Frozen Earth: The Once and Future Story of Ice Ages

by Doug Macdougall

  Agassiz’s lecture at the Neuchâtel meeting and publication of Études sur les glaciers were the first major parries in the debate about continental-scale glaciation. Throughout, Agassiz never flagged in his efforts to convince others about the reality of an ice age. The debate raged on for much longer than he, or for that matter anyone else, could have predicted. He continued to spend summers studying Alpine ice, and in 1840, he and his colleagues set up a permanent camp and observatory on one of the major alpine glaciers in order to make continuous observations of temperature, ice movement, the nature of the rocky moraines that characterize glaciers, and many other features of these “rivers of ice.” The little scientific encampment quickly became a magnet for visiting geologists and inquisitive travelers. As always, Agassiz needed money for his venture. His friend and patron from Paris days, Alexander von Humboldt, suggested to the king of Prussia that Agassiz’s glacier research was a worthy cause, and funds soon appeared. With characteristic indifference to his finances, Agassiz spent the money almost instantly on supplies, salaries for assistants, and equipment. He seemed always to be in debt.

  Summers may have been spent on the ice, but during the rest of the year, Agassiz found time to travel widely within Europe, mainly in connection with his work on fossil fish. He visited museums and private collections wherever they existed, usually with his faithful artist assistant, who would make detailed sketches. His reputation grew, and he was rewarded with grants and honors, and also with long-lasting friendships with leading figures in geology and paleontology. Agassiz also used his travels to search for signs of glacial action on the landscape throughout Europe. Britain was considered to be the center for geological research at that time, and Agassiz, together with prominent British naturalists, made field excursions in Scotland, northern England, and Ireland. They found abundant evidence of past glacial activity, especially in the form of moraines, erratic boulders, and glacially scratched rock surfaces. These efforts won him some converts, but while many were willing to concede that the glaciers of the Alps had once been more extensive, it was much more difficult to persuade them that a great ice sheet had once covered much of the British Isles. For one thing, Britain is not a mountainous country, and glaciers were still inextricably linked to mountains in the minds of many geologists. For another, some large-scale landscape features that were being touted as glacial features, such as lakes and valleys, seemed just too vast to have been formed by the action of ice. It was really just a problem of imagination; most geologists simply could not imagine the scale of the long-gone continental ice sheets. They had been several kilometers thick and hundreds to thousands of kilometers in extent. Not a tree or a blade of grass survived where they stood; hills and valleys had been completely buried in a vast, monotonous white mantle of ice and snow. And they had moved, slowly but inexorably flowing under their own great mass. Agassiz once referred to glaciers as “God’s great plough.” He used the expression as a metaphor for natural catastrophe rather than a description of glacial erosion, but it is nevertheless an apt portrayal of the power of ice to shape the landscape, to scoop out lake basins, gouge out valleys, and pile up the loose debris in great mounds and ridges.

  Agassiz was in his late thirties, at the peak of his scientific career and much appreciated in Switzerland, but in March of 1846, he sought greener pastures: he departed for the United States. Ostensibly, the trip was to give a series of invited lectures at the Lowell Institute in Boston, and also to lead an expedition, once again with funding from the king of Prussia, to explore the natural history of America. Agassiz spoke of returning to Neuchâtel to continue his work there, but few believed him. A poor manager, he was deep in debt because of an ill-conceived scientific publishing venture he had established in Neuchâtel so that he would have control over the publication of his monumental series of volumes on fossil fish. Partly because of his financial problems and partly due to the demands of his scientific work, his personal life was also in turmoil—his wife had moved back to her native Germany with two of their three children. And, although Agassiz was already recognized as one of the preeminent naturalists of his day, he had realized for some time that he would have to move on if he were to achieve all of his ambitions. The previous summer he had quietly turned over responsibility for his permanent glacial observatory to another man, one Daniel Dollfus-Ausset. He seemed to be tidying up loose ends. For many, his departure from Neuchâtel had the aspect of a permanent farewell.

  The premonitions of his Neuchâtel friends and students were justified. Agassiz settled permanently in the United States, and only once returned, briefly, to the small town where he had accomplished so much of his best work. His departure from Switzerland also marked a turning point in his active work on glaciation. Although he lectured on the topic to great acclaim in the United States and made observations of glacial features both in North and South America, most of his time was occupied with work in zoology, and, increasingly, with administration and organizational activities. The days of slogging up Alpine icefields, measuring the slow, plastic flow of ice, or being lowered into a drain-hole that had been bored into a glacier by summer meltwater—he referred to this escapade as a “descent into hell”—were over.

  This is not to say that Agassiz’s personal interest in the subject ever flagged; it just didn’t occupy the same place in his life that it had during the decade from 1836, when he was first convinced by de Charpentier and Venetz about the reality of past glaciation, to 1846, when he left for America. Indeed, one of Agassiz’s first acts on reaching North America (after an Atlantic crossing so rough that rumors abounded in Europe that the ship, and Agassiz with it, had been lost) was to look for signs of glacial activity. En route to Boston, the ship had docked first at Halifax, Canada. Within minutes of stepping ashore, Agassiz had found what he was looking for on the hill overlooking the harbor: the same glacial grooves and scratches on the bedrock that he knew so well from the Alps. Such evidence bolstered his confidence. More than ever, he was sure that much of the northern hemisphere had once been covered by a deep ocean of ice. And in the coming years, he was to observe and describe signs of glaciation—moraines, glacial grooves and scratches, erratic boulders—throughout the northeastern United States. But as time went on his observations became more cursory and his speculations more grandiose. In 1865 and 1866, he conducted a long expedition to Brazil. He reported seeing “glacial drift” and erratic boulders deep in the Amazon basin, and within a week of returning to the United States, he presented a paper to the National Academy of Sciences claiming that large tracts of South America had been covered in ice during the ice age. He provided very little firm evidence for this hypothesis. Many geologists had already studied the deposits of the Amazon Basin without reporting any signs of past glaciation; Agassiz simply claimed that most glacial features had been destroyed by the tropical climate. In his enthusiasm for his idea, Agassiz was overreaching himself. He was right about a global ice age, but he was wrong about the Amazon Basin—in the Andes, and in the far south of South America, glaciers had indeed encroached far beyond their present boundaries, but the tropical lowlands of Brazil had not been glaciated.

  By this time in his career, with other pressing responsibilities, Agassiz was no longer active in science on a day-to-day basis. Younger zoologists and biologists, both in Europe and in North America, were exploring Darwin’s ideas about evolution and generally supporting them, but Agassiz continued to hold a catastrophist view of evolution, partly a holdover from his early experiences working with his mentor, Cuvier, in Paris. Colleagues and critics alike suspected that his claim about the ice age having affected much of South America might be influenced by his views on evolution—a truly global ice age would have had more far more extensive and catastrophic biological consequences than one that affected only some parts of the Earth. As we have already seen, Cuvier had believed that evolution occurred through catastrophic events that wiped out very large numbers of organisms simultaneously. In his view, the new spec
ies that later arose were completely new, with no connection to those that had gone before. Agassiz held similar beliefs. We now know that the ice age did have significant biological effects and caused species extinction even far from the ice-covered regions. But these effects were much different from the catastrophism espoused by Agassiz; they included environmental stress, loss of accustomed habitat, and the various environmental effects of climate change. Although the rate of extinction increased during the ice age, many species survived by adapting or migrating to more favorable regions.

  Figure 3.Louis Agassiz late in his career, as a professor at Harvard University. Agassiz cut an imposing figure and was a superb teacher. He changed the approach to teaching science in the United States by insisting that students “learn by doing.” Photograph courtesy Ernst Mayr Library of the Museum of Comparative Zoology, Harvard University. Copyright President and Fellows of Harvard College.

  In many respects, the later years of Agassiz’s career are a paradox and a disappointment—the man whose curiosity, superb observational abilities, and penchant for synthesis led to great advances in zoology and geology early in his career gradually became stubborn and dogmatic as the years passed. Many of the implications of his work in biology, especially as they impacted ideas about evolution, were left for others to work out.

  Although Agassiz’s scientific work in the United States never did quite match the achievements of his early career, he nevertheless left another sort of legacy. When he arrived in Boston, Agassiz promptly charmed the influential citizens of that city and, as he had done upon taking up his position in Neuchâtel, set about popularizing science through his lectures. They were hugely popular, and he became a sought-after speaker. Not only was he a working scientist and expert who was anxious to explain his ideas to the world, but he was also outgoing and, with his Swiss accent, slightly exotic. His enthusiasm was contagious and his talks about ice ages caught the imagination of the public. Agassiz was so successful as a speaker that—appropriately enough for a newly arrived American—he was able to retire a considerable part of the debt he had accumulated in Switzerland from his speaker’s fees.

  Before long he was also a familiar figure in the young capital, Washington. He became a professor at Harvard (figure 3) and founded the Museum of Comparative Zoology there, and he changed the way science was taught by insisting that his students do hands-on work in the laboratory and the field. He was instrumental in founding Cornell University, the National Academy of Sciences, and the American Association for the Advancement of Science. Longfellow wrote a poem for him on his fiftieth birthday, and Oliver Wendell Holmes wrote another on the occasion of his departure on his expedition to Brazil. He published eagerly awaited articles on natural history in the Atlantic Monthly. When he died, in 1873, the vice president of the United States, the governor of Massachusetts, and many other notables attended his funeral.

  Fittingly, Agassiz’s grave at the Mt. Auburn cemetery in Cambridge, Massachusetts, is marked with a large granite boulder, retrieved with considerable difficulty from a moraine of the Aar glacier in Switzerland, near the spot where he had set up his glaciological observatory in 1840. There was another memorial to Agassiz as well. By the time he died, the reality of ice ages was recognized by scientists around the world.

  CHAPTER FOUR

  The Evidence

  What, exactly, are the clues that betray the presence of extensive continental ice sheets in our planet’s recent past? Some have already been described in previous chapters, and if you live north of about 40 degrees latitude in North America, or a bit further north than that in Europe, or in a mountainous region almost anywhere, you have probably seen some of the effects of glaciers for yourself—although you may not have realized it. Today many more features are recognized as having originated in the glacial-interglacial cycles of the Pleistocene Ice Age than was the case in Agassiz’s day—everything from dead coral reefs in Indonesia now on dry land well above sea level to the rich soils of the central United States, developed on wind-blown silt called loess. The biosphere—the world of living things—was also strongly affected, although not to the degree that Agassiz thought. Careful examination and analysis of glacial effects, especially over the past few decades, has provided a remarkably detailed picture of how our planet has operated during the current ice age. As we shall see later in this book, there is even good (but circumstantial) evidence that the development of modern humans was influenced by the fluctuating climate of the glacial cycles.

  In the decades after the concept of an ice age was first introduced, much of the debate about its validity centered around interpretation of purported glacial features in places like Scandinavia, Scotland, or the northern United States, far from the Alpine glaciers that Agassiz had studied. Opponents of the ice age theory searched for alternative explanations, but for those who were convinced early on that an ice age had occurred, this was a period of intense exploration and discovery. By the late 1800s, there were credible glacial maps that showed the former extent of ice in Europe and North America. The makers of those maps also quickly came to the conclusion that there had been a series of ice advances and retreats, rather than a single period in which glaciers grew to a maximum extent and then declined.

  Both the initial proposal that there had been an ice age and the subsequent discovery that there had been multiple ice advances and retreats rested heavily on the significance of two types of glacial deposits: one that we have already encountered, erratic boulders, often referred to simply as erratics, and a second, glacial drift—a general term for the loose, rocky debris distributed across the countryside by glaciers. Erratic boulders are spread over large regions in Europe and North America; the most remarkable ones are very large and they are often quite different in makeup from the local bedrock. Large granite boulders like the one in figure 4 sit enigmatically on the local limestone in the Jura Mountains of Switzerland and western France, not far from where Louis Agassiz was born. How they got there was unknown before the glacial theory was developed—the nearest outcrops of granite are a hundred kilometers or more away. Some of them are so massive that they are difficult or impossible to move, and farmers clearing their land left them where they lay—great rocky sentinels sitting mutely in the middle of fertile fields, dispassionately surveying their surroundings. Similar features are common in the farmlands of the northern United States and Canada. In the northeastern United States, where there are outcrops of very distinctive rock varieties, trails of erratics of a specific type can often be traced for hundreds of kilometers, fanning out over the countryside “downstream” of their sources. Careful mapping of these erratic trails can provide an accurate picture of how the glaciers that carried the boulders moved across the land.

  Figure 4.A large erratic boulder in a field near Örebro, Sweden. Boulders like this one, very different in makeup from other rocks in the vicinity and much too heavy to have been carried by water, convinced Louis Agassiz that large tracts of Europe had once been covered by thick, flowing ice that carried the boulders far from their place of origin. Note the woman to the right of the erratic for scale. A boulder of this size probably weighs close to ten thousand metric tons. Photograph copyright Dr. John Shelton.

  Before the ice age theory was generally accepted, most geologists and naturalists argued that the erratics had been transported by water to their current resting places. They realized that even fairly small boulders would sink instantly in normal streams, but they also understood enough about rare natural phenomena such as tsunamis (“tidal waves”) and great storms to know that water could transport heavy objects under extreme conditions. In the late eighteenth and early nineteenth centuries, memory of the great Lisbon earthquake of 1755 was strong. It had actually occurred off the coast of Portugal, not in Lisbon itself, but it had generated large tsunamis that scattered heavy objects far inland as though they were matchsticks. It was also well known that a raging mountain stream could carry very large rocks, especially when swollen with the
downpour of a violent storm. But even such extreme events could not easily explain the massive granite erratics in the Jura Mountains, especially the ones that were perched high on valley walls, far above the streams below. Nor could they account for the presence in northern England of erratic boulders that appeared (based on their mineral makeup) to have originated across the North Sea in Norway, or those in the German lowlands that were hundreds of kilometers from their source. Compounding the problem of interpreting these deposits, however, was the fact that at a few localities in Britain, where much of the most detailed research into the ice age controversy was being conducted, the fine-grained drift that accompanied the erratic boulders contained seashells. Critics of the ice transport hypothesis seized on this; they claimed it was conclusive evidence that the ocean was involved. They argued that the erratics must have been transported by great, violent floods coursing over the land, and they said that there were simply no modern-day counterparts. They knew that the sea had covered parts of the continents in the past, because fossilized fish were found throughout Europe. The marine shells in “glacial” drift, they asserted, were proof that the sea had invaded the land yet again and left the drift behind when it receded. Actually, until about the 1820s, there was widespread belief that all of the loose sand and boulders strewn across the land surface had been deposited there by one or more floods, probably by the one described in the Bible. It was not until much later that the true origin of the seashells in drift was realized. James Croll, a Scottish scientist whom we shall encounter later in this book, deduced that they too had been transported by glaciers, scraped up by the ice along with sediments from the shallow seas around Britain and carried inland. However, before their origin was understood, the shells were a serious difficulty for those who argued that drift and erratics were ice age deposits.