Earth Sciences / 69his native Switzerland and there he might have remained, the most eminent ichthyologist of his
generation, were it not for what began as a secondary interest.
Switzerland is noted for its glaciers and the Swiss are very familiar with them. In various parts of
Europe there are boulders and gravels made of rock quite different from that of the region in which
they are found, and in the 1830s some Swiss scientists were speculating that these rocks had been
pushed into their present positions by glaciers. If so, it implied first that glaciers move and, second,
that they once extended much further than they do now. Agassiz was sceptical, but decided to test the
idea and in 1836 and 1837 he spent his summer holidays studying Swiss glaciers.
He soon discovered piles of rock to either side of glaciers and where they terminated, and
noticed that some of these rocks were scoured with lines, as though small stones had been
dragged across them under great pressure. Continuing with his visits to glaciers, in 1839 he
found a hut, built on a glacier in 1827, a mile from its original location. His final test involved
fixing a line of stakes across a glacier from side to side. Two years later, in 1841, he found they
had moved and now formed a U shape, because the stakes near the centre of the glacier had
moved further than those near the sides.
Already persuaded that glaciers do move, Agassiz published his ideas in 1840, as Études sur les
glaciers, shortly before his rival, Jean de Charpentier (1786–1855), published his own version of the
same theory. This proposed that in the fairly recent past all Switzerland and all those regions of
Europe in which unstratified gravel occurs had been covered by sheets of ice similar to those that
still cover Greenland. Agassiz extended his studies to other parts of northern Europe, and concluded
that the ‘Great Ice Age’ had been very extensive. In 1846 he was invited to lecture in the United
States, mainly because of his work with fossil fish, but he used the opportunity to deliver popular
lectures about the Great Ice Age and to seek, and find, evidence for glaciation in North America. He
remained in the United States, spending most of his time at Harvard, and became an American
citizen.
Boulders and unstratified gravel, unrelated to the underlying rocks, have clearly been transported.
Agassiz supplied an explanation for the mechanism of their transport, but an alternative
explanation already existed. Many scientists believed the Earth had once lain beneath water,
perhaps the biblical flood. In overturning this conjecture, Agassiz had a profound influence on
our ideas of Earth history.
The Great Ice Age was soon accepted, but in modern times the original concept has been greatly
modified. Figure 2.24 shows the extent to which ice sheets have covered the Earth at one time or
another during the last 2 million years. As the map indicates, recent ice ages have affected both
hemispheres, although glaciation has been more marked in the northern hemisphere, where land
extends into higher latitudes. Much of what appears as open sea between ice sheets was in fact
frozen, as the sea around the North Pole is frozen today.
Conventionally, the ice ages are said to occupy the Pleistocene Epoch, which began about 2 million
years ago and ended about 10000 years ago, with the end of the last glaciation and the
commencement of the Holocene (or Recent) Epoch, in which we now live. In fact, glaciation
began somewhat earlier, rather more than 3 million years ago, as glaciers advanced and then
retreated again several times (GENTRY AND SUTCLIFFE, 1981), and it is possible that the date
of the beginning of the Pleistocene may be revised. Indeed, it may be that our use of the word
‘Holocene’, or ‘Recent’, is premature. Most palaeoclimatologists (scientists who study the climates
of the distant past) agree that we are presently living in an interglacial, called the Flandrian, and
that one day (no one can say when) this will end and there will be another ice age. If this is so,
perhaps the Pleistocene Epoch has not yet ended!