Chapter 5

Fig. 19.—Vertical (structure) section from the Rocky Mountains to Omaha, Nebraska, illustrating a widespread underground porous rock layer (the Dakota sandstone) charged with water under pressure, the clay formation acting as a cap rock. (After Darton, U. S. Geological Survey.)

We still have to consider a third mode of occurrence of waters within the earth. Many formations, like granite and other types of crystalline rocks are neither in definite layers, nor are they sufficiently porous to allow water to really flow through them. Where such rocks extend far down from or near the surface, how does rain water descend? It does so along cracks or fractures (both joints and faults) which we have learned are almost universally abundantly present in all hard rocks in the upper (or zone of fracture) portionof the earth’s crust. Joint cracks are generally very irregular in direction and spacing, while fault fractures are usually fairly regular and straight. Many cracks are not wide enough to allow anything like good passageways for water, while others are sufficiently open to allow water to travel along them for hundreds, or even thousands of feet. In canyons of the West, springs not rarely emerge from the bottoms of great, nearly vertical ledges of granite and other hard crystalline rocks, the waters certainly having entered the rocks hundreds, or even some thousands of feet, higher. In rocks of the kind here considered it is evident, then, that the movements of subterranean waters must be mostly exceedingly irregular and usually not in great quantities. In many deep mines of the world, underground water causes little or no trouble except often near the surface. Occasionally a shaft or tunnel strikes a prominent joint or fault fracture filled with water.

What we might really call underground streams may occur only under exceptional conditions in rocks other than limestone, but in limestone they are not uncommon because the slow solubility of the rock allows underground waters to slowly enlarge the passageways to form distinct channels. Echo River, which flows through Mammoth Cave, is a fine case in point.

Most water by far which emerges as springs, was at one time surface water. A simple, but common case is where rain water soaking through porous soil (e.g., sand) or rock, sinks to the top of an underlying impervious layer (e.g., clay) along whose surface it flows until it reaches the side of a valley where a spring results. In fact, whereverthe water table is crossed by the surface of the ground, water must either seep or flow out. Where underground streams which are common in limestone regions reach the surface on hill or valley sides, springs result. Another source of springs is where under proper conditions of slope a porous rock layer, charged with water well below the surface, appears at a lower level than its source of water. Still another type of spring is where a fissure or fracture crosses a water-bearing layer in which the pressure is great enough to cause the water to rise to the surface along the relatively open fissure or fracture.

In various localities we hear of springs in seemingly paradoxical situations on tops of hills and even mountains. Such a mystery is not difficult to clear up. In the first place, such springs are rarely at the summit of the hill or mountain. A case well known to many persons is the small, but never-failing spring a little below the summit of Mount Whiteface, a peak in the Adirondacks, rising 3,000 feet above the general level of the immediately surrounding country. In this case, a mass of highly fractured rock, subjected to much rainfall and lying above the level of the spring, is sufficiently large easily to contain and give forth enough water to account for several such springs. In rare cases, however, springs or flowing wells are located on summits, and in such places it is only necessary to bear in mind some of the principles above set forth, but mainly the facts that water may travel under pressure long distances underground, and that the point of emergence may be on a hill which is actually lower than the source of the water far away.

The economic significance of underground waters is forcibly brought to our attention when we realize that 75 per cent of the people of the United States depend upon wells for their water supply. Many city supplies, most farm supplies, and much irrigation water come from wells. The 3,000,000 people of Iowa, for example, are dependent upon underground waters from wells varying in depth from a few feet to several thousand feet.

Fig. 20.—Ideal section illustrating the chief requisite conditions of artesian wells. A, a porous stratum; B and C, impervious beds below and above A, acting as confining strata; F, the height of the water level in the porous bed A, or, in other words, the height of the reservoir or fountainhead; D and E, flowing wells springing from the porous water-filled bed A. (After U. S. Geological Survey.)

Most wells are simply dug to depths a little below the water table. In humid climate regions the depths seldom exceed fifty feet. The water encountered in such wells is rarely under pressure. In some regions of deep soils or loose formations, wells are actually bored with an auger to depths of as great as 200 feet. Deep wells in relatively hard rocks are always drilled to depths of even thousands of feet. In such cases the purpose is to strike either a porous rock layer charged with water, or a crack or fissure filled with water, the water almost always being under pressure (sometimes very great), under such conditions. These are called artesian wells whether the water under pressure actually flows out at the surface or not.

We may now inquire as to the necessary conditions for artesian wells. This may best be done bythe aid of diagrams.Figure 20illustrates a very common case where a porous layer, lying between impervious layers, passing under a valley, comes to the surface of the hills on each side where the water enters the porous layer. On sinking a well to the water-charged layer, the water rushes through the hole to a greater or less distance above the surface. InFigure 21the porous and impervious layers are simply tilted, and the water under pressure rises through the free opening to the surface. Wells of this kind are also common in the Atlantic Coastal Plain of the United States. In another case, less comprehensible to the layman, the porous water-bearing stratum curves downward under a hill or mountain, water entering it where it is exposed on each side. Under such conditions a flowing artesian well cannot be drilled at or near the summit, but since the water is under pressure it will rise in the hole to a level approaching that of the lowest part of the outcrop of the porous layer on either side of the hill or mountain. This is essentially the condition of things toward the interior of Iowa, where water from the deeper wells rises 2,000 feet or more in the holes, but does not reach the surface.

Fig. 21.—Section illustrating the thinning out of a porous water-bearing bed. A, inclosed between impervious beds B and C, thus furnishing the necessary conditions for an artesian fountain at D. (After U. S. Geological Survey.)

The drilling of deep wells, where records, including samples of rock materials brought up, have been kept, has been a great aid to the geologist in determining,or rendering more precise, the knowledge of not only the kinds of rocks underground, but also the thicknesses and structural relations of the formations.

In yet another way deep wells are of special significance, that is in regard to the light which they throw upon the subterranean temperature of the earth. Very recently the deepest well in the world was drilled near Fairmont, West Virginia, to a depth of 7,579 feet, in quest of oil or gas. At a depth of 7,500 feet, the temperature was found to be 168 degrees F. Allowing for a near-surface temperature 50 degrees, this means an average rate of increase downward of 1 degree in 62 feet. The second deepest well is near Clarksburg, West Virginia, sunk to a depth of 7,386 feet, with a temperature of 172 degrees at the 7,000-foot level, or at the rate Of 1 degree in 57 feet, allowing for a near-surface temperature of 50 degrees. It is a remarkable fact, that little or no water was encountered all the way down. A well 7,348 feet deep in southeastern Germany gave a temperature of 186 degrees at the bottom, or a rate of increase of 1 degree in 54 feet. These three records are about the average for the deep holes of the world. Next to the deepest mining shafts in the world are in the copper mining region of northern Michigan, where over 5,000 feet (counted vertically, not down the slope) down the temperature is nearly 90 degrees the year round. The rate of increase is here less than in most wells of such depth, because of the cooling air currents. Many years ago a rather remarkable experiment in well drilling was tried by the city of Budapest, Hungary, the attempt being to get a supply of water at the brewing temperature of 176 degrees in orderto encourage the manufacture of beer. After getting a good supply of water at a depth of 3,120 feet and a temperature of 158 degrees, work was stopped. In building the two great tunnels (St. Gotthard and Simplon) through portions of the Alps, such high temperatures were encountered that work was continued only under great difficulties. In the famous Comstock gold and silver mine of Nevada, over forty years ago, temperatures as high as 157 degrees were encountered in the shafts at a depth of only 2,000 feet or a little over, the exceptional temperature for such depth no doubt being due to occurrence of the ores in geologically recent igneous rocks which have not yet cooled to the normal temperature for the depth of 2,000 feet.

From the sanitary standpoint, wells are of very great significance, especially in view of the fact that such a large proportion of people depend upon well water. It is generally understood that typhoid fever is more common in the country than in cities, in spite of what might reasonably be expected. What are some of the causes leading up to such a situation? The idea that water purifies itself after flowing a relatively short distance is, in many cases, far from being true, especially when we are dealing with underground water. Actual observations prove that germ-laden water may travel surprisingly far underground. Germ-laden water from barns, cesspools, or outhouses spreads notably on sinking to the water table and it is easy to see how so many wells become contaminated. On general principles, a geologist is especially wary of water from a well in a barn yard. The well for human use at least should be located out of reasonable range of such contamination. Under the condition of the diagrama well or spring some distance down the side of the hill may actually be unfit for use, though a serious situation is much less likely to develop there. Nor should one assume that by locating the well on the uphill side of the house, and the outhouses or cesspool on the downhill side, safety is assured. From what we have learned in regard to earth movements, and the tilting of strata from their original positions, we know how the movement of water in the saturated zone near the surface may be downhill roughly following the hill slope, while in a tilted porous layer of rock farther below the surface the movement of water may be in just the opposite direction. A well drilled into the solid rock for safety on the uphill side of a house might derive its water from this very same porous layer, whose water has been contaminated from a cesspool or other source down the side of the hill. Such a case is by no means a theory or a rarity. There is also real danger of contamination in cases where the water flows more like streams underground through cracks or fissures in hard or dense rocks, or through channels developed by solutionin limestone. It may happen that water becoming contaminated from barn sites, cesspools, or outhouses finds its way along such a channel to the side or bottom of a well. The author well remembers the case of a farmer whose house, barn, and well were close together on a little limestone terrace and who continued to use the well water although he complained of its disagreeable taste, especially after a rain when he could “taste the barn in it.”

Fig. 22.—Diagram illustrating a danger of contamination of wells by impure underground water. S, soil; B, bedrock; P, porous stratum. Impure water from cesspool moves through porous layer to bottom of well. (By the author.)

Finally, in this connection, it may be said that wells should be located in the light of the principles above explained, best of all upon the advice of some one with geological training, and that, to insure safety to health from the well water, sanitary analyses (at small cost) should be made once or twice a year. A bad well should be abandoned and a new one sunk.

A large amount of money has been wasted upon, and much mystery and superstition has surrounded so-called “water witches,” or those who claim some special or supernatural power of locating supplies of underground water. Most common of all devices used is the so-called “divining rod,” which is a forked stick of willow, witch-hazel, or other wood, according to the seemingly special requirement of the operator. Certain mechanical and electrical devices are also employed. With one fork of the divining rod grasped in each hand and the main part of the stick upright, the operator walks about until, due to some “mysterious” influence, a place is found where underground water pulls the upright portion of the stick downward in spite of the grasp of the holder. Some operators even claim to know just how deep a well must be sunk. Without any attempt to question the honesty of all operators, geologistsare in full accord with the following quotation from a paper published by M. L. Fuller, for the United States Geological Survey: “The uselessness of the divining rod is indicated by the facts that it may be worked at will by the operator, that he fails to detect strong water currents in tunnels and other channels that afford no surface indications of water, and that his locations in limestone regions where water flows in well-defined channels are no more successful than those dependent upon mere guesses. In fact, its operators are successful only in regions in which ground water occurs in definite sheets in porous material, or in more or less clayey deposits, such as pebbly clay or till. In such regions (which are extremely common) few failures can occur, for wells can get water almost anywhere. Ground water occurs under certain definite conditions, and just as surface streams may be expected wherever there is a valley, so ground water may be found where certain rocks and conditions exist. No appliance, either mechanical or electrical, has yet been devised that will detect water in places where plain common sense will not show its presence just as well. The only advantage of employing a “water witch,” as the operator of the divining rod is sometimes called, is that crudely skilled services are thus occasionally obtained, since the men so employed, if endowed with any natural shrewdness, become, through their experience in locating wells, better observers of the occurrences and movements of ground water than the average person.”

It should not be assumed, however, from the above statement that the location or foretelling of underground water is mostly hopeless from a scientific point of view. In most regions the kinds ofrocks which would be pierced by wells can be more or less accurately foretold by careful studies of the rocks exposed at the surface. But foretelling the underground water is often much more uncertain. Where the geologic structure or arrangement of rocks in a region is fairly regular, as in the case of most sedimentary rocks, and a few scattering deep wells have been drilled, with records preserved, the geologist, by combining such data with his surface studies, can do much toward putting the facts regarding the underground waters of the region on a scientific basis. There are many such regions, an excellent case in point being Iowa, regarding which State the United States Geological Survey has published a report containing data by the use of which it is possible to foretell almost exactly what formations would be pierced by drilling from 1,000 to 3,000 feet or more, the thickness of each, which ones are water-bearing and, in many cases, even the character of the mineralization of the water for almost any part of the State. Such knowledge, through the years, is worth untold millions of dollars to the State. Where the rocks are igneous and rather uniformly dense, usually little or nothing can be accurately foretold about the underground water supplies, because in such rocks the water follows exceedingly variable and irregular cracks and fissures. In metamorphic rocks the difficulties are usually about as great. In limestone regions, with humid climate, much water travels in channels underground, but these are so exceedingly irregular that there is no way of locating them by surface studies. In humid climates it seldom happens, however, that a well does not reach at least a fair supply of water within a few thousand feet even in rockformations in which the water travels along irregular cracks and channels.

Certain other important features of the geological work of underground water should be brought to the attention of the reader. One of these is its power to dissolve mineral substances of many kinds more or less rapidly. As already pointed out, limestone is especially susceptible to solution in water, both surface and underground.

The carbonate of lime taken into solution from limestone is the principal substance which causes so-called “hard water.” Most of the solution takes place in the upper part of the zone of fracture of the earth’s crust and the dissolved substances are carried along generally to the lower levels where they tend to deposit (and crystallize), filling fissures, cracks, and even tiny spaces between mineral grains. Cracks and fissures thus filled by mineral matter from solution are called “veins.” In many mining regions valuable ores and other substances have been deposited from underground water solutions and concentrated in veins. In many places underground waters with certain substances in solution travel through various rocks or encounter solutions of other substances and, as a result of chemical action, many new mineral combinations result. Such actions through the millions of years of geologic time have effected great changes in many rock formations. In the case of petrification, like that of petrified wood, the buried organism slowly decomposes cell by cell, and particle by particle it is replaced by mineral matter from underground water solutions. In this manner the remarkable so-called petrified forests (not really forests) of Arizona and the Yellowstone Park wereformed, the petrifying material there having been the very common substance called silica which is the same in composition as the familiar mineral quartz. Mineral matter carried in solution in surface streams is derived from ground waters which reach the surface. An idea of the tremendous quantity of mineral matter thus removed may be gained from the statement that by careful determination the Mississippi River carries 120,000,000 tons in solution into the Gulf of Mexico each year.

Fig. 23.—Structure section and part of landscape in a limestone region showing how caves and natural bridges are formed by the dissolving action of underground water. AA, limestone; BB, sink holes; DD, caves and galleries; and an arch (natural bridge) which is the remnant of a large cave. (After Shaler, U. S. Geological Survey.)

One interesting effect of the dissolving power of underground water in limestone regions is the development of caves or caverns. Most remarkable of all is Mammoth Cave, Kentucky, with its hundreds of miles of passageways and galleries. This marvelous work of nature is all a result of the action of underground water which has dissolved and carried away vast quantities of limestone. Echo River, which flows through the cavern, is still carrying on the work aided by various underground tributaries. The stalactites and stalagmites, which are so strikingly displayed in many caves, as at Luray, Virginia, in which water with carbonate of lime drips or oozes from the roof and, due mainly to evaporation, deposits the lime. Many wonderful and fantasticeffects are thus produced. Where part of the roof of a cave is dissolved out, or falls in, a “sink hole” results. Where all but a portion of the roof of a cave or underground channel has fallen in, a natural bridge, like the famous one in Virginia, results, though natural bridges are also formed by other means.

In concluding this chapter we shall briefly discuss hot underground waters, hot springs, and geysers. There are two well-known ways by which underground waters may become heated. One is by the movement of water downward into the normally heated portion of the earth, the rate of increase downward being, as above stated, 1 degree F. for about 50 to 60 feet. Water descending two miles would, therefore, attain a temperature of about 200 degrees F. In some regions such a temperature may be reached at depths considerably less. Such water (under pressure) taking a short course to the surface (forming springs) at a lower level would retain much of its heat taken up far below the surface. In regions where there are great down-folds of the strata (i.e., synclines), as in the central to southern Appalachians, conditions appear to be favorable for such warm or hot springs, as, for example, at Hot Springs, Virginia. A second cause of the heating of underground water is by the descent of surface waters into contact with masses of still hot igneous rock of relatively recent geologic age. In some such cases the water does not go more than some hundreds of feet down and when, under proper conditions, it returns to the surface hot and even boiling springs may result.

Plate 7.—An Upbend Fold(anticline)in the Appalachian Mountain Strata Near Hancock, Maryland. The strata were deposited in horizontal layers upon the sea bottom, covering the region many millions of years ago in middle Paleozoic time. At the time of the Appalachian Mountain revolution, near the end of Paleozoic time, this and many other folds developed well below the surface. Removal of overlying material by erosion has laid bare the fold as we see it to-day. (Photo by Russell, U. S. Geological Survey.)

Plate 8.—(a)A Ledge of Igneous Rock(Granite)in Northern New York.This illustrates so-called “joints” or natural cracks, commonly separating most hard rock masses into more or less prismatic blocks. (Photo by the author.)

Plate 8.—(b)A Fault Fracture in a Ledge at East Canada Creek in the Mohawk Valley, New York.The Ordovician limestone formation in thin layers on the right has sunk hundreds of feet along vertical fault to the left of middle, bringing it sharply against the older (Cambrian) massive formation on the left. The hole is artificial. (Photo by Darton, U. S. Geological Survey.)

Geysers are periodically eruptive hot springs found only in a few of the volcanic regions of the world. They are most wonderfully displayed in the Yellowstone National Park, where they send columns of hot water to all heights up to 250 feet at various intervals of time. Almost incredible amounts of hot water are sent into the air every day in the geyser basins of Yellowstone Park. The single geyser “Old Faithful,” which erupts at intervals of about seventy minutes, sends a column of water several feet in diameter to heights of from 125 to 150 feet. During each eruption about 1,500,000 gallons of water are sent forth, or every day enough to supply the need of a fairly large city. A very brief explanation of the cause of geyser eruptions may be stated as follows: The very irregular, narrow, geyser tube extends nearly vertically downward into yet uncooled lava. The tube is more or less rapidly filled by underground water. The bottom, or near-bottom, portion of the water gradually becomes heated by the lava until finally the boiling point is reached for that depth. But, because of the pressure of the overlying water column, the boiling point at that depth is considerably greater than for the surface. A little steam develops far down and this causes the whole column of water above it to lift slightly, thus relieving the pressure on the superheated water far down. This relief of pressure allows much of the superheated water far down to flash into steam, which violently forces the column of water out of the geyser tube.

CHAPTER X

HOW MOUNTAINS COME AND GO

MOUNTAINS constitute the grandest relief features of the earth, and some of the most profound lessons of earth changes may be learned by studying them. To the layman who views great mountains in all their grandeur and massiveness, the expression “everlasting hills” seems appropriate. But the geologist knows that even the loftiest mountains are only temporary features on the face of the earth. Like organisms, they come and go. For example, where the great Rocky Mountains now stand was only a few million years ago (in late Mesozoic time) the bottom of an interior sea. Where the Appalachians now stand there were no mountains late in the Paleozoic era (not less than ten or twelve million years ago), but instead sea water covered the district. Then the Appalachians were formed, lifting their heads much higher than at present, after which they were cut down almost to sea level, and then once more upraised. The Coast Range Mountains of our Pacific Coast have come into existence since the middle of the present (Cenozoic) geologic era. Every mountain, like every organism, has a life history, in some cases simple, and other cases complex. All pass through stages of birth, youth, maturity, old age, and death. Some rear their heads and disappear after a short (geological) existence. Others continue their growthand persist much longer, while still others undergo periods of profound rejuvenation.

Among the various processes by which mountain ranges have been formed, the folding and accompanying general uplift of strata are the most important. In fact, in most of the great mountain ranges of the world the folded structure is conspicuously developed, so much so that they may well be called “folded mountains.” Very commonly, however, mountains of this type have also been subjected to more or less fracturing of the rocks (faulting), and not uncommonly they have also been subjected to igneous activity, including both intrusion and extrusion of molten material. It is among the folded mountains of greater or less degree of complexity that the “greatest exhibitions of geologic phenomena are seen and the lessons which geology as a sciences teaches may be learned. If one desires to know the history of a region, one turns naturally to its mountain ranges, for here may be found the upturned and dissected strata, a study of whose kinds, thickness, and fossils throws light upon past events, while their foldings and dislocations show the nature and results of those great dynamic agencies which, from time to time, have operated upon the outer portion of the earth, and given to it the broad distinctive features which characterize it to-day.” (L. V. Pirsson.) Among the great mountains we may also see wonderful exhibitions of the results of weathering and erosion, especially the work of rivers and glaciers.

We can, perhaps, best convey to the reader some of the main facts and principles regarding folded mountains by considering certain observations which may be readily made in a short trip across afolded range of not too complex kind—for example, across the Appalachian range along the line of the Baltimore and Ohio Railroad, west of Washington, or the Pennsylvania Railroad, west of Philadelphia. It would be most evident that the mountains consist of strata, that is sedimentary rocks, such as sandstone, shale and limestone, which were deposited under water. A few measurements would reveal the fact that thousands of feet in thickness of strata are represented. Careful measurements by geologists have, in fact, shown that the strata were originally piled up layer upon layer to a thickness of 25,000 to 30,000 feet. The fact that they are strata of such great thickness proves that sediments must there have accumulated under water for some millions of years at least. Closer examination of a few good exposures (i.e., outcrops) would further reveal the presence of fossil shells and impressions of marine organisms, thus definitely leading us to conclude that the strata were accumulated under sea water, which, of course, means that the present site of the mountain range was once sea floor.

Examination of the rock materials also establishes the fact that the strata are such as were deposited in relatively shallow sea water—that is to say, none are at all of the sort which are now forming under really deep ocean water. Most of the strata represent original sands (and even gravels) and muds which could have accumulated only relatively near shore, that is within about 100 miles, which harmonizes with a statement made in a preceding chapter to the effect that very little land-derived sediment is at present depositing more than 100 miles out from shore. The coarse materials(sands and gravels) could not, of course, be carried many miles out, while many of the strata are covered with ripple marks, thus positively proving their shallow-water origin. We conclude, therefore, that the Appalachian strata are of marine, shallow-water origin. But we have already stated that these strata are at least 25,000 feet thick. How, then, do we reconcile these two seemingly paradoxical statements? All that is necessary is to realize that the floor of the shallow sea, in which the sediments eroded from adjacent land were being deposited, slowly, though more or less irregularly, subsided or sank during the long ages (millions of years) of their accumulation. It would carry us too far afield to really attempt an explanation of this remarkable type of geologic phenomenon, and it must suffice to suggest that, starting with the earth’s crust in equilibrium, the very weight of accumulating strata would tend to destroy that equilibrium and so cause subsidence.

In our trip across the mountains it would be strikingly evident that the strata are no longer in their original horizontal position, as they were piled up layer upon layer, but that they have been notably disturbed and thrown into folds (Plate 7), large and small, some masses of the strata having been bent upward (anticlines) and others downward (synclines). Such folded structures could have been developed only by a great force of lateral compression in the earth’s crust within the zone of flowage. Under compression the strata were mashed together, notably bent into curves (folds), and more or less upraised. It would also be readily observed that the main axes of the folds extend essentially parallel to the main trend of the mountain range,thus proving that the force of compression was exerted at right angles to the trend of the range.

Fig. 24.—Diagrammatic sections illustrating the development of a typical folded mountain range. Upper figure: A, the old land eroded to furnish sediments deposited under the adjacent sea at C. Middle figure: strata (C) folded as they would appear if unaffected by erosion, and a down-warp (B) between A and C. Lower figure: condition after profound erosion, and filling of B with sediment. (Drawn by the author.)

Using a biological analogy, a brief history of a typical folded mountain range may be stated as follows: First, there is the prenatal or embryonic stage when the materials of the range are gathering, that is when the sediments are piling up layer upon layer relatively near shore on a sinking sea bottom. Next comes the birth of the range when, due to the great lateral compressive force, the strata are thrown into folds and forced to appear above sea level, the range thus literally being born out of the sea. During the next, or youthful stage, therange grows (with increasing altitudes) because of continued application of the compressive force. Even during the youthful growing stage weathering and erosion attack the range and tend to reduce it. Then comes the stage of maturity, when the upbuilding (compressive) force and the tearing down (erosive) force about counterbalance each other. At this time the range has reached its maximum height and ruggedness of relief, with ridges and valleys higher and deeper than at any other time. The old-age stage sets in when the upbuilding power wanes or actually ceases, and erosion dominates or reigns supreme. Slowly but surely, unless there be a renewal by an upbuilding power, the range is cut down until little or nothing of it remains well above sea level, or, in other words, until a peneplain is developed. This last stage may truly be called the death of the range. Usually, however, some local portions of the disappearing range, which are more resistant or more favorably situated against erosion, are left standing to at least moderate heights above the general level of the plain of erosion.

The above normal order of events may be disturbed at any stage, especially after maturity, by renewed uplift when the streams are revived in activity and increased ruggedness results. Even after the whole range as a relief feature has been planed away, the site of the range may be uplifted and a new cycle of erosion started.

By the use of two well-known examples we shall not only illustrate the above principles of mountain history, but also show that no less than a few million years must be allowed for the growth and decay of a great folded range. During the last (Permian) period of the Paleozoic era the Appalachian stratabegan to buckle and the yielding to pressure continued till well into the succeeding (Triassic) period. The climax was reached about the close of the Permian. Then, throughout the Mesozoic era, erosion reduced the central Appalachians to a great plain (peneplain) near sea level, after which, about the beginning of the present (Cenozoic era), the site of the former range was distinctly upraised (without folding of the rocks), causing the revived streams to begin their work of carving out the present ridges and valleys, this work still being in progress.

In the case of the Sierra Nevadas, the strata were folded into a lofty mountain range relatively late in the Mesozoic era and, by the middle of the Cenozoic era, the old-age stage of erosion was well advanced when the range was not more than a few thousand feet high. Then (in the middle of the Cenozoic era) uplift, accompanied by faulting on a large scale, but not by folding, took place, and the range was notably rejuvenated to about its present height. All the remarkably deep canyons of the Sierras have been carved out since the rejuvenation.

How is the geological birthday of a mountain range determined? In the preceding paragraph we stated that the Appalachians were folded and born out of the sea about the close of the Paleozoic era. This is readily proved by calling attention to two facts. First, the youngest strata involved in the folding are of Permian, or late Paleozoic Age in the geologic column, as proved by their fossil content, etc., and obviously the folding must have taken place after they had been deposited. Clearly, then, the folding could not have taken place before very late Paleozoic time. Second, the oldest strata resting upon the folded rocks are of early (not the very earliest) Mesozoic Age, and these strata are somewhat tilted but not folded. Obviously, then, the folding must have occurred before the nonfolded strata were deposited, which means that the folding must have been essentially completed in not later than early Mesozoic time. Or, in the case of the Rocky Mountains, we know that strata were there folded late in the Mesozoic era or very early in the Cenozoic era, because folded rocks as late in age as late Mesozoic (Cretaceous) have resting upon them, in some places, nonfolded strata of early Cenozoic (Tertiary) Age. The figure clearly shows how the Ordovician strata must have been folded before the next (Silurian and Devonian) strata were deposited upon them in southeastern New York.

Fig. 25.—Diagram illustrating the topography and folded structure of the Appalachian Mountains west of Harrisburg, Pennsylvania. The valleys have been etched out of belts of weak rocks, while outcropping resistant rocks stand out to form ridges. Note the course of the Susquehanna River across the mountain ridges, this being a “superimposed river” (see text,p. 233). (Drawn by A. K. Lobeck.)

Fig. 26.—Only slightly tilted strata of Silurian and Devonian ages resting upon folded strata of Cambrian and Ordovician ages in an east-west section across the Catskill Mountains and Hudson Valley of New York. The folding took place at the time of the Taconic Mountain Revolution toward the end of the Ordovician period. (Drawn by the author.)

As already suggested, however, folding is not the only method by which mountains are formed. Many ranges are either entirely due to the tilting of earth blocks by faulting or fracturing of the earth, or their present altitude, at least, is a direct result of faulting. Such may be called block mountains. They are wonderfully represented by the various north-south ranges rising some thousands of feet above the general level of the Great Basin region of Utah and Nevada. These ranges are, in short, somewhateroded edges of approximately parallel-tilted fault blocks lying between the Sierra Nevada Range and the Wasatch Range. In southeastern Oregon a series of nearly parallel block mountains, up to forty miles in length and over 1,000 feet in height, show very steep eastern fronts only slightly modified by erosion.

Another mode of origin of mountains is by the rise of molten material to the surface, especially where a chain of volcanoes is located. Thus the Cascade Mountains from northern California through Oregon and Washington, including Mounts Lassen, Shasta, Pitt, Baker, St. Helens, and Rainier, are very largely the result of volcanic action. The long chain of Aleutian Islands of Alaska, referred to in our study of volcanoes, is an excellent example of a great mountain range now being built up out of the sea by volcanic action. More locally molten rocks under pressure may not reach the surface but instead simply bulge or dome the strata over them, as in the case of the group known as the Henry Mountains of Utah, and also in other parts of the West.

In still other cases mountains of considerable area and altitude have resulted from erosion of uplifted regions where the uplift has been practically unaccompanied by either folding, faulting, or igneous activity. Any low-lying area, regardless of the character of its rocks, structure, or previous history, may be notably upraised and simply subjected to erosion. An excellent illustration is afforded by the Catskill Mountains of New York, where numerous deep valleys and narrow ridges have been carved out of upraised nearly horizontal strata. The so-called “Bad Lands” region of parts of SouthDakota and Wyoming is also essentially of this type, where deep, narrow valleys and sharp ridges have been etched out of high, relatively soft, nearly horizontal strata, resulting in an almost impassable maze of mountains. In the high, recently upraised Colorado Plateau of parts of Arizona, New Mexico, Colorado, and Utah, nearly horizontal strata are being etched out, the result being numerous buttes, mesas (flat-topped hills and mountains) and deep canyons, including the Grand Canyon with its maze of peaks and pinnacles, many of them rising like mountains out of the canyon depths.

Mountains of the pure types just described are not the prevailing ones of the earth. Most mountains and their structures, as we see them to-day, are products of two or more of the processes of folding, faulting, igneous action, and erosion. A few well-known examples will suffice to make this matter clearer. Thus, the Appalachian Mountains originally developed by severe folding of thick strata. After considerable erosion, numerous small and large thrust faults developed, some of the dislocations amounting to miles. Then the whole range was cut down nearly to sea level by erosion, after which the district was upraised (without folding) mostly from 2,000 to 4,000 feet, and the present long, narrow mountain ridges and valleys have been carved out by stream erosion. Thus folding, faulting, and erosion all enter into the height and structure of the Appalachians.

A lofty mountain range still more complex in its history is the Sierra Nevada of California. First, thick strata were highly folded, upraised, and intruded by great masses of molten granite. Erosion then proceeded to cut the range down to hills, afterwhich a great fracture (fault) developed along the eastern side and the Sierra Nevada earth block was notably tilted with steep eastern front and long western slope. Erosion has considerably modified the eastern fault face, and the deep canyons like Yosemite, King’s River and American River, have been carved out of the western slope of the great tilted fault block. Geologically recently the central to northern portion of the range has been affected by volcanic action, streams of lava in some cases having flowed down the valleys.

CHAPTER XI

A STUDY OF LAKES

LAKES are ephemeral features on the face of the earth. Compared to the tens of millions of years of known earth history, lakes, even large ones, are very short lived. They may, in truth, be regarded as merely results of the temporary obstructions to drainage. Lake basins are known to originate in many ways, and there are various means by which they are destroyed. Not attempting an exhaustive, scientific treatment of the subject, our present purpose may be well served by describing and explaining some of the better known and more remarkable lakes of the world.

Even a cursory examination of a large map of the world reveals the fact that the regions of most numerous lakes are those which were recently occupied by glaciers—either the vast ice sheets of the Glacial epoch or mountain (or valley) glaciers. This is because more lakes of the present time have come into existence as direct or indirect results of glaciation than by any other cause. A considerable number of these lakes occupy rock basins which have been eroded or excavated by the direct action of flowing ice. Small lakes of this sort are commonly found in the upper parts of valleys formerly occupied by mountain or so-called Alpine glaciers, because there the excavating power of such glaciers was especially effective. More rarely rock basinshave been scoured out by glaciers farther down their valleys. Many lakes occupy rock basins excavated by ice in the high Sierra Nevada and Cascade Ranges, in the Rocky Mountains from Colorado into Canada, in the Alps, and in the mountains of Norway. Few, if any of them are, however, large or famous. Other lakes, some of very considerable size, occupy rock basins scoured out by the passage of the great ice sheets of the Glacial epoch in North America and Europe, though they are less common than formerly supposed. Some of the many lake basins of Ontario, Canada, are quite certainly of this origin, as might well be expected, because the power of the great ice sheet was there in general notably greater than south of the Great Lakes where the tendency was to unload or deposit the eroded materials as shown by the great accumulations of glacial débris (moraines).

Where the ice walls of certain existing glaciers form dams across valleys, waters are ponded, a small lake of this kind occurring alongside the Great Aletsch Glacier of the Alps, where its wall is slowly moving past a tributary valley. Lakes of this kind also occur in Greenland and in Alaska, but none are of considerable size. During the Great Ice Age, however, literally thousands of large and small lakes were formed, both during the advance and the retreat of the ice, wherever the glacier wall blocked valleys which sloped downward toward the ice. New York State furnishes many fine examples of large and small lakes of this sort. Thus, when the great glacier was melting in northern New York, waters hundreds of feet deep and many miles long were ponded between two ice lobes—one retreating eastward and the other westward from the MohawkValley. An ice dam lake was also formed a little later, when an ice wall blocked the northern part of the Black River Valley just west of the Adirondack Mountains and caused a lake covering about 200 square miles. One of the largest of all known ice dam lakes has been called Lake Agassiz, which attained a maximum length of over 700 miles and a width of 250 miles in the Red River of the North region of eastern North Dakota, western and northwestern Minnesota, and northward into Canada, most of its area having been in Canada. It began as a small lake with southward drainage into the Mississippi when the great northward retreating ice sheet formed a dam across the valley of the Red River of the North. The retreating ice continued to block the northward drainage until the vast lake, covering a greater territory than all of the present Great Lakes combined, was developed. Beaches, bars, deltas and the outflow channel of this remarkable lake are wonderfully well preserved. Lake Winnipeg is a mere remnant of great Lake Agassiz.

Many ponds and small lakes occupy basins formed by irregular accumulations of glacial (morainic) materials. Still others lie in depressions which formed by the melting of masses of ice which became wholly or partly buried by ice deposits, or by sediments washed into bodies of water which were held up by ice dams. Depressions of the latter kind are commonly found as pits or so-called “kettle holes” below the general level of sand flats or sand plains of glacial lake origin.

Most common of all lake basins of glacial origin are those formed by accumulation of glacial débris or morainic materials acting as natural dams acrossvalleys. This is, in fact, the most common of all ways by which existing lake basins, some of them very large, have been formed. Most of the thousands of ponds and lakes of Minnesota, Wisconsin, and northern New York belong in this category.

In the Adirondack Mountains, for example, most of the lakes, like the well-known Lake Placid, Saranac Lakes, Long Lake, and Schroon Lake, have their waters ponded by single dams of glacial débris across valleys. In some cases a series of such dams blockades a valley and forms a chain of lakes like the well-known Fulton Chain in the Adirondacks. Less commonly the lake may have its waters ponded by two natural dams of glacial débris, one across a valley at each end of a lake. A very fine, large scale example of the last-named type is the famous Lake George in the southeastern Adirondacks. It is over 30 miles long and from 1 to 21/2miles wide. It lies in the bottom of a deep, narrow mountain valley, mountain sides rising very steeply from a few hundred feet to 2,000 or more feet above its shores. There are many islands, especially in the so-called “Narrows,” thus greatly enhancing the scenic effect. The valley itself has been produced by a combination of faulting and erosion. There was a preglacial stream divide at the present location of the “Narrows.” This divide was somewhat reduced by ice erosion when the deep, narrow body of ice plowed its way through the valley during the Ice Age. During the retreat of the ice heavy morainic accumulations were left as dams across the valley at each end of the lake.

Another remarkable body of water, similar to Lake George in its origin, is Chautauqua Lake of western New York, famous for its Chautauqua assemblies.It lies 1,338 feet above sea level, with its northern end near the edge of the steep front of the plateau overlooking Lake Erie. Chautauqua Lake really consists of parts of two valleys, one sloping north and the other sloping south, each dammed by glacial deposits.

The famous Alpine lakes—Garda, Como, and Maggiore—have resulted from deposition of glacial morainic materials under conditions different from those above described. In these cases great mountain or valley glaciers once flowed down the valleys and spread out part way upon the Italian plain. Great accumulations of glacial débris took place around the borders of the glacier lobes, and, after retreat of the ice, the glacial deposits acted as dams ponding the waters far back into the mountain valleys.

The origin and history of the Great Lakes constitutes one of the most interesting and remarkable chapters in the recent geological history of North America. Most of the salient points have been well worked out and they may be very briefly summarized, as follows: Before the Ice Age the Great Lakes did not exist, because the region, prior to that time, had been land subjected to erosion for millions of years—a time altogether too long for any lake to survive. Their sites were occupied by broad, low, stream-cut valleys which were quite certainly locally somewhat deepened by ice erosion during the Ice Age. Ice erosion is, however, altogether insufficient to account for the great closed basins. The two most important factors entering into the formation of the basins of the Great Lakes were doubtless the great glacial (morainic) accumulations acting as dams along the south side, and the tilting of theland downward on the north side of the region. In support of this explanation it has been established that the great dumping ground of ice-transported materials from the north was in general along the southern side of the Great Lakes and southward. It has also been well established that, late in the Ice Age, the land on the southern side of the Great Lakes region was lower than at present, as proved by the tilted character of beaches of the well-known extinct glacial lakes which were the ancestors of the present lakes. Such a down-warp of the land must have helped to form the closed basins by tending to stop the southward and southwestward drainage of the region.

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