favored that the water may here descend in a sort of vertical shaft until it meets a bedding plane extending laterally and offering more favorable conditions for corrosion. Its journey now begins in a lateral direction, and solution of the rock continuing, a tunnel may be etched out and extended until another joint is encountered which is favorable to its further descent into the formation. By this process on alternating shafts and galleries the water descends to near the surface of the water table by a series of steps, and is eventually discharged into the river system of the district (Fig. 191). Within the larger caverns the water at the lowest level usually flows as a subterranean river to emerge later into the light from beneath a rock arch.

From the plan of a system of connecting caverns it may often be observed that the galleries of the several levels are alike directed along two rectangular directions which indicate the master joint directions within the limestone formation. This is especially clear from the map of the galleries in the explored portions of the Mammoth Cave (Fig. 192).

Swallow holes and limestone sinks.—Above the caverns of limestone formations there are selected points where the water has descended in the largest volume, and here funnel-shaped depressions have been dissolved out from the surface of the rock. In different districts such depressions have become known as “sinks”, “swallow holes”, entonnoirs, and Orgeln. Wherever the depressions have a characteristic circular outline, there can be little doubt that they are the product of solution by the descending water, and have relatively small connections only with the subterranean caverns. They have thus naturally collected upon their bottoms the insoluble clay which was contained in the impure limestone as well as a certain amount of slope wash from the surface. Inasmuch as the clays are impervious to water, the bottoms of these swallow holes are better supplied with moisture than the surrounding rock surfaces, and by nourishing a more vigorous plant growth are strongly impressed upon the landscape (Fig. 193).

Certain of the depressions above caverns are, however, less regular in outline, and their bottoms are occupied by a mass of limestone rubble. In some instances, at least, these depressions appear to be the result of local incaving of the cavern roofs. An incaving of this nature may close up an earlier gallery in the cavern and divert the cave waters to a new course. The destruction of the roofs of caverns through this process of incaving may continue until only relatively small remnants are left. From long subterranean tunnels the caves are thus transformed into subaËrial rock bridges that have become known as “natural bridges.” The best-known American example is the Natural Bridge near Lexington, Virginia. Much grander natural bridges have been formed in sandstone by a totally different process, and must not be confused with these limestone remnants of caverns.

The sinter deposits.—Just as water can dissolve the calcareous rocks with the formation of caverns, it can under other conditions deposit the material which has thus been taken into solution. Its power to hold carbonate of lime in solution is dependent upon the presence of carbonic acid gas within the water. Water charged with gas and dissolved lime carbonate is said to be “hard”, and if the gas be driven off by boiling or otherwise, the dissolved lime is thrown out of solution and deposited in a form well known to all housekeepers.

Hard water flowing in a surface stream, if dashed into spray at a cascade, may deposit its lime carbonate in an ever thickening veneer wherever the spray is dashed about the falls. This material, when cut in section, has waving parallel layers and is known as travertine or calcareous sinter. Some of the most remarkable deposits of this nature may be seen at the cascade of Tivoli near Rome, and most of the Roman buildings have been constructed from travertine that has been quarried in the vicinity.

The growth of stalactites.—Water, after percolating slowly through the crevices of limestone, where it becomes charged with the carbonic acid gas and with dissolved carbonate of lime, may trickle from the roof of a cavern. Emerging from the narrow crevice, it may give off some of its contained gas and is usually subject to evaporation, with the result that the lime carbonate is left adhering to the rock surface from which evaporation took place. If the water collects upon the cavern roof so slowly that it can entirely evaporate before a drop can form, the entire content of carbonate will be left adhering to the roof. Evaporation is most rapid near the margins and over the center of each drop as it develops, and the deposit which is left thus takes the form of tiny white rings at those points upon the crevice where there is the easiest passage for the trickling water. To the outer surface of these rings water will first adhere and then evaporate, as it will also slowly ooze through the passage in the ring, but here without evaporation until it reaches the lower surface. A pendant structure will, therefore, develop, growing outward in all directions by the deposition of concentric layers which are thickest near the roof, and downward into the form of a rock “icicle” through evaporation of the water which collects near the tip. These pendant sinter formations are known as stalactites and are thus formed of concentric layers arranged like a series of nested cornucopias with a perforation of nearly uniform caliber along the axis of the structure (Fig. 194).

Formation of stalagmites.—Wherever the water percolates through the roof of the cavern so rapidly that it cannot entirely evaporate upon the roof, a portion falls to the floor, and, spattering as it strikes, builds up a relatively thick cone of sinter known as a stalagmite, and this is accurately centered beneath a stalactite upon the roof. In proportion as the cavern is high, the dropping water is widely dispersed as it strikes the floor, with the formation of a correspondingly thick and blunt stalagmite. As this rises by growth toward the roof, it often develops upon its summit a distinct crater-like depression (Fig. 194, lower figure). When the process is long continued, stalactites and stalagmites may grow together to form columns which may be ranged with their neighbors like the pipes of an organ, and like them they give out clear tones when struck lightly with a mallet. At other times the columns are joined to their neighbors to form hangings and draperies of the most fantastic and beautiful design (Fig. 195).

In remote antiquity limestone caverns afforded a refuge to many species of predatory birds and animals as well as to our earliest ancestors. The bones of all these denizens of the caves lie entombed within the clays and the sinter formations upon the cavern floors, and they tell the story of a fierce and long-continued warfare for the possession of these natural strongholds. The evidence is clear that these cave men with their primitive weapons were able at times to drive away the cave bears, lions, and hyenas, and to set up in the cavern their simple hearths, only in their turn to be conquered by the ferocity of their enemies. Some of the European caves have yielded many wagonloads of the skeletons of these fierce predatory animals, together with the simple weapons of the primitive man.

The Karst and its features.—Most so-called limestones have a large admixture of argillaceous materials (clays) and of siliceous or sandy particles. Such impurities make up the bulk of the clays and muds which are left behind when the soluble portions of the limestone have been dissolved.

Swallow holes we have found to be characteristic features within such districts. When limestones are more nearly pure, as in the Karst region east of the Adriatic Sea, similar features are developed, but upon a grander scale, and certain additional forms are encountered. In place of the sink or swallow hole, there appears the “karst funnel” or doline, a deep, bowl-shaped depression having a flat bottom. Such funnels may be 30 to 3000 feet across and from 6 to 300 feet in depth (Fig. 196). Though in one or two instances known to be the result of the break down of cavern roofs (Fig. 197), yet like the swallow holes of other regions these larger funnels appear generally to be the work of solution by the descending waters. Where they have been opened in artificial cuttings along railroads or in mines, the original rock is found intact at the bottom, with small crevices only going down to lower levels. Over the bottoms of the dolines there is spread a layer of fertile red clay, the terra rossa, like that which is obtained as a residue when a fragment of the limestone has been dissolved in laboratory experiments.

A desert from the destruction of forests.—Between the dolines is found a veritable desert with jutting limestone angles and little if any vegetation. The water which falls upon the surface either runs off quickly or goes down to the subterranean caverns by which so much of the country is undermined. Hence it is that the gardens which furnish the sustenance for the scattered population are all included within the narrow limits of the doline bottoms. Although to-day so largely a barren waste, we know that the Karst upon the Adriatic was in remote antiquity a heavily forested region and that it supplied the myriads of wooden piles upon which the city of Venice is supported. The vessels which brought to this port upon the Adriatic its ancient prosperity were built from wood brought from this tract of modern desert. In the days of Venetian grandeur the fertile terra rossa formed a veneer upon the rock surface of the Karst and so retained the surface waters for the support of the luxuriant forest cover. After deforestation this veneer of rich soil was washed by the rains into the dolines or into the few stream courses of the region, thus leaving a barren tract which it will be all but impossible to reclaim (plate 6 A).

Upon the steeper slopes over the purer limestones, the rain water runs away, guided by the joints within the rock. There is thus etched out a more or less complete network of narrow channels (Fig. 190, p. 181), between which the remnants rise in sharp blades to produce a structure often simulated upon the fissured surface of a glacier that has been melted in the sun’s rays (Fig. 401). These almost impassable areas of karst country are described as Schratten or Karrenfelder (Fig. 198).

The ponore and the polje.—To-day large areas of the Karst are devoid of surface streams, nearly all the surface water finding its way down the crevices of the limestone into caverns, and there flowing in subterranean courses. The foot traveler in the Karst country is sometimes suddenly arrested to find a precipice yawning at his feet, and looking down a well-like opening to the depth of a hundred feet or more, he may see at the bottom a large river which emerges from beneath the one wall to disappear beneath the other. These well-like shafts are in the Austrian Karst known as Ponores, while to the southward in Greece they are called Katavothren.

Elsewhere the karst river may emerge from its subterranean course in a broader depressed area bounded by vertical cliffs, from which it later disappears beneath the limestone wall. Such depressions of the karst are known as poljen, and appear in most cases to be above the downthrown blocks in the intricate fault mosaic of the region. Some of these steeply walled inclosures have an area of several hundred square miles, and especially at the time of the spring snow melting they are flooded with water and so transformed into seasonal lakes (Fig. 199 and p. 422). It appears that at such times the cave galleries of the region with their local narrows are not able to carry off all the water which is conducted to them; and in consequence there is a temporary impounding of the flood waters in those portions of the river’s course which are open to the sky and more extended. The rush of water at such times may bring the red clay into the subterranean channels in sufficient quantity to clog the passages. The Zirknitz Lake usually has high water two or three times a year, and exceptionally the flooding has continued for a number of years. It has thus in some districts been necessary to afford relief to the population through the construction of expensive drainage tunnels.

The conditions which are typified in the Karst area to the east of the Adriatic Sea are encountered also in many other lands; as, for example, in the Vorarlberg and Swiss Alps, in Lebanon, and in Sicily.

The return of the water to the surface.—Water which has descended from the surface and been there held between impervious layers, may be under the pressure of its own weight or “head”; and will later find its way upward, it may be to the surface or higher, where a perforation is discovered in its otherwise impervious cover. Such local perforations are produced naturally by lines of fracture or faulting (widened at their intersections), and artificially through the sinking of deep wells. The water, which at ordinary times reaches the surface upon fissures, is usually concentrated locally at the intersections of the fracture network, where it issues in lines of fissure springs (Fig. 200); but at the time of earthquakes the water may rise above the surface in lines of fountains (p. 83), or occasionally as sheets of water which may mount some tens of feet into the air.

In contrast to the flow of surface springs, which varies with the season through wide ranges both in its volume and in temperature of the water, the volume of fissure springs is but slightly affected by the seasonal precipitation, and the water temperature is maintained relatively constant. Rock is but a poor heat conductor, and the seasonal temperature changes descend a few feet only into the ground. Thus water which rises from depths of a few hundred feet only is apt to be icy cold, while from greater depths the effect of the earth’s internal heat is apparent in a uniform but relatively higher temperature of the water. Such “warm” or thermal springs are apt to contain considerable mineral matter in solution, both because the water is far traveled and because its higher temperature has considerably increased its solvent properties.

It has long been recognized that lines of junction of different rock formations at the base of mountain ranges are localities favorable for the occurrence of thermal springs. These junction lines are usually within zones where by movement upon fractures the widest openings in the rock have formed, and the catchment area of the neighboring mountain highland has supplied head for the ground water. A map of the hot springs within the Great Basin of the western United States would present in the main a map of its principal faults.

Artesian wells.—From the natural fissure spring an artesian well differs in the artificial character of the perforation of the impervious cover to the water layer. The water of artesian wells may flow out at the surface under pressure, or it may require pumping to raise it from some lower level. Ideal conditions are furnished where the geological structure of the district is that of a broad basin or syncline. The water which falls in a neighboring upland is here impounded between two parallel, saucer-like walls and will flow under its head if the upper wall be perforated at some low level (Fig. 201, 3).

A monoclinal structure may furnish artesian conditions when the generally pervious layer has become clogged at a low level so as to hold back the water (Fig. 248, 2). Pumping wells may be used successfully even when such clogging does not exist, for the slow-moving underground water flows readily in the direction of all free outlets (Fig. 201, 1).

Hot springs and geysers.—Thermal springs whose temperature approaches the boiling point of water are known as hot springs. A geyser is a hot spring which intermittently ejects a column of water and steam. Both hot springs and geysers are to be found only in volcanic regions, and appear to be connected with uncooled masses of siliceous lava. In two of the three known geyser regions, Iceland and New Zealand, the volcanoes of the neighborhood are still active, and the lavas of the Yellowstone National Park date from the quite recent geological period which immediately preceded the so-called “Ice Age.”

Wherever found, geysers are in the low levels along lines of drainage where the underground water would most naturally reappear at the surface. Their water has penetrated to considerable depths below the surface, but has been chiefly heated by ascending steam or other vapors. The water journey has been chiefly made along fissures, as is shown by the cool springs which often issue near them. Though some hot springs and geysers may disappear from a district, others are found to be forming, and there is no good reason to think that geysers are rapidly dying out, as was at one time supposed.

The action of a geyser was first satisfactorily explained by the great German chemist Bunsen after he had made studies of the Icelandic geysers, and the mechanics of the eruption was later strikingly illustrated in the laboratory by an artificial geyser constructed by the Irish physicist Tyndall. In many respects this action is like that of the Strombolian eruption within a cinder cone, since it is connected with the viscosity of the fluid and the resistance which this opposes to the liberation of the developing vapor. In the case of the geyser, a column of heated water stands within a vertical tube and is heated near the bottom of the column.

Though the water may at its surface have the normal boiling temperature and be there in quiet ebullition, the boiling point for all lower levels is raised by the weight of the column of superincumbent liquid, and so for a time the formation of steam within the mass is prevented. In Fig. 202 is shown a cross section of the Icelandic Geysir from which our name for such phenomena has been derived, and to this section have been added the actual observed temperatures of the water at the different levels as well as the temperatures at which boiling can take place at these levels. From this it will be seen that at a depth of 45 feet the water is but 2° Centigrade below its boiling point. A slight increase of temperature at this level, due to the constantly ascending steam, will not only carry this layer above the boiling point, but the expansion of the steam within the mass will elevate the upper layers of the water into zones where the boiling points are lower, and thus bring about a sudden and violent ebullition of all these upper portions. Thus is explained the almost universal observation that just before geysers erupt the hot water rises in the bowls and generally overflows them.

The water ejected from the geyser is considerably cooled in the air; and after its return to the tube must be again heated by the ascending vapors before another eruption can occur. The measure of the cooling, the time necessary to fill the tube, and the supply of rising steam, all play a part in fixing the period which separates consecutive eruptions. If the top of the tube be narrowed from its average caliber, as is commonly observed to be true of the geysers within the Yellowstone National Park, the escape of the steam is further hindered, and frequent geyser eruption promoted.

An artificial geyser for demonstration of the phenomenon in the lecture room is represented in Fig. 203. The cut has been prepared from a photograph of an apparatus designed by Professor B. W. Snow of the University of Wisconsin. In this design the tube is contracted so as to have a top diameter one fourth only of what it is at the bottom, where heat is directly applied by multiple Bunsen lamps. The water once sufficiently heated, this artificial geyser erupts at regular intervals of time which are dependent upon the dimensions of the apparatus and the quantity of heat applied.

In case of natural geysers a considerable quantity of heat escapes between eruptions in steam which issues quietly from the bowl of the geyser. If this heat be retained by plugging the mouth of the tube with a barrowful of turf, as is sometimes done with the geyser Strokr in Iceland, eruption is promoted and so takes place earlier. Another method of securing the same result is to increase the viscosity of the water through the addition of soap, as was accidentally discovered by a Chinaman who was utilizing the geyser water in the Yellowstone Park for laundry operations. After this discovery it became a common custom to “soap” the Yellowstone geysers in order to make them play; but this method was prohibited under heavy penalty after the disastrous eruption of the Excelsior Geyser.

The deposition of siliceous sinter by plant growth.—Geysers are known only from areas of siliceous volcanic lava, and this may perhaps have its cause in the easier solution of the geyser tube from such materials. The silica dissolved in the heated waters is again deposited at the surface to form siliceous sinter or geyserite. This material forms terraces surrounding the geysers or is built up into mounds which are often quite symmetrical, such as those of the Bee Hive and Lone Star geysers of the Yellowstone Park (Fig. 204).

The greater part of this separation of silica from the heated geyser waters is due to the action of plants or algÆ that are able to grow in the boiling waters and which produce the beautiful colors in the linings to the hot springs. The wonderful variety of the tints displayed is accounted for by the fact that the algÆ take on different colors at different temperatures. The silica is deposited from the water in the gelatinous hydrated form, which, however, dries in the sun to a white sand. The growth within the pools goes on in a manner similar to that of a coral reef, the algÆ dying below and there becoming encased in the rock lining while still continuing to grow upon the surface. Whereas sinter of this nature, when deposited by evaporation alone, can produce a maximum thickness of layer of a twentieth of an inch each year, the growth from alga deposition within limited areas may be as much as eight inches during the same period.

Reading References for Chapter XIV

General:—

F. H. King. Principles and Conditions of the Movements of Ground Water, 19th Ann. Rept. U. S. Geol. Surv., 1899, Pt. ii, pp. 59-294, pls. 6-16.

C. S. Slichter. The Motions of the Underground Waters, Water Supply Paper No. 67, U. S. Geol. Surv., 1902, pp. 1-106, pls. 1-8; Field Measurements of the Rate of Movement of Underground Waters, ibid., No. 140, 1905, pp. 1-122, pls. 1-15.

M. L. Fuller. Occurrence of Underground Water, ibid.. No. 114, 1905, pp. 18-40, pls. 4; Bibliographic review and index of papers relating to underground waters published by the United States Geological Survey, 1879-1904, ibid., No. 120, 1905, pp. 1-128.

Caverns:—

E. A. Martel. Les abimes, les eaux souterraines, les cavernes, les sources, la spÉlÆologie. Delagrave, Paris, pp. 578. (Lavishly illustrated.)

H. C. Hovey. Celebrated American Caverns. Cincinnati, 1896, pp. 228; The Mammoth Cave of Kentucky. Louisville, 1897, pp. 111.

J. W. Beede. Cycle of Subterranean Drainage in the Bloomington Quadrangle, Proc. Ind. Acad. Sci., 1910, pp. 1-31.

Karst conditions:—

J. Cvijic. Das KarstphÄnomen, Geogr. Abh., vol. 5, 1893.

Émile Chaix. La topographie du desert de platÉ (Hautes Savoie), Le Globe, vol. 34, 1895, pp. 1-44, pls. 1-16, pp. 217-330.

W. v. Knebel. HÖhlenkunde mit BerÜcksichtigung der KarstphÄnomene. Vieweg, Braunschweig, 1906, pp. 222.

A. Grund. Die Karsthydrographie, Studien aus Westbosnien, Geogr. Abh., vol. 7, No. 3, 1903, pp. 200.

Émile Chaix-du Bois et AndrÉ Chaix. Contribution a l’Étude des lapies en Carniole et au Steinernes Meer, Le Globe, vol. 46, 1907, pp. 17-56, pls. 26.

P. Arbenz. Die Karrenbildungen geschildert am Beispiele der Karrenfelder bei der Frutt in Kanton Obwalden (Schweiz). Deutsch. Alpenzeitung, Munich, 1909, pp. 1-9.

F. Katzer. Karst und Karsthydrographie. Sarejevo, 1909, pp. 95.

M. Neumayr. Erdgeschichte, vol. 1, pp. 500-510.

E. de Martonne. TraitÉ de GÉographie Physique, pp. 462-472 (excellent summaries in this and the last reference).

E. A. Martel. The Land of the Causses, Appalachia, vol. 7, 1893, pp. 18-149, pls. 4-13.

Fissure springs:—

A. C. Peale. Natural Mineral Waters of the United States, 14th Ann. Rept. U. S. Geol. Surv., Pt. ii, 1894, pp. 49-88.

William H. Hobbs. The Newark System of the Pomperaug Valley. Connecticut, 21st Ann. Rept. U. S. Geol. Surv., Pt. iii, 1901, pp. 91-93.

Artesian wells:—

T. C. Chamberlin. Requisite and Qualifying Conditions of Artesian Wells, 5th Ann. Rept. U. S. Geol. Surv., 1885, pp. 131-173.

Hot springs and geysers:—

A. C. Peale. Yellowstone Park, Thermal Springs, 12th Ann. Rept. Geol. and Geogr. Surv. Ter. (Hayden), Pt. ii, Sec. ii, pp. 63-454 (many plates and maps).

W. H. Weed. Geysers, Rept. Smithson. Inst., 1891, pp. 163-178.

Arnold Hague and W. H. Weed (on hot springs and geysers of Yellowstone National Park), C. R. Cong. GÉol. Intern., Washington, 1891, pp. 346-363.

W. H. Weed. Formation of Travertine and Siliceous Sinter by the Vegetation of Hot Springs, 9th Ann. Rept. U. S. Geol. Surv., 1889, pp. 613-676, pls. 78-87.

M. Neumayr. Erdgeschichte, vol. 1, pp. 500-510.

Arnold Hague. Soaping Geysers, Trans. Am. Inst. Min. Eng., vol. 17, 1889, pp. 546-553.

John Tyndall. Heat as a Mode of Motion, New York, 1873, pp. 115-121 (artificial geyser).

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