DEFINITION OF PLEISTOCENE

The Pleistocene is the older of two epochs of

the Quaternary period, lasting from 1.8 million years ago

until 10,000 years ago, at the beginning of the Holocene

epoch. Charles Lyell formally proposed the name Pleistocene

in 1839, after earlier informal proposals, based on the

appearance of species of North Sea mollusks in Mediterranean

strata. The Pleistocene is recognized as an epoch of

widespread glaciation, with glaciers advancing through much

of Europe and North America, and across the southern continents.

Glaciers covered about 30 percent of the northern continents,

most as huge ice sheets that advanced across Canada,

the northern United States, and Eurasia, and smaller alpine

glaciers that dissected the mountain ranges forming the

glacial landforms visible today including horns, aretes, Ushaped

valleys, and giant eskers and moraines. Some of the

ice sheets were up to three kilometers thick, forming huge

bulldozers that removed much of the soil from Canada and

scraped the bedrock clean, depositing giant outwash plains in

lower latitudes.

The continental ice sheets are known to have advanced

and retreated several times during the Pleistocene, based on

correlations of moraines, sea surface temperatures deduced

from oxygen isotope analysis of deep-sea cores, and magnetic

stratigraphy. Eighteen major glacial expansions and retreats

are now recognized from the past 2.4 million years, including

four major glacial stages in North America. The Nebraskian

glacial maximum peaked at 700,000 years ago, followed by

the Kansan, Illinoisan, and the Wisconsin maximums. Ice

from the Wisconsin glacial maximum only retreated from

northern United States and Canada 11,000 years ago and

may return in a short amount of geological time.

Many species became extinct or otherwise changed in

response to the rapid climate changes in the Pleistocene.

Many species lived in the climate zone close to the glacier

front, including the woolly mammoth, giant versions of

mammals now living in the arctic, rhinoceros, and caribou.

Further from the ice, giant deer, mastodons, dogs and cats,

ground sloths, and other mammals were common. Both

humans (Homo sapiens) and Neandertals roamed through

Eurasia, but the nature of the interaction between these two

hominid species is unknown. Many of the giant mammals

became extinct in the latter part of the Pleistocene, especially

between 18,000 and 10,000 years ago, and there is currently

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