Early Man in the New World :: 4 THE GREAT ICE AGE

4 THE GREAT ICE AGE

Speak to the earth, and it shall teach thee. —JOB 12:8

Our Part of the Geologic Time Scale

The dead hand of another system of classification lies across a still larger area than the Stone Age itself or the Age of Man. This area is the entire life of our earth since it took sufficient shape to support cellular life. As it is so large an area and much of it is so remote in time, changes in the definition of most of its various divisions do not much affect the present discussion.

Once upon a time there were four great divisions, neatly numbered in Latin as the Primary, the Secondary, the Tertiary, and the Quaternary. The first two went by the board when newer scientists found older ages and stretched the life of the earth a couple of billion years. The Tertiary is still a respected appellation, but the good name of the Quaternary—the area of time with which this book is mainly concerned—is seriously questioned. Defined as the Age of Man, it was supposed to harbor all evidence of his existence; but hints of his presence in the Tertiary have rather sullied the scientific standing of the later period.

THE LIFE STORY OF THE EARTH

This summary of the story of the earth is a combination of charts in Arthur Holmes’ Principles of Physical Geology, Earnest A. Hooton’s Up from the Ape, and George Gaylord Simpson’s The Meaning of Evolution, with modifications by William C. Putnam and James Gilluly. *The divisions marked with an asterisk used to be called, respectively, Secondary and Primary.

PALEONTOLOGICAL DIVISIONS		GEOLOGICAL DIVISIONS	DURATION IN YEARS	CUMULATIVE TOTALS (Round numbers)
CENOZOIC (“recent life”)
	Quaternary Age of Man	Holocene (“wholly recent”)	25,000
		Pleistocene (“most recent”) or Great Ice Age	1,000,000	1,000,000
	Tertiary Age of Mammals	Pliocene (“more recent”)	11,000,000
		Miocene (“less recent”)	16,000,000
		Oligocene (“little recent”)	11,000,000	75,000,000
		Eocene (“dawn of recent”)	19,000,000
		Paleocene (“ancient recent”)	17,000,000
MESOZOIC (“middle life”)		Three periods	130,000,000	205,000,000
	*Age of Reptiles
PALEOZOIC (“ancient life”)		Six or seven periods beginning with the Cambrian	300,000,000	505,000,000
	*Age of Fishes, Amphibians, and Primitive Marine Invertebrates
PROTEROZOIC (“earlier life”)		Pre-Cambrian	1,250,000,000	1,750,000,000
	Age, presumably, of soft-bodied animals
ARCHAEOZOIC (“primordial life”)
EOZOIC (“dawn of life”)
	Problematic signs of life, indicated by presence of carbon
Unrecorded Interval Since the Origin of the Earth			Unknown	2,000,000,000 to 10,000,000,000

In this book we are concerned with two divisions of the Quaternary which are also growing vaguer in outline, less precise in time. They are the Pleistocene, or Glacial Period, or Great Ice Age, and the Holocene, Recent, or Postglacial Period in which we now live. (If your Greek is rusty, you will be amused to discover that those scientific-sounding terms are merely translations of “wholly recent” and “most recent.”) Most geologists believe that these two areas of time covered about 1,000,000 years; but some give them half a million more, and a few limit them to the 600,000 years, or even 300,000 years, of the last four glaciations. Some start the Postglacial 25,000 years ago, when the ice began to shrink toward its present limits; some start it 9,000 years ago, when a relatively modern climate appeared. Some geologists say we are still in the Pleistocene, and merely enjoying a warm spell before another glaciation.

By definition—or lack of it—the Pleistocene is rather vaguely bounded, and quite as much at its beginning as at its end. To the paleontologist, the Pleistocene is the time of certain large and picturesque mammals that are now extinct. To the geologist, it is the time of the waxing and waning of the great glaciers. The beginnings and the ends of these two definitions of the Pleistocene do not correspond too closely. We shall use the term as little as possible, substituting the Great Ice Age.

The Glacial Hypothesis Appears

It is hardly more than a century since science began to realize that large parts of Europe and North America once were covered with glaciers. The discovery came from attempts to explain certain disturbing things called “erratic blocks.” These were large masses of stone—sometimes weighing as much as 10,000 tons—which had no business being where they were, because the native rock in their neighborhood was entirely different. Some of the erratic blocks, for example, should have been hundreds of miles away. The common explanation was that they were water-borne, perhaps by the biblical flood. An American cotton manufacturer accounted for the wearing away and the scratching of such boulders by supposing that they had been embedded in the lower surfaces of icebergs and then swept scraping across the earth by the tumultuous waters on which the Ark had ridden. In 1802 John Playfair, a professor of mathematics at Edinburgh, ventured the theory that the blocks had been transported by glacial ice.[1] This idea had occurred to a mountaineer named Kuhn in 1787, and Saussure echoed it in 1803; they knew and interpreted correctly the moraines of loose stones and boulders which they saw at the foot and the sides of the glaciers. From 1821 to the middle thirties various French, Swiss, and German scientists—Brard, Venetz, Charpentier, and Schimper—discussed and amplified this idea. Though A. Bernhardi, an obscure German professor of forestry, suggested in 1832 that “the polar ice once reached clear to the southernmost edge of the district which is now covered by those rock remnants,”[2] it was not until 1837 that the glacial theory took definite shape. Then Louis Agassiz, speaking before a Swiss society, launched the glacial hypothesis that there had been a period of great cold just before the advent of recent life. By 1840, when Agassiz published his Studies of the Glaciers, the idea was pretty generally accepted; he had “added the Glacial Epoch to the geological time-table.” The theory has been much amplified since then.

Adolphe Morlot, in 1854, discovered fossils of temperate plants between layers of glacial deposits, and advanced the theory that there had been warm periods as well as cold ones during the Great Ice Age. In his “Notice sur le Quaternaire en Suisse” he suggested three separate glaciations with two warm interglacial periods between. In 1874 James Geikie, the geologist of Edinburgh, brought out his The Great Ice Age and Its Relation to the Antiquity of Man, building upon Morlot’s work; and his Prehistoric Europe, in 1881, expanded the glaciations to six. Yet for thirty more years some stubborn scientists still believed in a single glaciation.

It was not until the turn of the century that the work of Albrecht Penck and Eduard BrÜckner established the history of the Alpine glaciations on a solid scientific foundation that has endured pretty well till today. They found four major glaciations and named them in neat alphabetical order after four Alpine valleys—GÜnz, Mindel, Riss, and WÜrm.[3] They divided the WÜrm glaciation at first into two periods of activity, and later into a number of smaller oscillations toward the end. There has been some controversy over the subdivisions of the WÜrm, and one to three Danubian glaciers have been suggested hundreds of thousands of years before the GÜnz; but the general hypothesis brought forward by Agassiz and the amplifications of his successors are now definitely established. With all this goes much knowledge of the ice sheets that covered Scandinavia, northern England, and Germany as far south as Dresden, and North America from ocean to ocean and down to Long Island and the Ohio and Missouri rivers.

The End of the Great Ice Age

Authorities agree that the last melting of the ice sheets and glaciers in the Alpine region began somewhere between 20,000 and 15,500 years ago. After considerable shrinkage and oscillation, the ice increased again for about 5,000 years, and then began to shrink once more. There is some disagreement as to when the Great Ice Age ended; a recent and very minor Daun glaciation has been rather rashly dated as late as only 3,500 years ago. These calculations are only for the Alpine region, and we must remember, of course, that the great ice sheets of northern Europe and North America behaved somewhat differently.

THE ICE FIELDS OF THE LAST GLACIATION

At the height of the last glaciation 5,000,000 square miles of North America were covered with ice, as against 2,500,000 in Eurasia. The volume of ice was three times as great. The shore lines are those of the present rather than glacial time. (Map after Flint, 1957; Antevs, 1928; and Flint and Dorsey, 1945; estimates from Daly, 1934.)

We have some fairly exact knowledge about the retreat of the ice across Sweden. This has resulted from the theory of Baron Gerhard de Geer that the varves—layers of alternately coarse and fine clays deposited in lakes in front of the retreating glaciers—represent the summer and winter sediments released by the melting ice. (We have a somewhat similar index in the tree-ring count of wide and narrow rings originated by A. E. Douglass and improved upon by Harold S. Gladwin. Both tree rings and varves may reflect changes in solar radiation.) De Geer counted the varves and determined that the ice sheet began to retreat in southernmost Sweden some 14,000 years ago, and Ragnar Liden determined that it had disappeared by 6840 B.C.

De Geer’s Swedish-American pupil Ernst Antevs applied the same system in North America, and found the ice beginning to retreat from Long Island 36,500 years ago.[4] A calculation of the time required for the wearing away of the postglacial Niagara Gorge has produced about the same result, but this has been seriously challenged by Richard F. Flint.[5]

The picture of glaciation is more complicated in North America than in the Old World. Europe had two main areas of ice—a small one in the Alps, a much larger one in Scandinavia, the British Isles, northern Germany, and Poland—but they were self-contained. North America had three ice centers—the Labradoran east of Hudson Bay, the Keewatin west of the bay, and the Cordilleran in the Canadian Rockies; these three sheets of ice did not always grow or shrink at the same time or at the same rate, and they occasionally overlapped (see maps on pages 26 and 27).

Incidentally, most of the ice of the glacial period was in the New World. The area of land covered was almost twice as great as in the Old World, and the bulk of ice three to five times as great.[6]

River Terraces and Beach Lines

There are other evidences of glaciation besides varves, erratic blocks, moraines of stones and mixed debris along the sides and fronts of the ice streams, and scratches and polish on the native rock over which the glaciers passed. Four raised terraces are found along the sides of many river valleys. Four raised beach lines, first found in the Mediterranean region, have now been noted in the Americas and Australia. Submerged beach lines and land-bridges have been found at certain places under the ocean, as well as deep channels prolonging present rivers far out to sea.

Naturally enough the Great Ice Age was a time of notable changes of climate. Vegetation advanced and retreated widely. The level of the sea rose and fell some hundreds of feet. Whether or not there was more rain and snow—a moot point with science—the many rivers of the world grew in volume, and often in speed, at certain times and became low and sluggish at others. These profound alternations created the river terraces which have aided so much in determining the age of man and his various cultures. As the ocean sank, while the glaciers grew, the slopes of river beds became steeper, and the rivers themselves grew swifter. The turbulent rivers cut deeper channels and carried the displaced materials far down their valleys and ultimately even into the sea. During the cold, dry period at the climax of each glaciation, the dying trees and brush and grasses released their grip on gravels and silts, the intermittent flood waters of the melting glaciers carried away the debris and—because the rivers lost in slope and grew sluggish as the sea level rose—they deposited the gravels and silts in their beds. As the glaciers grew again and the oceans sank, the rivers once more became swifter and more turbulent, cut deeper channels, carried away part of the gravels and silts, and left the rest as terraces. Thus the passing of each glaciation meant the adding of a new and a lower terrace to the river valleys. Four such sets of river terraces are found just outside the areas where the glaciers have been active—in the valleys of the Rhine, the Thames, the Somme, the Isar, and other rivers.

THE AGE OF RIVER TERRACES

These simplified sections of a river valley show how successive channels were cut deeper and deeper, leaving the older deposits of gravels and silts in the higher terraces at the sides. The discovery of this process of nature was of the greatest value in determining the age and the succession of the cultures of early man in Europe. The oldest flint tools were found in the gravels of the highest terraces, and the newest in the lowest.

Farther to the south, periods of great rainfall helped to produce similar river terraces in valleys like the Nile. The concentration of masses of ice in central and northern Europe upset the zones of climate of Africa and other parts of the earth and caused great climatic disturbance. Rainfall belts moved far south, and the rain increased in abundance. Such periods of rain are called pluvials. There is still a good deal of argument about whether pluvial periods occurred principally in glacial or interglacial periods. This affects the dating of early man, and it is particularly important to us in the New World.

The raised beaches and the submerged beaches were obviously caused primarily by the lowering and raising of the sea level and not of the land. There were land movements, of course—as there are even now—but they were either too small or too irregular to account entirely for the systematic arrangement of old beaches in many parts of the world.

There has been much controversy about other glacial matters, but there can be no question that the submerged beaches and the land-bridges were a by-product of glaciation. The great masses of ice—estimated to have averaged half a mile to two miles thick in North America and somewhere between those figures in northern Europe—depressed somewhat the parts of the earth on which they lay; the rest of the land tended consequently to rise a bit, though not enough to account for the now sunken beaches and for the land-bridges that united Africa and Europe, England and the Continent, and Alaska and Siberia at various times. It was the immense amount of sea water drawn up and locked in the glaciers that reduced the area of the ocean and created new shore lines and the land-bridges. Estimates of how much the seas were lowered from their present level range from 70 to 1,800 feet; the best are 200 to 300 feet. There is still enough glacial ice to raise the ocean more than 100 feet if it all melted.

The raised beaches belong to a later discussion of the cause of the Great Ice Age as a whole.

The Cause of Glaciation

Most geologists believe that a comparatively slight drop in temperature would bring back the glaciers and the ice fields. The German geologist BrÜckner calculated that summers in the last glaciation were only 4° centigrade, or about 7° Fahrenheit, colder than they are today.[7]

What could have caused this slight drop in temperature in the Great Ice Age? Most of the explanations are not satisfactory. One is that the earth happened to pass through a dust laden nebula that reduced solar radiation. Another is a hypothetical decrease in the amount of carbon dioxide in the atmosphere. Other explanations have to do with changes in the altitude of land, shifts in air currents and ocean currents, volcanic eruptions filling the air with dust that screened the rays of the sun. All these adventitious causes would have had to be repeated with the curious and complex rhythm which is characteristic of the waxing and waning of the ice sheets.

One theory seems to have a good deal of cogency. It depends on three known alterations in the relation of the earth to the sun. The first is a slow, regular change in the shape of the earth’s orbit through a cycle of 92,000 years. The second is a shift in the inclination of the earth’s axis through 40,000 years. The third is what a layman would call the wobble of this axis through 21,000 years. The first change increases or decreases the distance of the earth from the sun. The other two alter the angle of the sun’s rays and thus also increase or decrease the warmth given a particular area of the earth at certain seasons. No single unfavorable position would have had a great deal of effect in lowering summer temperature in the northern hemisphere, but two occurring at the same time—let alone three—would have appreciably diminished the sun’s heat.

This astronomical theory of the cause of glaciation goes back a hundred years. As long ago as 1842 the French mathematician and astronomer J. AdhÉmar suggested that changes in the earth’s axis increased rainfall and provided the floods which he thought had moved the erratic blocks. Between 1864 and 1875 James Croll combined the wobble of the earth’s axis and the change in the earth’s orbit. A number of other men worked unsatisfactorily on the problem. The Serbian astronomer and physicist Milutin Milankovitch combined all three, and, between his first publication in 1913 and his latest in 1938, calculated the variations of solar radiation for the past 650,000 years.[8] In 1924 W. KÖppen and A. Wegener applied Milankovitch’s early figures to the glaciation question, and Frederick E. Zeuner has lately used the revised figures of Milankovitch. Zeuner’s results, somewhat simplified, appear on page 55. They are fairly close to the geological estimates of B. Eberl and W. Soergel; his last two glaciations extend further back than those of Penck and BrÜckner.[9] Zeuner’s dates do not agree, of course, with those of an extremist like the geologist Kirtley F. Mather, who dates the first, or GÜnz, glaciation as ranging from 2,000,000 to 1,500,000 years ago.[10] Zeuner’s findings work out well enough for the American glaciations, except that there is no New World equivalent for his first WÜrm maximum of 115,000 years ago. Many authorities refuse to accept any such condition in the Old World. Because of this glaciation Zeuner moves back the appearance of Homo sapiens a good 50,000 or even 75,000 years.

THE FOUR GREAT GLACIATIONS

Six varying estimates of their duration made by five authorities. Fisk’s are of the New World glaciers, which are generally equated with those of Europe.

There is one serious objection to Zeuner’s theory. Two of the three movements of the earth on which it is based would have reduced the warmth of summer in the northern hemisphere, but they would at the same time have increased the temperature of the southern hemisphere, thus alternating glaciation in the two hemispheres. Unfortunately, it is fairly well established that glaciers north and south of the equator have waxed and waned at the same time over a considerable number of years.

It is not enough, of course, to find the cause of the individual glaciations. There must be a cause for the glacial period as a whole. The Great Ice Age was an almost unique event in the history of the earth. We have to go back 200,000,000 years, to the time of the reptiles that preceded the dinosaurs, before we come again on major glaciations.

Zeuner states frankly that the astronomic theory “does not provide the cause of the Ice Age” as a whole.[11] Some added factor must be found. One which he considers is a migration of the north pole from the direction of the Pacific to its present location; Zeuner and others think the movement occurred before the Great Ice Age.

Two geologists, Maurice Ewing and William L. Donn, have accounted for the beginning of the Ice Age by accepting the theory that the north and south poles had moved from the north Pacific and the south Atlantic to their present positions. They account rather ingeniously for the advance and the retreat of the four glaciations. With the north pole where it is now, the ice-free Arctic Ocean would supply moisture by evaporation. This moisture, owing to cold over the northern areas of Asia and North America, would fall as snow to nourish glaciers. But how to stop this process and reverse it? It happens that the sea floor forms a rather high sill between the Atlantic and the Arctic Oceans. When the sea level dropped, as its water piled up in the great glaciers, the sill came too close to the surface to allow much of the warmer water from the Atlantic to reach the Arctic Ocean and to keep it from freezing over. Once the ice pack formed, evaporation diminished abruptly. The glaciers lost their nourishment. The summer melt returned their waters to the ocean. Sea level rose. The currents from the Atlantic could flow over the sill again and melt the ice pack. Then conditions would be ripe for a second advance of snow and ice across the northern world. The shallow waters of Bering Strait probably had little effect upon the Arctic Ocean.

This theory has been criticized adversely by various authorities, despite the geologic, oceanographic, and meteorological evidence that Ewing and Donn have brought to bear upon each step of their reasoning. Their theory is particularly attractive to the archaeologist; it requires an ice-free Arctic coast when the land-bridge at Bering Strait would have been available to early man. Climatic conditions at that time would have been severe along the land-bridge and coastline, but not impossible for the survival of early man.[12]

Another explanation is a general decrease in solar energy; Zeuner holds this in reserve for lack of evidence. But some present-day geologists seize on the possibility that the heat of the sun may have changed from time to time, and use the theory in a curious, almost paradoxical way. The author of this hypothesis, Sir George C. Simpson, believes that the great masses of ice resulted from an initial increase instead of a decrease in temperature.[13] As the weather grew slightly warmer, cloudiness and rain and snow increased, the snow-line fell, and glaciation resulted. As the weather grew still warmer, the ice melted. (Simpson demonstrated the basic principle of this through an ingenious laboratory experiment.) His glacial theory, which is explained in more detail on the opposite page, postulates two increases in solar energy, and draws from them a meteorological pattern that provides the four glacial and three interglacial periods. The first and last interglacials would be warm and wet, the second cold and dry. Zeuner objects to this theory on the ground that the last interglacial—which, according to Simpson, should have been warm and wet—was mainly cool and dry.[14] But, while it may have been cool and dry in the German area which Zeuner has most closely studied, other areas probably had other climates. Simpson’s hypothesis would account for the heavy rains, or pluvial periods, of Nilotic Africa, which may link up with the glaciations of Europe; but he provides only two pluvials, and there are evidences of three or more in Africa.

GLACIATION THROUGH WARMTH

A somewhat modified graph of Simpson’s theory of the cause of the Great Ice Age. The following summary by Carl Sauer includes quotations from Simpson: [First] “Increased solar radiation received by the earth leads to increase in the general circulation of the atmosphere, which forms a great cloud blanket and causes increased precipitation in appropriate areas. In particular, in high latitudes and altitudes there is increased snowfall or glaciers. [Second] ‘As the radiation increases still further, the ice melts away and we have overcast skies and much precipitation but no ice accumulation.’ [Third] ‘When the solar radiation decreases, conditions are reversed and the whole sequence is gone through in reverse order.’” (After Simpson, 1938; quotation from Sauer, 1944.)

Zeuner has an explanation of why the periodic decrease in the heat from the sun produced glaciation during the last million years and not for 200,000,000 years before. He introduces the geological factor called Eustatism,[15] meaning by it simply a progressive drop in sea level. According to the hypothesis, this began before the Great Ice Age, and was caused by the sinking of very deep portions of the sea floor. As the sea level sank, the temperature of the mountains and plains dropped also, for the higher we rise above the surface of the ocean the cooler the air grows. The snowline fell, the mountain glaciers grew larger, and the snow and ice on the northern plains could not be completely melted by the reduced summer heat, and gradually grew deeper and more extensive. Thus the lowering of general temperature made it possible for the periodic decrease in solar radiation to cause the glaciations of the Great Ice Age. The theory of a general and steady lowering of the sea level is based on a series of four raised beaches occurring uniformly in many parts of the world. Other students believe these terraces were products of a regional rise of land.

Considering that the Great Ice Age ranges back at least 600,000 years—and probably 1,000,000, if we credit evidence of three earlier Danubian glaciations—it is small wonder that scientists are not entirely agreed on many factors in its story. “The difficulties are such,” says the French archaeologist A. Vayson de Pradenne, “that after fifty years of study to which the greatest geologists have devoted all their energies, there is no certainty yet as to the exact number of glaciations and the way in which the faunal changes are related to them.”[16]

Much more important, of course, than the cause of glaciation is its effect on early man. Ice covered 27 per cent of the earth’s surface during the WÜrm-Wisconsin period, according to Flint. This created the land-bridge over Bering Strait. It connected Santa Rosa Island with the coast of California. It broadened the Isthmus of Panama, so that man did not have to pass through a semi-mountainous jungle, which suggests that he came south during the Wisconsin glaciation. It seems to have been the ice that urged man to the south in the Americas and provided freeways.

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