THE PLEISTOCENE ICE AGES
For more than 200 million years from the end of the Permian to about 40
million years ago, no major ice caps covered the world.This suggests that the
existence of ice sheets at both poles during the Pleistocene was a unique event
in Earth history. Analysis of deep-sea sediments and glacial ice cores provides
a historic record of the ice ages.They began about 3 million years ago, when
a progression of glaciers sprawled across the northern continents.
During this time, the surface waters of the ocean cooled dramatically.
Diatoms (Fig. 192), a species of algae with shells made of silica, sharply decline
in the Antarctic.This presumably occurred when sea ice reached its maximum
northern extent, thus shading the algae below. In the absence of sunlight for
photosynthesis, the diatoms vanished.Their disappearance marks the initiation
of the Pleistocene glacial epoch in the Northern Hemisphere.
The latest period of glaciation began about 115,000 years ago. It inten-
sified about 75,000 years ago, possibly due to the massive Mount Toba erup-
tion in Indonesia. It peaked about 18,000 years ago. During the height of the
Ice Age, glaciers up to 2 or more miles thick enveloped Canada, Greenland,
and Northern Europe (Fig. 193).
In North America, the largest ice sheet, called the Laurentide, blanketed
an area of 5 million square miles. It extended from Hudson Bay, reaching
northward into the Arctic Ocean and southward into eastern Canada, New
England, and the upper midwestern United States. A smaller ice sheet, called
the Cordilleran,originated in the Canadian Rockies. It engulfed western
Canada and the northern and southern parts of Alaska. It left an ice-free cor-
ridor down the middle, used by people migrating from Asia into North Amer-
ica. Glaciers also invaded small portions of northwestern United States.
The largest ice sheet in Europe, called the Fennoscandian, fanned out
from northern Scandinavia. It covered most of Great Britain as far south as
London and large parts of northern Germany, Poland, and European Russia.
A smaller ice sheet, called the Alpine, centered in the Swiss Alps,covered parts
of Austria, Italy, France, and southern Germany. In Asia, ice sheets draped over
the Himalayas and blanketed parts of Siberia.
In the Southern Hemisphere, only Antarctica had a major ice sheet. It
expanded about 10 percent larger than its present size and extended as far as
the tip of South America. Small ice sheets capped the mountains of Australia,
New Zealand, and the Andes of South America. Elsewhere, alpine glaciers
topped mountains that are presently ice free. Excess ice with nowhere to go
except into the sea calved off to form icebergs. During the peak of the last ice
age, icebergs covered half the area of the ocean. The ice floating in the sea
reflected sunlight back into space,thereby maintaining a cool climate with
average global temperatures about 5 degrees Celsius colder than today.
QUATERNARY GLACIATION