The Ice Age: Mountains of Ice

At the start of the Quaternary period, about 2.5 million years ago, continental ice sheets began to form in northernmost Canada. Throughout this period, the northern half of North America has been periodically covered by continental glaciers that originated in northern Canada (Figure 1.16). The Quaternary period is divided into two epochs: the Pleistocene and Holocene. During the Pleistocene, ice sheets advanced south and retreated north several dozen times. The Holocene epoch is the most recent (and current) period of retreat, called an interglacial interval. The most recent glacial advance in North America reached its maximum extent 25,000 - 18,000 years ago, while the beginning of the Holocene is considered to be 11,700 years ago, or about 9700 BCE.

The entire United States was affected by the cooling climate during the most recent ice age. A cooling climate contributes to the growth of continental glaciers: as more snow falls in the winter than melts in the summer, the snow packs into dense glacial ice. In this case, as snow and ice continued to accumulate on the glacier, the ice began to move under its own weight and pressure. The older ice on the bottom was pushed out horizontally by the weight of the overlying younger ice and snow. Glacial ice then radiated out from a central point, flowing laterally in every direction away from the origin (Figure 1.17). As a result, the continental glacier that originated in Canada migrated southwards toward the United States.

In the South Central, continental glaciers extended southward into northeastern Kansas and northern Missouri, as evidenced by quartzite erratics, till, glacial lake deposits, and loess. During this time, the ice advanced as far south as the Missouri River, and the meltwater from the glaciers likely cut through the landscape to form the river’s original valley. The predominant effects of the Pleistocene ice age on the Coastal Plain were the rise and fall of sea level, subsequent erosion and deposition, and changes in weather, drainage patterns, and the distribution of plant and animal species. At the peak of the last glacial maximum, sea level dropped more than 100 meters (328 feet) below the current level (see Figure 1.15). Widely fluctuating sea levels drastically affected the erosion and deposition of sediment on the Coastal Plain, creating scarps and river terraces and steepening stream gradients, which resulted in more rapid streambed erosion.

Figure 1.16: Extent of glaciation over North America.

Figure 1.16: Extent of glaciation over North America.

Glaciers indirectly affected the majority of the South Central in a variety of ways. Meltwater from the north was filled with sediment that accelerated erosion, carving most of the region’s modern river valleys into the landscape. As the flowing water approached the coast and slowed, it dumped its load of sediment, helping to build the Coastal Plain out into the Gulf of Mexico. Quaternary-aged floodplains along the banks of the Mississippi and Missouri Rivers were also composed of a significant amount of glacial sediment. Rock flour from these floodplains was blown by the wind, covering much of Missouri and Mississippi in layers of loess. Deposits of this silt provide the foundation for the rich soils that exist in much of the central United States. Glacial meltwater and sediment also constructed the outermost 80 kilometers (50 miles) of the Coastal Plain, especially the Mississippi Delta. Nearly all of Louisiana’s surface is Neogene- or Quaternary-aged, unconsolidated or semi-consolidated sediment deposited by water.

Figure 1.17: Continental glaciers originating in Canada spread across North America, including the northern portion of the South Central region, during the Quaternary period.

Figure 1.17: Continental glaciers originating in Canada spread across North America, including the northern portion of the South Central region, during the Quaternary period.

See Chapter 6: Glaciers to learn more about glaciation and the ways in which the last ice age affected the South Central.

See Chapter 9: Climate to learn more about how climate change affects the environment.

The ice age continues today, but the Earth is in an interglacial stage, since the ice sheets have retreated for now. The current interglacial period has slowed both erosional and depositional processes in the South Central—this and a higher, more stable sea level allowed coastal features such as barrier islands and lagoons to form, resulting in the landscape we know today. The glacial-interglacial cycling of ice ages indicates that the world will return to a glacial stage in the future, but the impacts of human-induced climate change might radically shift the direction of these natural cycles.

Why was there an ice age?

What led to the formation of large continental glaciers in the Northern Hemisphere 2.5 million years ago? Movement of the Earth’s plates may have been a direct or indirect cause of the glaciation. As plates shifted, continents moved together and apart, changing the size and shape of the ocean basins, and altering ocean currents that transported heat from the equator to the poles. Sufficient precipitation in northern Asia and North America also enabled continental glaciers to grow and flow outward. The rise of the Himalayas exposed new rock that trapped carbon dioxide through chemical weathering; in turn, the decreased levels of carbon dioxide led to a global cooling. Finally, the presence of continental landmasses (Antarctica and to some extent Greenland) over or near the poles was also a major factor in the development of continental glaciers, as precipitation could now be deposited on these landmasses.