Geology Reference
In-Depth Information
isotope data (the ratio of O
18
to O
16
) from deep-sea cores
shows that 20 major warm-cold cycles have occurred during
the last 2 million years.
More than 70 million km
3
of snow and ice blanketed
the continents during the maximum glacial coverage of the
Pleistocene. The storage of ocean waters in glaciers lowered
sea level 130 m and exposed large areas of the present-day
continental shelves, which were soon covered by vegetation.
Lowering of sea level also affected the base level of rivers and
streams. When sea level dropped, streams eroded downward
as they sought to adjust to a lower base level.
From such glacial features as terminal moraines
The Effects of Glaciation
Glaciers moving over Earth's
surface have produced distinctive landscapes in much of
Canada, the northern tier of states, and the mountains of the
West (see Chapter 14). Sea level has risen and fallen with the
formation and melting of glaciers, and these changes in turn
have affected the margins of continents. Glaciers have also
altered the world's climate, causing cooler and wetter condi-
tions in some areas that are arid to semiarid today.
◗
(
Figure 23.15), erratics, and drumlins (see Chapter 14),
it seems that at their greatest extent, Pleistocene glaciers
covered about three times as much of Earth's surface as
◗
Figure 23.13
Pleistocene Deformation and Volcanism
a
These deformed Pliocene-age rocks are only a few hundred meters from the San Andreas fault in
southern California.
b
View of the Cima volcanic fi eld in Mojave Desert National Preserve in California, which was active
between 7.6 million (Late Miocene) and 10,000 years ago (Late Pleistocene). Basalt lava fl ows and
about 40 cinder cones are present here.
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