What Were the Ice Ages?

By: Philip G. Chase

Originally Published in 1992

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Ice sheet in Antarctica.
Figure 1. The largest remaining ice sheet in the world covers most of Antarctica. While glaciers on land endure until overcome by melting and evaporation, glaciers flowing to the ocean end abruptly, “calving off” icebergs than can be carried by ocean currents. This picture gives some idea of the enormous scale of glacial phenomena. Photo by P.W. Chase.

Humans, or at least the first members of our biological family, first arrived in Eu­rope at least 500,000 years ago. Since the last Ice Age officially ended only about 10,000 years ago, most of Eu­ropean prehistory took place against the backdrop of Ice Age climates, and much of Ice Age archaeology consists of understanding these cli­mates and their effects on the lives of early Europeans. These effects must have been varied, because the climates themselves were varied.

We tend to think of the Ice Ages (or the Pleistocene, the geologically correct term for the height of the Ice Ages) as a time of bitter cold, a time long past. The Ice Ages, which began over 13 million years ago when the great Antarctic ice sheet formed, of­ficially ended when the last of the great ice sheets retreated toward the poles or into the mountains almost 10,000 years ago (Fig. 1). Neverthe­less, the Ice Ages have almost surely not ended.

Nor were the Ice Ages a time of unvarying cold. They were instead marked by great swings in the world’s climate, from frigid to much warmer than today. London, for ex­ample, is known for its fogs and chilly damp, yet in the very midst of the “Ice” Ages, hippopotami wal­lowed in the Thames. What is more, the present warm spell is still young, as geologists reckon time. Europe saw its first major glaciers over 3 mil­lion years ago. During these 3 mil­lion years, in between the cold times, there have been many warm spells that lasted longer than today’s. It is only because our lives are short that we have no memory, even in history, of anything but warmth; and it is for this reason that we think of warmth as permanent instead of transitory. Though we ourselves were born in the midst of a warm spell, it will not last.

What Caused the Ice Ages?

If we consider the mechanism that caused world glaciation, we can see why it is likely that the great gla­ciers will return. The most probable explanations involve forces so huge as to dwarf even the greenhouse ef­fect, and are of a kind that are likely to reoccur. Furthermore, whatever the mechanism, it will inevitably trig­ger a snowball effect. Most of the earth’s warmth comes from the sun. Glaciers, being white, reflect sun­light; as they grow, the amount of sunlight reflected increases. As a consequence, temperatures fall and the glaciers grow at an even faster pace. The opposite occurs when they begin to shrink. Less light is reflect­ed, more heat is trapped, and the glaciers retreat ever more rapidly.

The most commonly accepted ex­planation for the Ice Ages is a hy­pothesis named for its main elaborator, Milton Milankovitch. This hypothesis invokes several cyclical changes in the relationship between earth and sun (Fig. 2). First, the angle of the earth’s axis of rotation increases and decreases over a period of 41,000 years. This angle is responsible for the changing length of night and day from summer to winter. The fact that it increases and decreases means that sunlight received at different latitudes varies though the 41,000-year cycle.

Second, the earth’s orbit also changes cyclically every 100,000 years. This orbit is not circular but elliptical, so that the distance between sun and earth varies during the year. The gravitational pull of other planets alters the elongation of this ellipse. When, as now, the orbit is close to circular, average distance from the sun is low, and annual solar radiation is high, but the orbit is elliptical, the earth spends part of the year farther from the sun than normal, and receives less heat.

Third, the point along the earth’s orbit at which the solstices or equinoxes occur varies through two superimposed cycles of 19,000 and 23,000 years. If the orbit were circular, this variation would make no difference, but since it is elliptical, this means that sometimes the (northern hemisphere’s) long summer days will fall near the sun, while at other times they will occur far from the sun. (For a fuller discussion of this theory, see Imbrie and Imbrie 1979.)

Although most scholars accept the solar explanation, there are some who disagree. For example, it has been argued that periods of massive volcanic activity could have reduced sunlight enough to trigger the feedback between glaciation and reflection of sunlight described above. It has even been suggested that climatic change is not orderly but random. If this is so (and it is true that fluctuations in weather are greater from decade to decade than from year to year), than the Ice Ages have no explanation; they happened by chance alone. For scientists, however, such a hypothesis is not terribly satis­factory. By its very nature, it can never be disproven—and the possi­bility of disproof is what makes a hy­pothesis scientific.

Diagram of the Milankovich hypothesis.
Figure 2. The Milankovich hypothesis accounts for the cyclical cooling and warming of Ice Age climates in terms of several characteristics of the earth’s relationship to the sun. First, the tilt of the earth’s axis varies. Second, the earth’s orbit around the sun varies from an almost circular ellipse (dashed line) to an elongated one, so that the averaged distance from earth to sun changes. Finally, the point on the orbit where any one solstice or equinox occurs also changes. The winter solstice occurs at point A today, but occurred at point B 5500 years ago and at point C 11,000 years ago. All of these cycles affect the amount of solar radiation received by different parts of the earth’s surface.

The Geological Legacy of the Ice Ages

The geological effects of glaciers are twofold: erosion and deposition of eroded material. As the ice flows—and slips and grinds—across the underlying bedrock, it leaves tell­tale traces. The rock is often pol­ished and grooved. Under continen­tal ice sheets (see Fig. 1), whole regions can bear witness to such treatment. Most of Canada and large parts of the United States today con­sist of great expanses of smooth, flat­tened rocky landscapes where the great North American ice sheets spared few hills and little soil, and numerous lakes fill the shallow de­pressions scoured out by the passing ice. The effects of mountain glaciers are more restricted in scope, but more spectacular in terms of scenery (Fig. 3).

Labeled illustration of glaciers.
Figure 3. In mountains, glaciers originate in amphitheater-like hollows called “cirques” (C). They often descend in hollowed-out steps that, when the glacier is gone, trap lakes of water (called “Pater Noster: lakes because of their resemblance to the beads of a rosary). In cold periods, mountain glaciers formed in such numbers that they worked in teams to remove huge amounts of rock. The narrow ridges of steep and irregular peaks left behind give a distinctive look to glaciated mountain ranges such as the Alps. Equally distinctive are the valleys between the mountains with their U-shaped bottoms (as opposed to the V-shaped or flat-bottomed valleys produced by water). Since the depth of a glacial valley is determined only by the size of the glacier it contains, the mouths of tributary valleys often hang above the main valley floor (H).

Glacial deposits, too, are charac­teristic. At the point where a glacier finally terminates, it dumps the ma­terial it has eroded. This material, called “till,” will mark the presence of a glacier for future geologists (Fig. 4). When this point is stable—when the end of a mountain glacier or the periphery of an ice sheet does not move for a long time—till is deposit­ed in ridges called “moraines.” When the ice has disappeared, these moraines remain behind, drawing a map of the vanished glacier on the modern landscape.

Glaciers are also responsible for a windblown sediment called loess. Much of the enormous quantity of rock that glaciers erode is reduced to a very fine powder called rock flour. The meltwater of glaciers is often called glacial milk, because it runs white with its load of rock flour. In glacial times, cold and aridity discourage vegetation near the margins of the continental ice sheets, so that rock flour deposited by glacier-fed streams is poorly protected. Strong winds pick it up in huge quantities, depositing it downwind in thick blankets. These wind-deposited sedi­ments provide the fertile soil that covers vast areas of the northern hemisphere (Fig. 5).

Of course, glaciers are not the only markers of cold times. Changes in climate are also reflected in the geological record by variations in erosion, deposition, and weathering (Fig. 7). Precipitation also changed during the course of the Ice Ages, and it too affected the geological record.

A man and a dog sitting on a glacier.
Figure 4. The foot of a glacier in the Andes. Glacial sediments or till are marked by poor sorting of material by size. Water-deposited sediments are of uniform size in any one location, depending on the velocity of the stream. Glaciers, however, dump everything from large boulders to very fine powder in the same spot. (This photograph was taken by the author’s father, P.W. Chase, during a geological expedition to Peru in 1928.)
Archaeologists excavating in the loess of a vineyard.
Figure 5. The next time you eat a roast beef sandwich, give special thanks to the glaciers of the Ice Ages. Chances are that the wheat to make the bread and the corn to fatten the beef were both grown on a sediment called loess. Great loess deposits blanket both the North American Midwest and the northern plain of Europe—that continent’s breadbasket which stretches from northern France into the Ukraine. Here archaeologists are excavating the Upper Paleolithic site of Gruebgraben in the loess of an Austrian vineyard.

Frost, for example, leaves many traces. Caves are of special interest to archaeologists, for they are often rich in artifacts. The natural fill in which artifacts are found comes in large part from cave roofs. Much of this rock is pried off when water en­ters cracks, Freezes, then expands. The colder the climate, the deeper this freezing goes, and the larger the chunks of rock pried loose.

Climate also determines the rate of chemical activity. Since heat and humidity are conducive to the break­down and alteration of rock and sed­iment, warm periods are often indicated by heavily altered sedi­ments. Geologists call such chemical­ly altered horizons “soils,” and can recognize those produced in differ­ent environments—in grasslands as opposed to forests, for example.

Map of Europe showing the terrain at the height of the last glacial advance.
Europe in a cold spell (at the height of the last glacial advance). So much water was locked up in glaciers that sea levels were much lower than today: what are now islands were part of the mainland and what are now seas were lakes. Map drawn by Raymond Rorke after Flint 1971.

The glaciers had their most far-flung effect not on land hut in the oceans. The water they locked up as ice had to come from somewhere, and that was, ultimately, from the oceans. The glaciers were often so huge that sea levels were much lower than today (Fig. 6). Land that is presently inundated was, in glacial periods, dry. Twenty thousand years ago Britain was connected by a great plain to France and Denmark. But when the climate was warmer and the glaciers smaller than today, the oceans were deeper. Ancient beaches, complete with sea shells, are found far above today’s shorelines (Fig. 8).

A close-up of permafrost.
Figure 7. Permafrost leaves characteristic traces in the geological record. In summer, the surface of the ground may thaw, but an impermeable layer of ice remains below. Water, with nowhere to go, supersaturates the soil, which starts to act somewhat like a liquid. It may flow, creating swirling patterns as sediments from different layers are intermingled.

Rising and falling sea levels caused changes far inland as well. Rivers would alternately fill and erode their valleys, even far up­stream, as their mouths rose and fell through the millennia. Since rivers also tend to move from side to side, such cycles of building and destruc­tion created terraces, step-like fea­tures roughly parallelling the rivers. Terrace sequences can be quite com­plicated, and sometimes almost im­possible for the non-geologist to recognize, but this did not keep one single-minded French archaeologist from telling an unfortunate cab dri­ver to “turn left at the third terrace.”

Plants and animals must respond to climatic change, and both leave traces behind them. Sufferers from hay fever will be discouraged but not surprised to learn that pollen is re­markably resistant to the ravages of time. Pollen specialists, or palynolo­gists, recover fossil pollen from ar­chaeological sites by soaking dirt in acid strong enough to destroy it, but not strong enough to damage the pollen grains, which under a micro­scope are almost as recognizable as the plants which they come (Fig. 12). Thus palynologists can count grains rom different species in a sample of fossil pollen and reconstruct the vegetation of ancient landscapes. The job in not simple, though. Some kinds of pollen can travel hundreds of miles on the wind, and palynologists endlessly debate the theoretical niceties involved. Still, pollen is one of the most useful clues to climate in areas not directly affected by glaciers.

A map of Europe showing the sea levels during the last major glacial retrat.
Figure 8. Northwestern Europe during the last major glacial retreat. Note that sea levels were higher than they are today and that the environment differed drastically from that shown in Figure 6. Map drawn by Raymond Rorke after Flint 1971.
Microscope photo of a willow pollen grain.
Figure 12. Scanning electrion microscope photograph of a grain of willow pollen. The grain is about 25 thousandths of a millimeter long. Lakes and bogs are especially useful to geologists in providing fossil pollen records: they are relatively unaffected by erosion and they provide the acid conditions in which pollen survives best. Photo courtesy of Mary Kay O’Rourke with thanks to Suzanne Fish and Naomi Miller. Image slightly reduced.

Animals, too, respond to their en­vironment, and their bones survive almost as well as pollen. If the bones a paleontologist finds belong to a species that is still alive today, they provide a clue to ancient climatic conditions. Knowing the environmental preferences of the living species, the paleontologist assumes that their Ice Age counter­parts inhabited a similar environ­ment. In southern France, during sev­eral periods, people hunted reindeer more than any other animal. Obvi­ously, the climate then would not have attracted sun­bathers to the Cote d’Azur. Yet even long extinct species can provide infor­mation about cli­mate. Woolly mam­moths and woolly rhinoceri have been dead for millennia, yet we know they be­longed in a climate far colder than that of their living relatives. For one thing, we know they had warm coats of hair because frozen carcasses have been found with the hair intact. (Pictures paint­ed by prehistoric artists also depict this hair very clearly.) Anatomy pro­vides another clue. The teeth of woolly mammoths are specialized for grazing. For this reason, and because they are normally found together with the remains of another grazer, the horse, it is probably safe to say that they lived on grasslands or steppes. and the extent of prehis­toric grasslands can be judged in part by where their bones are found.

Dating the Ice Ages

The remains of a house at the base of a cliff.
Figure 9. Layering of archaeological sediments in the Upper Paleolithic of Laugerie Haute in southwestern France. The site lies at the base of a limestone cliff (to the left in this photograph). The rock to the right is actually a huge block that fell off the cliff. The house, which was built against this block and on top of the archaeological sediments, was once occupied by Denis Peyrony, one of the most influential paleolithic archaeologists of the early decades of this century.

The foundation of all geological dating is stratigraphy, the analysis of the layering of rocks or sediments. Since various factors determine the appearance of a sediment—where it comes from, whether it was eroded by ice or by water, whether it was deposited by wind or water, how much it has been weathered—changes in sediment re­flect changes in environment. When archaeologists dig a very old site, they are usually digging through a series of layers representing such changes (Fig. 9). bn principle, the oldest layers are at the bottom, the youngest ones on top. By keeping track of the layers where artifacts are found, an archaeologist should be able to determine at least the relative ages of these finds.

In actual practice, stratigraphy can become quite complicated. For one thing, sediments laid down today are likely to be eroded tomorrow, so that most sites are plagued by serious stratigraphic gaps. For another, layers can be time-transgressive: the same layer can be older in one place than in another. bf the point at which a glacier terminates and deposits till is stable, a moraine is formed; but if the tip of the glacier is retreating as the climate gradually warms, it will lay down not a ridge but a con­tinuous sheet of till that will be oldest at the lower end and centuries or millennia younger at its upper end. Fi­nally, in places such as caves and stream beds, sediments are not laid down in simple layer-cake fashion, but in a much more complicated, even chaotic, manner (Fig. 11).

There is another problem with stratigraphy. Although it is useful to re-create the history of deposits in one exca­vation, archaeologists also need to know, in many cases, which of two sites is older. If they happen to be close to one another, the same layers of sediment may be found in both, and a geologist may be able to recognize them. Since this is rarely the case, geologists must instead look at each layer at each site, try to determine the conditions under which it formed, and then to correlate them using arguments such as, ‘The cold spell represented by layers A and Upper B at site X is the same cold spell represented by layers 4b through 8 at site Y.’ Needless to say, this is a time-con­suming and complicated process.

Graphic of European climate data through stages of evolution and archaeology.
Figure 10. The stages of evolution and archaeology in Europe set against the climatic data provided by the oxygen-isotope curve from a pair of deep sea cores from the Indian Ocean, a curve that goes back almost 500,000 years. (The white lines mark 100,000-year intervals.) Notice that there is a certain cyclicity to the curve. Mathematically, the curve can be accounted for by four-simultaneous cycles of 100,000, 43,000, 23,000, and 19,000 years. Note how similar there are to the cycles predicted by the Milankovitch hypothesis.

During the last few decades, geologists have found a remarkable record of the world’s climate preserved under the deep oceans. In places underwater where erosion does not occur, geologists can drill through the sediments, recovering cores containing unbroken stratigraphic records. Fortunately, in such sediments they also find a good mea­sure of climate, the shells of plankton called Foraminifera. Like other animals, species of Foraminifera differ in their climatic prefer­ences, so a scientist with the patience to count them can determine the temperature of the water in which they lived. A more subtle but perhaps more accurate way to use these tiny animals is to measure the ratio of two isotopes of oxygen, oxygen-16 and oxy­gen-18, found in their shells, for this ratio is a good measure of the amount of water that was locked up as glacial ice at the time the animals lived.

We do not have deep sea cores going back more than a few hundred thousand years, but for this time period at least, they give a remarkably complete picture of varia­tions in the world’s climate (Fig. 10). They thus provide a scale to which geologists can try to fit their terrestrial stratigraphies. This is done by matching cold spell to cold spell and warm spell to warm spell. Unfortunate­ly, there are so many of both that it is often not clear which one goes with which. Fortu­nately, geologists can get help from other dating methods.

A stratigraphic drawing.
Figure 11. Part of an archaeologist’s measured drawing of the stratigraphy in the Upper and Middle Paleolithic site of Montgaudier, in west central France. Montgaudier is a huge cave with very complex stratigraphy. This drawing gives some idea of just how complex stratigraphic analysis can be.

The techniques described so far give only relative ages; they let us know only whether one layer is younger, older, or of the same age as another. However, methods are avail­able that give absolute ages, expressed in years before the present. Those that can be used on Ice Age materials are for the most part based on radioactive decay. Not to be confused with organic decay, radioactive decay involves the transformation of an unstable, ra­dioactive substance into less radioactive, more stable substances. Because it proceeds at a very regular rate, it can be used to measure time—if we can find a material that provides a known starting point. Organic substances such as bone or charcoal are one such material, since the amount of radioactive carbon in a plant or animal at death is known. Volcanic rock can also be used; its start­ing point is the moment when it cooled and crystallized.

There is a problem with these methods, however. Ra­dioactive carbon decays so fast that it is useful only after about 40,000 years ago, and the radioactive potassium in volcanic rock decays so slowly that it is useful only before about 150,000 years ago. This leaves a huge gap of time not covered. Physicists are constantly developing other techniques. Perhaps the most promising of these is thermoluminescence, a way of measuring how long ago a rock like flint was heated. Since flint was often used for making tools, it frequently found its way into campfires, where it was heated enough to establish a starting point for thermoluminescence.

Thus, although geologists are beginning to understand the climatic framework of Ice Age chronology, and are beginning to flesh it out by associating particular sites with particular cold or warm spells, an enormous amount of work remains to be done.

Ice Age Climates and Ice Age People

The fastest great movement of the Ice Age glaciers probably averaged less than 10 miles in a lifetime. Ice Age people at best could have been only vaguely aware of the climatic transformations that seem so large in hindsight. For small tribes living on wild foods, with only the spoken word to link them to the past, such changes would probably be com­pletely hidden by changes on a much smaller scale, fluctuations from year to year or from decade to decade.

We know a little about how Ice Age peoples adapted to their envi­ronment, although much of what we know is puzzling. The earliest inhabi­tants of Europe seem to have been confined primarily to more temper­ate environments, but by Middle Pa­leolithic times, Neanderthals (the immediate predecessors of modern humans in Europe) were also able to live in cold if not truly arctic condi­ tions. In warm environments, the an­imals they hunted were forest dwellers (red deer, fallow deer, wild pigs and cattle), while in harsher en­vironments the animals were species that inhabited cold prairies or steppes (reindeer, horses, bison, and woolly mammoths). We have to re­member that we are tropical ani­mals. It is possible that Ice Age Europeans evolved some biological defenses against the cold that both we and our earlier African ancestors lacked (see the article by Nancy Min­ugh-Purvis, this issue), but it is doubtful that they could have lived even in temperate climates without fire, clothing, and shelter.

An assortment of bone needles.
Figure 13. Remarkably modern-looking bone needles from the Upper Paleolithic site of Morin, France. Such artifacts indicate that by the later Upper Paleolithic Europeans were regularly making clothing and probably other artifacts such as tents or bags. Although needles have not been found in early Upper Paleolithic sites, bone awls have been recovered. Photo courtesy of Arthur Jelinek.
A reindeer antler.
Figure 14. A reindeer antler, still attached to part of the skull, from the Middle Paleolithic layers of Combe Grenal in southwestern France. Cut marks on the bone around the base of the antler probably indicate that this animal was skinned. However, evidence for sophisticated tailoring of either garments or tents is lacking from the Middle Paleolithic archaeological record.

There is good evidence for the regular use of fire in Ice Age Eu­rope. What is surprising, though, is that before the Upper Paleolithic, when fully modern humans appear in Europe, we have less evidence of either shelters or clothing than we might expect, given the cold cli­mates people inhabited. There are a few stone pavements or stone “cir­cles” that have been interpreted as shelters, but they are rare and usual­ly controversial. By contrast, in the Upper Paleolithic, evidence of tents, paved floors, storage pits, and the like are both much more common and indubitably real. Likewise, in the Upper Paleolithic, we find the kinds of bone needles and awls we would expect if people were making clothes (Fig. 13). But in the Lower and Mid­dle Paleolithic, we find only ambigu­ous clues that may point to the use of clothing: a few broken bones with polished tips that may have been used as awls, and in two sites in France, the foot bones of leopards or bears, sometimes with cut marks that may indicate that these animals were skinned (Fig. 14). It is hard to imagine that these ice Age people lived without clothing or shelter; but the lack of specialized tools indicates that their clothing may have been crude by the standard of more re­cent cold-climate hunters such as Es­kimos, and the lack of traces of shelters implies that these, too, were crude and flimsy.

Because the Ice Age climate was so variable, archaeologists cannot speak in general terms about its im­pact. Even biological, evolutionary developments are hard to link to cli­matic change. This is not because the changes occurred too slowly, but because there were so many of them. One cannot explain an evolutionary development by a change in climate when the same change had already occurred many times with no biolog­ical effect.

In any case, it was in a varying cli­mate and environment that the peo­ple described in this issue lived their lives. This is something we share with them, although we, like they, live our daily lives unaware of the larger cy­cles of change. The Ice Ages have in large measure shaped the world we know today, and we are indebted to them not only for spectacular moun­tain landscapes and 10,000 Minneso­ta lakes, hut also for the many species that evolved during them; reindeer, horses, polar bears—and Homo sapiens.

Cite This Article

Chase, Philip G.. "What Were the Ice Ages?." Expedition Magazine 34, no. 3 (November, 1992): -. Accessed February 29, 2024. https://www.penn.museum/sites/expedition/what-were-the-ice-ages/


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