American Genesis: The American Indian and the Origins of Modern Man

50 AMERICAN GENESIS
sea level by over 300 feet, more than enough to turn the shallows
of the Bering Strait (even today only 140 feet deep ) into a broad
bridge of land connecting the two continents. Geologists have
made correlations of sea levels, ice accumulations, and landbridge
width during the last two ice ages (the Wisconsin and the
Illinoian, from 260,OOO? to 170,000 years ago), so it is known
when the bridge existed, as well as when conditions were best for
man to cross it. Although this bridge was last exposed from 23,000
to 8,000 years ago, geologists believe it was also exposed from
35,000 to 27,000 years ago, as well as 170,000 years ago, with the
possibility of a fourth exposure 70,000 years ago.- Probably there
were exposures of the bridge even earlier than 170,000 years ago,
but not enough data has been collected to fix actual dates.
GLACIAL PERIODS OF THE PLEISTOCENE EpOCH IN NORTH A~,rERlCA
Recent epoch
Pleistocene epoch
Wisconsin glaCial period
(Interglacial stage)
Illinoian glacial period
(Interglacial stage)
Kansan glacial period
(Interglacial stage)
Nebraskan glacial period
years ago
10,000 to present
1,000,000 to 10,000
70,000 to 1U,OOO
260,OOO? to 170,000
1,000,000 to ?
? to 430,Ooo?
During times of exposure, where the Bering Sea now exists
there was a broad land, called Beringia by geologists, some one
thousand miles wide when the ice sheets were at maximum size.
During some of these exposures, ice age animals moved freely
across the so-called Bering Bridge. It is generally accepted that
across this causeway the ancestors of the modern horse, camel,
wolf, fox, and woodchuck-all of which evolved in the Americas
-migrated to EuraSia; whereas the mammoth, bison, musk ox,
deer, elk, and moose, which are Eurasiatic, migrated into the