periods of glaciation over the past 2 to 3 million years, these large mammals
succumbed to extinction following the Ice Age.
During this time, Ice Age peoples occupied many parts of North America.
Their spear points have been found resting among the remains of giant mam-
mals, including mammoths, mastodons, tapirs, native horses, and camels. These
people crossed into North America from Asia over a land bridge formed by the
draining of the Bering Sea and moved through an ice-free corridor east of the
Canadian Rockies. Instead of migrating to North America in several waves, how-
ever, small bands of nomadic hunters probably crossed the ancient land bridges
in pursuit of big game and ended up in the New World purely by accident.The
human hunters arriving from Asia sped across the virgin continent, following
migrating herds of large herbivores, which they apparently hunted to extinction.
GLACIAL GEOLOGY
Much of the landscape in the northern latitudes owes its unusual geography
to massive glaciers that swept down from the polar regions during the last ice
age. Glaciers excavated some of most monumental landforms. Outwash
streams from glacial meltwater carved out many peculiar landscapes. The
power of glacial erosion is well demonstrated by deep-sided valleys carved out
of mountain slopes by thick sheets of flowing ice (Fig. 199) a mile or more
thick. Glacial erosion radically modified the shapes of stream valleys occupied
Figure 198The woolly
mammoth went extinct at
the end of the last ice age.QUATERNARY GLACIATION