8 THE PLEISTOCENE ICE AGE AND THE ARRIVAL OF HUMANS IN NORTH AMERICA

䊳 20.8 THE PLEISTOCENE ICE AGE AND THE ARRIVAL OF HUMANS IN NORTH AMERICA

The aforementioned sequence of tectonic events had cre-

Figure 20–18 Basalt rises above the Columbia River to form the Columbia Plateau. Each layer is

a separate basalt flow. (Donald Hyndman)

Pleistocene Ice Age. By 2 million years ago, large ice their maximum size during the Pleistocene Ice Age, so sheets as much as 3000 meters thick had formed in both

much water was stored in continental glaciers that global the Northern and Southern Hemispheres and were rapidly

sea level fell about 130 meters. As a result, the shallow spreading outward. At its greatest extent, the ice covered

shelf beneath the Bering Strait was exposed as a low- about one third of North America. The flowing ice

lying, swampy isthmus connecting Asia to North scoured rock and soil from the land, and when the gla-

America. Known as Beringia, this region was above sea ciers melted, they deposited huge piles of sand and gravel.

level at least twice during the past 150,000 years and an Thus, glacial erosion and deposition greatly modified the

unknown number of times previously during the landscape of North America.

Pleistocene Ice Age (Fig. 20–19). Mammoths, bison, The Bering Strait is part of a shallow-water shelf ly-

caribou, moose, musk ox, mountain sheep, and several ing between Alaska and Siberia. When glaciers were at

other mammals that we now associate with North

Norwegian Sea

British Isles

+ Pole

Bering Sea

ait

eu Str Brooks Range tia

ing

Ber

Baffin Bay

Hudson Bay

Figure 20–19 Beringia was a landmass measuring 1200 by 2300 kilometers and connect-

370 CHAPTER 20 T H E G E O L O G I C A L E VO L U T I O N O F N O RT H A M E R I C A

America migrated across Beringia to this continent dur- ing Pleistocene time.

Humans also migrated across Beringia to North America, although it is difficult to determine precisely when they first arrived. Coastal areas where the earliest migrants probably lived are now submerged beneath seas that rose as the glaciers melted. Thus, traces of their vil- lages and camps may never be found. However, people probably migrated to North America in several waves. The oldest uncontested remains of humans on this con- tinent are dated between 13,000 and 14,000 years ago. The most recent may have happened only 10,000 years ago, shortly before melting glaciers inundated Beringia.

As they arrived, humans and other animals spread out from the Alaska coast, following several paths (Fig. 20–20). Note how far north several of the pathways are. Why would humans and other mammals migrate north-

Figure 20–20 Humans and other animals migrated across ward during times when glaciers were at their maximum

Beringia and spread throughout the new world by several growth? And why would they cross to Alaska, a place

routes in Pleistocene time.

known for its cold climate and glaciers even now during

a nonglacial interval? Some geologists suggest that local climatic patterns created ice-free corridors that directed the migration paths of humans and animals. One corri-

grasses, heather, and sedge. Small groves of aspen and dor extended along the northern edge of the continent,

larch grew in sheltered areas. The terrain was perfect for while another ran along the west coast. A third may have

grazing animals. The flat plains and great animal popu- extended along the eastern side of the Cordillera, be-

lations made for easy hunting. When people who con- tween the alpine glaciers of the mountains and the great

tinued southward arrived in the vicinity of the modern continental ice sheet. If these ice-free corridors existed,

United States–Canada border, they found the end of the they must have been mostly treeless, vegetated by

ice and a fabulously rich new land.