Geology Reference
In-Depth Information
they do now (Figure 23.14a). That is, they covered more
than 40 million km
2
, and like the vast ice sheets now pres-
ent in Greenland and Antarctica, they were probably 3 km
thick. Geologists have identifi ed four major Pleistocene gla-
cial episodes that took place in North America—the
Nebras-
kan, Kansan, Illinoian,
and
Wisconsinan,
each named for the
states in which the most southerly glacial deposits are well
exposed. The three interglacial stages are named for localities
of well-exposed soils and other deposits (Figure 23.14b). In
Europe, six or seven major glacial advances and retreats are
recognized.
Recent studies show that there were an as-yet undeter-
mined number of pre-Illinoian glacial advances and retreats
in North America, and that the glacial history of this con-
tinent is more complex than previously thought. In view of
this evidence, the traditional four-part subdivision of the
North American Pleistocene will have to be modifi ed.
Stream channels in coastal areas were extended and
deepened along the emergent continental shelves. When sea
level rose with the melting of the glaciers, the lower ends of
stream valleys along the East Coast of North America were
flooded and are now important harbors, whereas just off
the West Coast, they form impressive submarine canyons.
Great amounts of sediment eroded by the glaciers were
transported by streams to the sea and thus contributed to
the growth of submarine fans along the base of the conti-
nental slope.
We noted in Chapter 10 that as the Pleistocene ice sheets
formed and increased in size, the weight of the ice caused
the crust to slowly subside deeper into the mantle. In some
places, Earth's surface was depressed as much as 300 m be-
low the preglacial elevations. As the ice sheets retreated by
melting, the downwarped areas gradually rebounded to their
former positions.
During the Wisconsinan glacial stage, many large lakes
existed in what are now dry basins in the southwestern
United States. These lakes formed as a result of greater pre-
cipitation and overall cooler temperatures (especially dur-
ing the summer), which lowered the evaporation rate. At the
same time, increased precipitation and runoff helped main-
tain high water levels. Lakes that formed during those times
are
pluvial lakes,
and they correspond to the expansion of
glaciers elsewhere. The largest of these lakes was Lake Bonn-
eville, which attained a maximum size of 50,000 km
2
and a
depth of at least 335 m (
◗
Figure 23.14
Pleistocene Glaciers
Figure 23.16). The vast salt depos-
its of the Bonneville Salt Flats west of Salt Lake City, Utah,
formed as parts of this ancient lake dried up; Great Salt Lake
◗
is simply the remnant of this
once-vast lake. Another large
pluvial lake (Lake Manly) existed
in Death Valley, California, which
is now the hottest, driest place in
North America.
In contrast to pluvial lakes,
which form far from glaciers,
proglacial lakes
are formed by
the meltwater that accumulates
along the margins of glaciers. In
fact, in many proglacial lakes,
one shoreline is the ice front it-
self, whereas the other shorelines
consist of moraines. Lake Agassiz
was a large proglacial lake cover-
ing about 250,000 km
2
of North
Dakota, Manitoba, Saskatchewan,
and Ontario. It persisted until
the glacial ice along its northern
margin melted, at which time
the lake drained northward into
Hudson Bay.
a
Centers of ice accumulation and maximum extent of Pleistocene
glaciers in North America.
(a)
Wisconsinan
Glaciation
Sangamon soil
Sangamon
Interglacial
Illinoian
Glaciation
Yarmouth
Interglacial
Kansan
Glaciation
Aftonian
Interglacial
Yarmouth soil
Nebraskan
Glaciation
Aftonian soil
Pre-Nebraskan
b
Standard terminology for Pleistocene glacial and interglacial stages in North America.
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