T

his chapter examines the Ice Ages and the present interglacial during
the Quaternary period. The Quaternary, from 3 million years ago to
the present,witnessed a progression of ice ages that came and went
almost like clockwork.The period is divided into the Pleistocene glacial
epoch and the Holocene interglacial, which is also commensurate with the
rise of civilization. Both the Tertiary and Quaternary are terms carried over
from the old geologic time scale in which the Primary and Secondary peri-
ods represented ancient Earth history.The pronounced unequal lengths of the
two periods is in recognition of the unique sequence of ice ages. Many geol-
ogists prefer to divide the Cenozoic into two nearly equal time intervals: the
Paleogene from 65 to 26 million years ago and the Neogene from 26 million
years ago to the present.
During the Pleistocene, the movement of continents to their present
locations and the raising of land to higher elevations made geographic con-
ditions ripe for a colder climate. Variations in the Earth’s orbital motions
might have provided the initial kick to trigger the growth of continental glac-
iers,which partly explains the recurrence of the ice age cycles about every

14

QUATERNARY

GLACIATION

THE AGE OF MODERN LIFE