migration of species, including humans, to various parts of the world.The great
weight of the ice sheets caused the continental crust to sink deeper into the
upper mantle. Even today, the northern lands are rebounding as much as half an
inch per year long after the weight of the glaciers has been lifted.
The lower temperatures reduced the evaporation rate of seawater and
lowered the average amount of precipitation, which caused the expansion of
deserts in many parts of the world (Table 11). The fierce desert winds pro-
duced tremendous dust storms. The dense dust suspended in the atmosphere
blocked sunlight, dropping temperatures even further. Most of the windblown
sand deposits, called loess (Fig. 194), in the central United States were laid
down during the Pleistocene Ice Ages.
The cold weather and approaching ice forced species to migrate to
warmer latitudes.Ahead of the ice sheets, which advanced perhaps a fewhun-
dred feet per year, lush deciduous woodlands gave way to evergreen forests
that yielded to grasslands.These grasslands became barren tundras and rugged
periglacial regions on the margins of the ice sheets.
THE HOLOCENE INTERGLACIAL
Perhaps one of the most dramatic climate changes in geologic history took place
during the present interglacial period known as the Holocene epoch.After
some 100,000 years of gradual accumulation of snow and ice up to 2 miles and
more thick,the glaciers melted awa y in only a few thousand years, retreating at
a rate of several hundred feet annually. About one-third of the ice melted
Figure 193The extent
of the glaciers during the
last ice age.QUATERNARY GLACIATION