Rancho La Brea - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 248

Rancho La Brea

B ACKGROUND : THE P LEISTOCENE
IN N ORTH A MERICA
By the onset of the Quaternary Period the
continents were close to their present
positions, although the mid-Atlantic ridge
was continuously spreading. At this time,
2.5 million years ago (Bowen, 1999), there
was a dramatic deterioration in the climate
which over much of North America and
northern Europe and Asia remained cold
to glacial during the whole of the
Quaternary, with temperate to warm
intervals of short duration. This is the
period of Earth's history known as
the 'Great Ice Age', when the climate was
the dominant geological force. Icebergs
began to appear in northern oceans and
vast continental ice-sheets covered much
of the northern continents. The North
American ice cap covered 13 million
square km (5 million square miles) of
the continent; it carved the landscape
of northern Canada, with meltwaters
carrying the debris south as far as the
Great Lakes.
The Quaternary Period, lasting for only
2.5 Ma, is much shorter than any other
geological period, and is too short to be
subdivided on the traditional basis of
faunal and floral evolutionary changes.
Instead it is subdivided on the basis of
climatic changes, which were dramatic at
this time. The term 'Ice Age' often gives
the wrong impression; the Quaternary was
not one continuous glaciation, but was a

period of oscillating climate with advances
of ice and growth of glaciers punctuated
by times when the climate was not very
different from that of today.
In North America there were four
major cold periods during the Quaternary
(compared to six in the British Isles).
The Nebraskan, Kansan, Illinoian, and
Wisconsinan glaciations alternated with
intervening warmer periods of the
pre-Nebraskan, Aftonian, Yarmouthian,
and Sangamonian interglacials. The
Nebraskan glaciation began around 1
million years ago and lasted about 100,000
years, but it was the final Wisconsinan
glaciation, which began about 100,000
years ago, that included the coldest time
during the whole of the Ice Age.
Sea level was much lower during the
glaciations because millions of cubic
kilometres of water from the oceans were
turned into ice, lowering sea level
eustatically by as much as 120 m (400 ft).
The Bering Strait, which today is a
shallow sea separating Alaska and Siberia,
emerged periodically as a land bridge
connecting north-eastern Asia and north-
western North America. Although this
prevented exchange of marine organisms
between the Pacific and the Arctic
oceans, it permitted terrestrial species to
migrate between North America and
Eurasia. North America species, such as
the camel and horse, migrated to Eurasia,
while Eurasian mammals, such as