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. 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 saber-toothed cats. Several theories have been proposed to explain why these animals became extinct.