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ice ages The term ice ages usually refers to the recurring
periods of glaciation during the Pleistocene and Holocene
epochs of the Quaternary period. During these times, glaciers
advanced from the north across North America and Eurasia.
The Quaternary period has been one of the coldest in all
of Earth history, but glaciations also occurred many times in
the past. For example, glaciations occurred in Gondwana (see
continental drift) 355 million to 280 million years ago and
in Antarctica beginning 33 million years ago. Perhaps the most
extensive ice ages occurred during the Precambrian time
(see Snowball Earth). This entry focuses upon the recurring
Northern Hemisphere ice ages of the Quaternary period.
An interglacial period followed each ice age, in which
the glaciers melted and retreated. Starting about 950,000
years ago, each ice age plus interglacial period lasted about
100,000 years. During the Pleistocene epoch, about 20 ice
ages occurred, with the Holocene epoch consisting of the
time since the peak of the most recent glaciation. In many
cases, a more recent glaciation has obliterated the geological
evidences of an earlier one. Several of the more extensive
glaciations can be directly studied from geological deposits.
The recent glaciations have been named for the location of
their maximum extent, the same glaciations receiving different
names in Europe and in North America (see table).
Major credit for the discovery of the Ice Ages goes to
Louis Agassiz (see Agassiz, Louis) but Agassiz had predecessors.
Jean de Charpentier, for example, observed that boulders
that were out of place had to be moved by some massive
force that did not involve the movement of the crust itself; he
believed such a force could only have been caused by glaciers.
The botanist Karl Schimper invented the concept of Eiszeit,
which translates to Ice Age. Most scientists, such as Alexander
von Humboldt, Roderick Murchison, and even Charles
Lyell (see Lyell, Charles), the father of uniformitarianism,
opposed the idea when it was first proposed. Before this
theory could be credible, a believable cause needed to be pro-
I
0
North American and European Glaciations
Approximate duration
(thousand years ago) North American European
75–10 Wisconsinan Würm
265–125 Illinoian Riss
435–300 Kansan Mindel
650–500 Nebraskan Gunz
posed and evidence for it presented. This was the same problem
that evolutionary theory had, at about the same time.
Despite the mystery surrounding what may have caused the
ice ages, Agassiz assembled the evidence that at least one ice
age had occurred. His evidence, including everything from
boulders out of place to the rubble and bones found in caves,
turned the tide of opinion. Interestingly, Agassiz believed in
catastrophism, that Earth history consisted of a series of
catastrophes. Earlier catastrophists had maintained that the
Flood of Noah was the most recent of these catastrophes;
Agassiz changed his mind, and those of other catastrophists,
by concluding that this “flood” had been ice, not liquid
water.
During the 1860s, scientific journals received papers
about physics from a certain James Croll of Anderson’s University
in Glasgow. One of these papers suggested that variations
in the orbit of the Earth caused ice ages to come and go.
The calculations were oversimplified, but provided a major
insight. It turns out Croll was not a professor, but a janitor
who spent a lot of time in the library.
The Serbian mathematician Milutin Milanković undertook
the necessary detailed calculations in the early 1900s
to connect the ice ages to variations in Earth’s orbit. He
performed these calculations, which would take even a