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(Pleistocene Epoch)

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The Ice Age (Pleistocene Epoch)

Several times in the history of the earth huge sheets of ice, or glaciers, covered large portions of its

continents. The most recent episode of glaciation, the Pleistocene epoch, is commonly referred to as

the Ice Age and began approximately 1.6 million years ago. During that time there were a number

of advances and retreats of the glaciers, which are termed glacial and interglacial stages,

respectively. The glaciers of Greenland and Antarctica are remnants of the last glacial advance, and

we presently live in an interglacial stage termed the Holocene epoch. With the end of the

Pleistocene (and the beginning of the Holocene) about 11,000 years ago, the Ice Age ended in

name only. It is very likely that the earth will experience another glacial advance, perhaps in the

next 10,000 to 20,000 years, and that the glacial/interglacial cycles will continue. Geologic history

shows, however, that ice ages eventually come to a complete end and do not occur again for

several hundred million years.

There is debate in the scientific community about what caused the glaciers to advance and retreat,

but current theory attributes it to astronomical causes. Because of variations in the earth's orbit,

average summer and winter temperatures change with time. There are periods when winters are

colder and summers hotter, followed by periods when winters are warmer and summers cooler. The

latter are thought to produce glacial advances, because the cooler summers are not adequate to

melt all of the previous winter's snow. The result is that snow and ice begin to accumulate from one

winter to the next. During periods when summers were warmer, and winters cooler, the glaciers are

thought to have retreated. During the Pleistocene epoch, two or more centers of glaciation in

Canada probably joined to form one large sheet of ice.

As the glaciers moved south they scraped and bulldozed rock and soil in front of them, and ground

and crushed the material beneath them. Soil and rock that was pushed in front of the glaciers, or

dropped as they melted, commonly formed long ridges called moraines. Moraines are a common

feature in the midwestern and northern U.S. Many of the boulders scattered throughout the region

are granites carried down from Canada. In fact, diamonds that were transported south from an as

yet undiscovered source in Canada, have been found in the Great Lakes region of the U.S. Exposed

bedrock in many places has long scratches, or striations, that were formed as the glaciers, carrying

an assortment of rocks from other areas, ground over the bedrock. This grinding also produced

large quantities of gravel, sand, and silt. An unsorted mixture of these materials brought by the

glaciers is called till, which forms much of the soil of the northern states.

The southern limit all of the glacial advances, not including the Rocky Mountains, is approximately

the present day location of the Ohio and Missouri Rivers, although the last (Wisconsin) advance did

not reach as far south as some of the previous advances. The glacial advances and retreats also

had a dramatic effect upon the geology and ecology of areas south of the glaciers. It took so much

water to form the glaciers that the world sea level fell approximately 140 meters (425 feet). This

exposed the flat continental shelves, now covered with water, as dry land. This lowering of sea level

has the same effect as raising the land, which caused rivers to begin eroding deep valleys. The

Lower Mississippi, Tombigbee-Alabama, and Red River systems formed deep cuts in the existing

land surface. As the glaciers melted and sea levels rose again, the valleys were filled with sediment

carried by meltwater-swollen rivers. Most of the melt water south of the glaciers flowed into the

Upper Mississippi, Ohio, and Missouri River basins. This sediment laden water fed into the Lower

Mississippi River forming a huge flood plain that is over 500 miles long, and in some places almost

200 miles wide, which is often referred to as the Mississippi Delta. Not all of the melt water ran

directly into the rivers. The retreating glacier sometimes left large depressions, which filled with

melt water to form lakes. The Great Lakes were formed in this manner, but they are not nearly as

large as some that have since drained and disappeared. Lakes Winnipeg, Reindeer, Athabasca,

Great Slave, and Great Bear in Canada are also remnants of this glacial lake system.

The leading edge of the glaciers was an ice cliff, sometimes hundreds of feet high, and cold dry

winds swept down from the glaciers. As the glaciers slowly spread south, they pushed the climatic

zones farther south. Just south of the glaciers was a zone of tundra, next was a zone

of shrub tundra, then scrub birch forests, then boreal forests, and finally mixed deciduous forests.

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Approximately 18,000 years ago, when the Wisconsin stage glaciers were at their maximum

southern extent, the Gulf Coast climate was colder and drier. The mean annual rainfall in southern

Louisiana was possibly as much as 40 inches less than it is today. Boreal forest ,

similar to those now found in Canada and the northern U.S., extended as far south as northern

Louisiana, Mississippi, and Alabama. The north central Gulf Coast was probably covered by sparse

forests of northern pine, similar to portions of southern Wisconsin or New England. Oak and hickory

forests, similar to those now found in Kentucky or Missouri, probably covered the river bottoms.

Florida was drier and its mean annual temperature may have been as much as 10 F colder. It was

covered with sparse, scrubby vegetation, sand dunes, and steppe-like open grasslands. There were

some scattered pines and broadleaf trees. Central Texas was probably covered by tall grass prairie,

with pine and aspen growing in the river bottoms. The high plains of west Texas

were most likely covered by short grass prairie and semidesert. These prairies were

probably similar to those of the present day Canadian Provinces of Saskatchewan and eastern

Alberta.

The harshest climatic conditions occurred in a zone adjacent to the ice edge. Cold temperatures and

strong winds, due to the proximity of the ice, created an Arctic desert, devoid of virtually all plants

and littered with rock debris and fine-grained sediment deposited by the glacial meltwaters. The

fine silt- and clay-sized sediment was picked up by the strong winds blowing from the glaciers and

deposited in sometimes thick, extensive layers called loess. Loess deposits cover

much of the Midwest and extended south into Louisiana and Mississippi. Loess deposits form many

of the present day bluffs along the lower Mississippi River (Natchez and Vicksburg, Mississippi), and

are the basis for much of the rich farm land in the Midwest.

Animals moved north and south with the vegetation zones. There are a number of animals that

existed during the Pleistocene epoch that are now extinct, such as: mammoths,

mastodons, camels, horses, ground sloths, giant bears and wolves, lions, and sabertoothed cats. Several theories have been proposed to explain why these animals

became extinct.

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