Frozen Earth: The Once and Future Story of Ice Ages (9 page)

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 species 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.