Stories in Stone

THE Quaternary period is the last of the geologic ages, the one in which we live. It includes the Pleistocene epoch, known generally as the Great Ice Age, in which primitive man appeared, and the Recent epoch, in which the human race attained supremacy over all other living beings.

Let us remember that Grand Canyon is cut in old beds of rock, Zion Canyon in beds of middle age, and Bryce Canyon in younger beds. But although the beds were laid down in times far apart, the forms and features of the land in all three canyons were produced by recent erosion.

Most of the land forms in all our national parks and elsewhere, which constitute the impressive scenery of the West, were produced by Pleistocene erosion or in part by Recent erosion, though several of the show places of America are notably Pleistocene. Perhaps first among these should be mentioned Glacier National Park (Plate XXXV), for its glaciers may be regarded as survivors from the Great Ice Age.

Yosemite Valley also is distinctively Quaternary, for its wonderful scenery was shaped by the glacial ice which once filled it. Conspicuous highlands like those of the Rocky Mountain National Park, whose charm depends on mountain peak and gorge, are doubtless due chiefly to Pleistocene action, although their shaping may have begun in an earlier epoch.

Glacier National Park

Glacier National Park, in northern Montana, although famous for its rugged mountains and gem-like lakes, is probably best known for its glaciers, some sixty in number, and for its steep-walled gorges, which were shaped by the powerful precursors of these relatively small bodies of ice, chiefly at a time when ice filled the valleys and covered much of the mountainous area.

Yosemite National Park

The ability of a stream of ice to shape a rock gorge is shown clearly in Yosemite Valley (Plate XXXVI). Those who have been entranced with its beauty and grandeur will doubtless agree with John Muir, who describes it as including—

innumerable lakes and waterfalls and smooth, silky lawns, the noblest forests, the loftiest granite domes, the deepest ice-sculptured canyons, the brightest crystalline pavements, and snowy mountains soaring into the sky twelve and thirteen thousand feet, arrayed in open ranks and spiry-pinnacled groups, partially separated by tremendous canyons and amphitheaters; gardens on their sunny brows, avalanches thundering down their long white slopes, cataracts roaring, gray and foaming in the crooked rugged gorges, and glaciers in their shadowy recesses, working in silence, slowly completing their sculptures; newborn lakes at their feet, blue and green, free from or encumbered with drifting icebergs like miniature Arctic Oceans, shining, sparkling, calm as stars.

This wondrous valley is the work of water and ice. It was once a tortuous river valley, in which the Merced River cut deep into the rocks. Then the valley was filled with ice, which broadened it, cut away its irregularities, and finally left it in the form which now delights the eye of every observer.

Rocky Mountain National Park

Striking natural scenes, great as well as small, may be the results of relatively recent erosion. The great ridges, peaks, and canyons of our western mountains, as well as the smaller features of the landscape, were shaped chiefly in the Quaternary period.

Impressive mountains and valleys are found in many of our national parks, but one park especially has been created as representative of the finest scenery in the Rocky Mountain region. On June 26, 1915, a part of the Front Range in Colorado, including Long's Peak (Plate XIII), which rises to an altitude of 14,255 feet, was set aside as the Rocky Mountain National Park. The appeal of this park is varied. Here the visitor may find many modern conveniences, or he may bivouac by rock and stream in the primitive forest.

Like Bunyan's pilgrims, who in the course of their progress came to the "delectable mountains," the modern pilgrims come—234,000 of them, in 1925—to find their hearts' desire in the delectable mountains of the Snowy Range. Here they realize the dreams that illuminated the weeks and months of vacation planning.

The Snowy Range is a part of the Continental Divide. Here craggy peaks and rock-ribbed gorges form many an inspiring scene. The mountain monarchs stand in calm dignity and stalwart nobility, cloud-encircled and snow-crowned.

This noble range is a succession of lofty summits that rise above the altitude at which trees can live. Several glaciers, preserved in sheltered gorges, remind us of the time, thousands of years ago, when ice filled all the gorges in the higher parts of the range.

The Rocky Mountain National Park is noted chiefly for the variety of its mountain landscape. The land forms are results of the action of stream and frost and ice as they carved the granite mass into peak and gorge.

A time there was when the waves of an open sea rolled unhindered by rock or shoal where Long's Peak now stands. When the mountains arose out of the sea, the covering of sedimentary rocks was eroded away and the underlying crystalline rocks were attacked and carved into the delectable mountains.

Caverns

Caverns have been mentioned as the fourth type of natural objects that may be regarded as distinctively Quaternary. America is rich in caverns. Many of them have been explored; others have never been entered.

Three types of caverns may be noted: those consisting of great, complicated solution cavities in limestone, slightly adorned, like Mammoth Cave in Kentucky; those represented by Carlsbad Cavern in New Mexico (Plate XXXIX), in which the solution cavities are so generally filled with onyx marble that they are attractive chiefly because of their adornments; and those formed in igneous rock during the closing stages of volcanic eruption, when molten rock flowed from beneath a solidified crust, leaving long, irregularly shaped subterranean galleries. As scoriaceous lava is a poor conductor of heat, ice accumulates in these caverns, so that many of them are known as ice caves. An example is one in New Mexico known as the Cerro de la Bandera ice cave.

Mammoth Cave

Probably every reader of this account knows something of Mammoth Cave. Many people have visited it and many others recall the wonder aroused by the meager accounts of this cave in school geographies.

Mammoth Cave, a cavern in limestone in south-central Kentucky, is the best known of the many caves in this land of Daniel Boone. In fact, the limestone over an area of some 8,000 square miles is so honeycombed by solution that surface streams are rare. The surface waters find their way through sink holes into underground streams, which thread their way through ramifying subterranean passages. It has been estimated that Kentucky has 100,000 miles of open caverns.

Mammoth Cave is entered through an inconspicuous opening on a forest-covered hillside overlooking Green River. Inside the opening the visitor pauses for a few minutes to light the lamps and get his "cave eyes." The first scenes of interest cluster about the old saltpeter vats, where nitrate was obtained for the gunpowder that saved America in the War of 1812. Many of the vats, which are built of split logs, have stood unchanged for more than 100 years and are still filled with the "peter dirt" from which the nitrate was leached.

At the inner end of the line of vats is a large open space known as Booth's Theatre (Plate XLI). This great unadorned space may be regarded as typical of Mammoth Cave. It is "grand, gloomy, and peculiar." The cave is mammoth but not beautiful. Its avenues are endless but unadorned. Mile after mile and hour after hour we walk between barren walls of limestone.

In the lowest chambers of the cave, 360 feet underground, is Echo River and its neighbor, the River Styx. Although there is nothing beautiful about them, these streams seem to have a peculiar fascination for visitors. The fascination seems to be due to the repulsive ugliness of the weird scenes. During times of flood the muddy back water from Green River enters the cave and deposits mud, which remains slimy for a long time after the flood waters have subsided.

The weird fascination of the place is enhanced by the blind fishes in the water and the presence on the walls of eyeless insects making their way over the rocks by means of long sensitive feelers. The timid, here, are wont to cower in fear, and the bolder spirits to recall Coleridge's lines

"Where Alph, the sacred river ran
Through caverns measureless to man
Down to a sunless sea."

PLATE XXXIII. A VOLCANIC CONE ON MOUNT SHASTA, CALIFORNIA. On this isolated peak (14,161 feet high) the storm clouds deposit snow which forms mountain glaciers. In the midst of the ice fields stands a volcanic cone of recent origin, built up about some vent from which lava was ejected. Here glacial frost and volcanic heat carry on an endless struggle for supremacy. Official photograph U. S. Army Air Service.

Probably the most interesting parts of Mammoth Cave are those that exhibit the so-called domes. The surface waters that make their way downward into the soluble limestone dissolve passageways, which which in some places are of magnificent proportions, as at Hovey's Cathedral (Plate XXII), in the new entrance to Mammoth Cave. These domes reach their climax in Mammoth Dome, a group of solution cavities which H. C. Hovey estimated to be 400 feet long, 150 feet wide, and 250 feet in maximum height.

Mammoth Cave has many neighbors which, although less widely known, are no less interesting. Some of these are in places adorned with furnishings of onyx marble. One of the most impressive of the adornments this group of caves is that known as Frozen Niagara (Plate XLIII), a mass of cave marble about 60 feet high.

Carlsbad Cavern

The most impressive of American caverns, the Carlsbad Cavern, in southeastern New Mexico, has only recently come prominently into notice. Long known locally, its unusual nature was disclosed to the readers of the National Geographic Magazine in January, 1924. Later in the same year the National Geographic Society sent an expedition under the leadership of the writer of this account to explore and study the cavern. The results were announced in the magazine in September, 1925.

Carlsbad Cavern has nothing to do with Carlsbad in central Europe. It is in southeastern New Mexico, in the foothills of the Guadalupe Mountains. The astonishing nature of this cavern was discovered by examinations made of proposed sites of reservoirs on Pecos River. The rocks that cause the reservoirs to leak contain the great cavern.

The first work of the explorers of this cave was done on the trail. Rough places were smoothed out and improvements were made that enabled us to reach the starting place of our work in little more than an hour's time, instead of two hours, as before. The chambers previously explored were used as points of departure for new discoveries. From them we pushed our way into the unknown parts.

We emulated Tom Sawyer in the use of kite strings. We used stretches of white twine, which were left as permanent markers. In time we had laid out a system of avenues marked with white twine that led to the exit.

Allow me now to take you on a personally conducted tour through this new wonderland. It must be a flying trip, for it would take days to examine all the strange features.

The cavern is approached from Carlsbad over a plain and up a mountain side to an opening where the roof collapsed. Near this opening a shaft had been dug and fitted with a windlass and wire rope having an iron bucket at the end. We climb into this bucket, the floor at the top of the shaft opens, and we descend 170 feet into Stygian darkness. Those who enter in the future will miss the ride on this bucket elevator, for a stairway has now been built in the natural opening.

After spending a little time in adjusting the lanterns and in getting our "cave eyes" we take the trail, picking our way over and among great blocks of rock that have fallen from the ceiling. We pass into a great opening, climbing up and down and over and finding our way around blocks of rock that have fallen from the ceiling, which is 250 feet above us, so far above that the lanterns only dimly illuminate it.

Three quarters of a mile from the entrance and more than 800 feet under the surface of the ground we reach the roughest place on the trail. Here were found parts of a human skeleton. Some cave man had lost his way and perhaps fell from rock to rock in the darkness to the shelf where his earthly career ended.

The spectacular part of the cavern begins just beyond this difficult part of the trail.

The first chamber to be entered is what is called Shinav's Wigwam, for the names of the features of the Cavern have been taken from Indian mythology. This chamber is nearly circular in outline, 200 feet across, 75 feet high, and wonderfully adorned. At the entrance to this glorified wigwam of the Navajo's wolf god hangs a large stalactite of gnarled appearance, which resembles a cave man's war club.

Around the walls of the wigwam are alcoves and niches and tributary chambers of marvelous character and amazing adornment. Had the author of "The Arabian Nights" seen Carlsbad Cavern he might have enriched his tale of Aladdin and his lamp with facts stranger than his fictions.

One of the most beautiful of these tributary chambers was discovered by an explorer who crept through a small hole in the wall of the Wigwam. Beyond the thin partition in which he found this hole a surprisingly spectacular view never before seen by human eye met his astonished gaze.

The new chamber thus discovered was named Avanyu's Retreat, for the wise serpent of Indian mythology, who is said to have lived in the waters of the underworld and who insisted on attending the councils of the gods.

We must not linger long in the Wigwam, for the most spectacular part of the cavern lies beyond. To reach it we climb over a pile of rocks 183 feet high, heaped on the floor of an enormous vault whose ceiling rises 350 feet above the floor. The inner slope of this hill leads down through a decorated archway into the Big Room, a great cavity half a mile long surrounded by tributary chambers and corridors, alcoves and niches.

These tributary avenues were searched in the hope of finding an exit through which an easy entrance to the cavern might be constructed. The search led to many new discoveries—too many to be even mentioned here.

Some of the most interesting discoveries were made in the basement of the cavern. A series of chambers was found 90 feet below the floor of the Big Room. The guide was lowered first on a rope through a hole in the floor. In the uncertain light of his kerosene torch, what he saw looked strange and unreal. All went well, however, until he reached water at the bottom of the hole. To his perturbed mind this seemed like an ocean. His frantic signals to be raised were misunderstood, and he was dropped unceremoniously into what proved to be a fountain. With his light gone, he passed an unhappy moment in the darkness before he discovered that he could touch bottom.

Later a wire ladder 90 feet long was built and lowered into the hole. The lower 75 feet of the ladder swung clear of the wall. A wire ladder thus suspended has an erratic disposition and an obstinate nature. Those who first descended were swayed and spun about in the darkness, and some whose nerves were weak had a sorry time.

The newly discovered chambers are extensive and wonderfully decorated. It is quite impossible to describe them. The only way to realize their marvels is to see them.

Carlsbad Cavern is believed to be the most spectacular underground wonderland in America, if not in the whole world. However, it may be excelled by some unexplored caverns in the Guadalupe Mountains.

For spacious chambers and for variety and striking beauty of decoration, Carlsbad Cavern is king of its kind.

The observer finds here many a stalagmite of impressive proportions and many a stalactite hanging gracefully from the decorated ceiling. The titanic proportions of some of the formations of cave marble, the grace and beauty of others, and the weird appearance of still others, suggesting gnomes and fairies and unearthly scenes, all combine to make the cavern a place of wonder, impressive and unforgetable.

Carlsbad Cavern is the work of water. Like many another well-known cave, it was made by the solution and removal of parts of the rocks. It differs from others in that the limestone rocks here contain beds of gypsum and rock salt. Here through long ages the underground water dissolved and carried away soluble material, leaving a great cavity deep down under the highlands.

PLATE XXXIV, A (top). "THE CORYPHODON WAS HORRIFIED." A restoration by Charles R. Knight of Coryphodon testes, a short-toed hoofed mammal whose fossil remains are found in rocks of Eocene age in Wyoming. By permission of the American Museum of Natural History. PLATE XXXIV, B (bottom). THE MAMMOTH, A RESTORATION BY CHARLES R. KNIGHT. Elephants of this kind ranged widely through the northern countries so recently that they are supposed to be contemporaneous with primitive man. By permission of the American Museum of Natural History.

PLATE XXXV. ICEBERG LAKE, IN GLACIER NATIONAL PARK. Showing a cirque with glacial ice and a small lake at the bottom. Blocks of ice broken from the edge of the glacier float in the water like icebergs in the sea, even in midsummer. Photograph taken Aug. 13, 1911, by M. R. Campbell.

In the course of time, after the cavities were formed, the process was reversed and parts of the carbonate of lime were deposited from solution in forms commonly called flowstone and dripstone. Thus were formed the decorations of cave marble.

Many a description of caverns emphasizes their great antiquity. We are likely to regard them as the dwelling places of cave men and to think of them in connection with the infancy of the human race, which, in comparison with the lifetime of a man, was long, long ago. But in terms of earth history the advent of the human race is a very recent event. Caverns may have been in existence long ages before the first men entered them and may yet be geologically very young.

The youth of a cavern considered from a geologist's point of view and the antiquity of the same cavern considered from the ordinary human standpoint are illustrated by a cavern in western New Mexico that was found in beds of recent lava. Geologically this cavern is very young; yet it contains the remains of prehistoric people.

As this is one of the most conspicuous ice caves yet discovered, the original description of it is repeated here.

AN ICE CAVERN IN NEW MEXICO¹

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¹Reprinted from the Geographical Review, Vol. 16, 1926, pp. 55-59, published by the American Geographical Society of New York.

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New Mexico is a land of natural wonders. To such features as the beautifully symmetrical crater cone of Mount Capulin and the spectacular Carlsbad Cavern, set aside as national monuments, is now added a new curiosity in the form of an unusually interesting ice cave.

The cave is situated in the western part of the State, about fifty miles southeast of Gallup and an equal distance from Grants. It is readily reached by automobile from either town. The writer's party made the visit from Gallup. Leaving that coal-mining town early one morning, we motored southward through the Zuni Indian Reservation to the Mormon town of Ramah.

A few miles farther east the ruins of some ancient cliff dwellings were visited. Little is known of the ruins in this part of New Mexico, but their number suggests that this country once supported a much greater population than it does at the present time. A stop was also made at Inscription Rock, now known as the El Morro National Monument, where the face of the massive sandstone is covered with inscriptions dating back to the early Spanish expeditions.

The Lava Country

Another run of half an hour brought us to the foot of a great volcanic cone, once used as a signal station and known locally as Cerro de la Bandera, or Flag Butte. The cone rises steeply many hundreds of feet above the plain on which it stands. Its summit has an altitude of 8300 feet according to our aneroid. The slope of loose volcanic cinders is as steep as unconsolidated material will lie. The lower part is covered with pine trees; the upper part is nearly barren. The great crater depression in the top of the cone was estimated as about 500 feet deep. The rim is broken away on one side as if the last flow of lava had broken through and carried away the material of the rim.

Cerro de la Bandera stands at the northern margin of a great lava field—rough, black malpais (bad country)—that stretches away as far as the eye can reach. It is known as Los Veteados, or the veined country, because of great cracks formed when the lava cooled. Many of these "veins" or cracks lead down into hollows where the molten lava escaped during the closing stages of flow, leaving the solidified crust arching over caves. These caves have long been used as places of refuge. Many a criminal has escaped capture by retiring to some such refuge in the malpais, where pursuit is practically impossible by those who are not intimately acquainted with the devious passages. The Apache warriors were wont to leave their women and children in these natural shelters when they went on the warpath or set out on foraging expeditions. Judging from the numerous rock shelters and fragments of pottery of ancient design, the custom was a very ancient one.

Features of the Ice Cave

The ice cave, which was our objective, is no exception to the rule. Evidences of former occupation were found on all sides of it. Doubtless this was a favorite refuge, because within the cave is a permanent supply of good drinking water.

The cave is located on the side from which the crater rim was carried away and is so situated that it is best examined in the morning light. It is about 50 feet below the surface and opens into a large depression formed by the collapse of the roof. Apparently the last flow of lava from the crater, perhaps the one that broke through the rim, was 50 to 75 feet deep. Where the cave is situated the lava crusted over to a depth of 50 feet or more, when the rock below, still in a fluid state, flowed out, leaving a long, irregularly shaped hollow. The crust above this cave collapsed in several places, but in other places it remains arched over the hollow. The largest depression thus formed is the entrance to the ice cave.

We clambered down the wall of jagged rocks into the large opening, made our way over the angular blocks of fallen rock that once formed the roof, and finally entered the dark cavern, where artificial light was needed. There we saw before us a perpendicular wall of clear blue ice extending entirely across the cave, a distance of about 50 feet, and rising 14 feet above the floor. Other visitors had been there before us, bringing with them the trunk of a small pine tree which served as a rude ladder. By means of it we climbed to the top of the ice.

The upper surface of the ice is level for about 30 feet back from the face and then gradually slopes inward toward the back of the cave. The total thickness of the ice is not known, for the bottom is nowhere exposed. Most of the ice is clear and has a bluish tint. It lies in horizontal layers separated by thin seams of impure ice. At one horizon near the top are several large blocks of rock, apparently fallen from the ceiling and later covered.

We took the temperature at several places in the ice cave. It was a warm day in August. The water standing in the pool at the base of the wall of ice, the air above the ice and in the cave back of the main mass, and the rock of the inner walls all showed a temperature of 32° F.

The effect of the summer heat is seen near the mouth of the cave. The ice is so far from the opening that the rays of sun never strike it; but a warm current of air occasionally reaches it. The winds at the surface cause shifting of air currents in the sink and to a less extent in the cave itself. The net result of the summer activity of the warm currents is shown in the form of the ice where the face is curved, suggesting swirling currents of warm air.

Origin of the Ice

The occurrence of perpetual ice in large quantity in caves is rare, although many small bodies of ice are known. Edwin Swift Balch describes a large number of such bodies in a volume on "Glacières or Freezing Caverns" (1900), and a recent publication by Georg Kyrle, "Grundriss de theoretischen Speläl;ologie," contains additional information. Also many ice caves in the lava fields of the Northwest have been examined but not described in print. None, however, that have come to the writer's notice excel that at Cerro de la Bandera in volume of ice.

The occurrence of ice formed in caves has given rise to much speculation and in some instances to wild conjecture. However, one need not look for extraordinary causes in explanation of the ice, nor is it necessary to appeal to chemical changes, exhalations of gas, or other rare phenomena. The ordinary changes in weather and the well-known characteristics of scoriaceous basalt appear to be quite sufficient to account for the ice in our ice cave, though no extended observations were made in it.

At the altitude of this cave, 7300 feet or more above sea level, the cold of winter is severe, and freezing weather lasts many months. During times of frost the cold air circulates among the rocks, cooling them below the freezing temperature. Water flowing into the cooled spaces congeals. During thawing weather warm air circulates through the open spaces and warms the rocks. If, on the whole, the warmth prevails, as it does in most places, the ice of winter melts in summer, and there is no perpetual supply. But in a few favored places the summer heat does not overcome the winter frost, and the ice formed during the cold season is not entirely melted during the warm season. In brief, there is a lagging of effect in the change of temperature. The "cold" of winter is conserved in the cave just as it is in an ice house. It is even possible that the temperature of the rocks to a considerable depth beneath the surface may be lowered so far below the freezing point during a long cold period that freezing may continue after all ice has melted from the surface. The accumulation of ice in spring and early summer has been noted in several places. Its formation in the Decorah Ice Cave, in Iowa, has been described by Alois F. Kovarik.¹

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¹A. F. Kovarik: The Decorah Ice Cave and Its Explanation, Scientific American Suppl., Vol. 46, 1898, November 26, pp. 19158-19159.

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PLATE XXXVI. YOSEMITE VALLEY, CALIFORNIA, AS SEEN FROM AN AIRPLANE. A typical ice-sculptured gorge, showing at the left the granite face of El Capitan, which rises about 3,000 feet above the bottom of the famous gorge and, at the right, the pinnacle of Sentinel Rock and the well-known form of Half Dome. Photograph by U. S. Army Air Service.

PLATE XXXVII. LOOKING A MOUNTAIN IN THE FACE. THE NORTH SLOPE OF MOUNT HOOD, OREGON, AS SEEN FROM AN AIRPLANE, SHOWING SEVERAL OF THE MOUNTAIN GLACIERS. Official photograph by U. S. Army Air Service.

Scientific Interest

It may not be out of place to call attention to the possibility of making scientific observations here of a timely nature. The lava in which this cave is situated results from a relatively recent volcanic eruption—how recent is not known. The liquefied rock flooded the lowland east of the Zuni Mountains and flowed northward to the San Jose River at Grants, where the congealed lava, as seen from the railway, appears quite fresh.

This is one of the lava flows said to be so recent that it might have taken place in historic time. Yet on this flow stand large pine trees, and in it is a cave with a body of ice of such nature that it must represent accumulation through many years. Also on the lava near the cave are the remains of a prehistoric people.

The banding of the ice in the cave suggests a possibility of working out a chronology. The mass is made up of layers of ice. Each layer may represent a year's accumulation or it may represent a climatic cycle. This could probably be determined by careful observation. It is not impossible that the climatic changes recorded in the ice might supplement the chronology obtained by studying the growth of trees. The large pine trees of this region offer an attractive start for such a comparison.

Basis of Geologic Time Division

As nearly all the land forms with which we are familiar were fashioned almost exclusively in the Quaternary period, it would be interesting to know what events brought the Tertiary period to a close and ushered in the Quaternary. An understanding of the subdivisions of rocks into systems and of geologic time into periods is necessary before this interest may be satisfied.

Sedimentary rocks were originally subdivided into systems because of physical differences. Later, when comparisons, which geologists call correlations, were made between rocks at widely separated localities, the fossils they contained were used as a means of correlation, sometimes to the exclusion of all other means.

Recently, however, a greater use of correlation by physical criteria has been advocated, and what is called diastrophism has been suggested as a basis for correlation. Diastrophism is a term used to denote such physical changes as upheaval and warping of the surface of the earth and the consequent shifting of seas. These physical changes are recorded in rock structure. Hence it is necessary to examine this structure in order to read earth history.

In reading earth history, as in reading other histories, we desire to know dates and lengths of periods. Some of the methods of estimating geologic time have already been stated, as well as some of the uncertainties in the conclusions. Other methods employed deal with glacial and post-glacial phenomena. The most widely known of these methods is the one employed by Taylor¹ and others in computing the time that has elapsed since the last continental ice sheet retreated from Niagara. They reckon time by the rate at which the river wears away the rocks.

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¹Taylor F. B., Niagara Falls Folio, U. S. Geological Survey, Folio 190, 1913.

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The old gorge was filled with rock débris when the ice moved over it. When the glacier retreated northward a new gorge was started. The falls are retreating upstream at a rate that has been determined by careful measurement. From this rate and the length of the gorge the time required for their erosion may be computed.

Another method of measuring the time that has elapsed since the ice retreated is based on a study of the carved clays of De Geer,¹ the laminated clays laid down near the retreating front of the glacial ice. The number of laminæ, or thin layers, indicate the age of the deposit in much the same way that the thin layers of silt in Egypt indicate the age of the Nile delta.

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¹DeGeer, Gerard, A Geochronology of the Last 12,000 years, Cong. Int. Geol., Compte Rendu, 1910, XI, pp. 241-253, 1912.

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These and all other methods of measuring geologic periods in terms of years give only approximate results. No method has yet been devised for measuring accurately even the last and shortest period. Nor is there uniformity of opinion as to what event shall mark the end of one period and the beginning of another. In geologic history, as in human history, there is a constant march of events, so that a "period" is only a term invented for convenience.

Geologists have found it difficult to establish a foundation, one based either on structure or on fossils, which is generally satisfactory for distinguishing Tertiary from Quaternary events. Some have maintained that there is no adequate means of separation. Others have maintained that a change of conditions that allowed ice to accumulate in continent-wide sheets was sufficient reason for introducing a new period name. This question may be left to the professional geologist.

However, it may be of interest to those who would understand modern landscapes to know that the Rocky Mountain region was elevated in early Quaternary time and that the carving of these elevated lands by erosion produced most of the scenic features which make the West famous.

The material removed from the highlands by erosion was spread out over the lowlands and in it were entombed the remains of the creatures of that time. From the fossil bones found we have learned something of the character of the animals that lived in the West while the northern lands were covered with ice. Among those living in southern Arizona at that time were large mastodons, camels, several kinds of horses, antelopes, deer, wolves, hyenas, tigers, and a great variety of smaller animals. Turkeys and many other kinds of birds were numerous. Among the turtles were some with shells five or six feet across. All of these beings, although they are here called by common names, belonged to species now extinct, and among them were creatures that have gone completely out of fashion. They have no known descendants. One of the strangest of these extinct forms is called Glyptodon. It was an armored animal something like the modern armadillo, with body armor or shell about five feet long and three feet wide.

In other parts of the country events entirely different in nature from those that affected the mountains were transpiring. While the Colorado was vigorously carving out the Grand Canyon, continental glaciers were forming in the more northerly parts of the country. Enormous sheets of ice, probably thousands of feet thick, covered large parts of North America as well as of Europe and Asia.

The occurrence of these vast quantities of ice in high latitudes—glacial ice like that which now envelops all of Greenland except its marginal parts—indicates that the Quaternary was a wintry age. Why the relatively mild climate of the preceding age should have changed to a world-wide winter is a mystery. This change was probably not sudden, nor was it necessarily very great; perhaps the average annual temperature was lowered only a few degrees. All that is necessary for the formation of a glacier is that more snow shall fall than is melted.

More of the events of the Quaternary period have been worked out for the glaciated regions than for other parts of the country. Hence, in order to know what was going on elsewhere while the canyons of the Colorado were being cut and while the mountains were being carved out, we must turn to the story we find in the more northerly regions. In North America there were three main centers of accumulation of this glacial ice. These were in Canada. Farther south there were numerous local centers on mountains or on other high lands. Some local glaciers were formed as far south as Arizona.

Great Ice Age

During the glacial epoch the ice from the northern lands spread southward and covered large parts of North America. In moving over the land it picked up soil and fragments of rock and carried them long distances. When the ice melted it laid down its load of rock waste in deposits called glacial till, which now forms the unassorted masses of clay, pebbles, and boulders seen in river bluffs and road cuts in glaciated regions.

The first stage of glaciation closed with the melting of the ice and the beginning of a warm interglacial time. A remarkable assemblage of animals invaded the glaciated region after the first ice had disappeared, and the bones of many of them have been found in the interglacial deposits. The remains of horses, camels, stags, elephants, mastodons, mammoths, and sloths have been identified. During the life of these animals the climate of North America was rather mild and vegetation was abundant. After this mild interglacial stage, ice again spread southward and invaded the northern part of the United States. This second stage of glaciation was followed by other warm interglacial stages and by later advances of the ice.

It should not be inferred from this rapid review that the events here related were unimportant or that the glacial epoch was short. The duration of the Great Ice Age, although short as reckoned in geologic time, must be measured in hundreds of thousands of years.

Animals of the Great Ice Age

The creatures of the Great Ice Age are particularly interesting because they are nearer to us in time than those of earlier periods and therefore more nearly like animals now living; yet many that lived in North America during this age were very different from those living here today. To find the descendants or near relatives of the Pleistocene animals of North America we must go to other continents, for some of them as far as India. They may have been scattered by the changes in climate which caused the repeated advance and retreat of continental ice sheets during the Great Ice Age.

The animals of early Pleistocene time were of various kinds. They were adapted to the mild climate which then prevailed, and they remained until the ice advanced southward. But they were driven away or exterminated before the end of the Ice Age, and their places were taken by animals such as are now found only in the frozen North. When the ice finally melted away and the climate became as mild as it is now, these arctic species followed the retreating ice front northward, and their places were taken by animals adapted to a temperate climate.

If a Pleistocene man could return and view the present day animals, some of these might seem as strange to him as those of the African jungle seem to an inhabitant of the Great Plains. The Pleistocene horses had a modern appearance; also many of the deer, bison, and smaller animals. But for some of the forms that were common in Pleistocene time—such as the mastodon, the woolly rhinoceros, the saber-tooth tiger, and the camels—the Pleistocene visitor would look in vain. They left no descendants in North America, and their nearest relatives can be found now only in far distant lands.

Most surprising, as well as most common in the North American landscape, is the presence of elephants and mastodons (Plate XXXIV, A). They ranged widely over the continent, and in great numbers. Among the most interesting of these curious giants is the Siberian mammoth, which probably came into North America across "Bering Land" when some parts of Bering Sea were above water. Along the same route from Asia to North America may have come primitive man, perhaps at the same time that the mammoths came.

Scarcely less strange in an American landscape are the woolly rhinoceros, the tapir, the camel, the llama, and other animals which now have no representatives on the continent. One of the camels was a peculiar creature with very long legs and neck, resembling a modern giraffe. Other grotesque figures of the time were giant ground-sloths with enormous claws, which were probably used to drag down branches of trees and to dig roots.

Fossil remains of land animals are not so likely to be preserved as those of water animals; yet great numbers of them have been discovered. Many have been found in ancient bogs, where the animals were mired; others have been found in caverns, where carnivorous animals made their lairs and to which they dragged their prey.

A curious and effectual trap for ancient as well as modern animals is a so-called tar pit or asphalt lake in California. Here many a heavy animal, such as an elephant, venturing forth in search of food or water, was mired in the soft tar and perished. Skulking wolves in search of food approached the carcass and were driven by some stronger enemy into the tar. From this asphalt deposit, which is at Rancho LaBrea, near Los Angeles, the skeletons of many animals have been obtained, especially bison, ground-sloths, extinct horses, camels, mastodons, mammoths, bears, and gigantic extinct lions, as well as those of many kinds of birds.

The asphalt here accumulated around oil springs. As the oil came to the surface and evaporated it left a sticky residue of semi-liquid asphalt, which gradually hardened except where the oil continually welled up from below. A film of dust covered the asphalt, and water accumulated in the hollows after rain. Animals came to drink and, attempting to cross, were caught in the tenacious asphalt as in quicksand. Thus caught, they lured beasts and birds of prey, and they in turn, coming near to devour the unfortunate victims, were caught in the soft, sticky asphalt and in turn served to attract still others.

After the mammoth, the saber-toothed tiger has probably appealed more strongly to the imagination than any other of the monsters associated with early man. It was the largest of the cat family—about as large as a modern grizzly bear. It had long, flattened saber-like tusks, which projected about seven inches from their sockets and which it probably used for stabbing its victims. With such weapons of offense, Saber-tooth was probably able to prey upon very large animals, even mastodons and giant sloths. Although these great tigers were so modern as to be contemporaries of early man, they have left no descendants. Their race has become wholly extinct.

But Quaternary history in the West is not written more plainly in its fossils than in its gorges, canyons, cliffs, and mesas. The records are written large in the land forms. In the landscapes are written the episodes of recent geologic history. The best of these stories in stone may be read in the national parks.

Recent Epoch

Some things seen near at hand seem larger and more impressive than similar things seen from a distance.

Recent events are likely to crowd from the mind larger events of ancient history. In these respects geology offers no exception to the rule. The earth processes of the present time—that is, of the epoch known as Recent—probably receive more consideration than those of all former epochs combined.

PLATE XXXVIII. THE TERRACED FACE OF THE HOT SPRINGS DEPOSITS IN YELLOWSTONE PARK, OVERLOOKING THE TOURIST CAMP. More than 154,000 people viewed this scene during the summer of 1925. Photograph by Willis T. Lee.

PLATE XXXIX. STALACTITES AND STALAGMITES IN CARSLBAD CAVERN, NEW MEXICO. Scene in the Big Room, hundreds of feet underground, to which no ray of light other than that from the explorer's lantern ever penetrates. This cavern is noted for natural decorations of great variety and beauty, ranging from delicate forms of spatterstone to pillars of titanic proportions. Photograph by Ray V. Davis.

A systematic summary of the events of the Recent epoch—the Age of Man—would require much space. But as no pretence at completeness is made here we may pass over it lightly. This seeming neglect of recent geologic events may not appear unwarranted if we reflect that we are dealing chiefly with things that are not usually emphasized in works on geology. The natural processes that affect our comfort and happiness receive almost daily consideration. Much of this consideration comes under the heading of geography, a subject that is growing ever more popular. This popularity is well illustrated by the National Geographic Magazine, which within a period of about 35 years has reached a circulation of more than a million copies.

Much of our modern economic activity is dependent directly on geologic and geographic knowledge. Without that knowledge little of the billion or more barrels of oil used last year would have been produced; nor would the billion and a quarter tons of coal mined during that year have been available for driving trains across the continent and steamers around the world. But while the activities of the present time rightly command most of our attention, the geologist is ever mindful that modern conditions are only the natural results of processes that have been in operation continuously through past ages.

From the point of view of the geologist there is little distinction between the Great Ice Age and the Recent epoch, the Age of Man. The former ended in the United States when the last continental glacier, in its retreat northward about 35,000 years ago, abandoned the United States. But in Greenland, where an area of more than 500,000 square miles is covered thickly with ice, the Great Ice Age is not yet dosed.

It is often said that geography is geology in the making. In this sense the geology of the Recent epoch is read in the physical geography of our time. It appears in the modern landscape, in the operating mine, and in the flowing well. Wind and frost, rain and river, the surging sea and all its straits and inlets are day by day making the geology of the future.

Now, as in all past time, the earth processes are building up masses of rock in some places and tearing down masses of rock in other places. The destructive process may be illustrated by a picture of Crater Lake, in Oregon, where sometime in the past a great mass of volcanic rock was heaped up. This mass is now being attacked by rain and frost and wind, and from it is being carved a landscape of great interest and beauty, shown in Plate XLIV.

Although most modern landscapes are the results of earth processes that reach far back in geologic time, the forms that lie immediately beneath the eye today were produced by erosion during the Recent epoch. The great mountain range in the landscape pictured opposite page 177 (Plate XLV) dates from some ancient time, and the dark volcanic cone in the middle ground was probably formed long before the beginning of Quaternary time. Yet the sculpturing of the mountain and the leveling of the plain, now covered with sage brush, was accomplished in very recent geologic time.

And so, in closing our rambling journey through the West, I would emphasize the fact that

To him who in the love of Nature holds
Communion with her visible forms, she speaks
A various language,

and invite you to learn the language of Nature and to read in mountain and canyon, in river and lake, in the fossils and in the rocks, those Stories in Stone which tell the romance of our world.