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 (Figures 1.13, 1.14). The Quaternary period is divided into two epochs: the Pleistocene and Holocene. During the Pleistocene, the 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 21,000–18,000 years ago, while the beginning of the Holocene is considered to be 11,700 years ago, or about 9,700 BCE.

Figure 1.13: Continental glaciers originating in Canada spread across North America, including nearly all of the Midwest, during the Quaternary period.

Figure 1.13: Continental glaciers originating in Canada spread across North America, including nearly all of the Midwest, during the Quaternary period.

Figure 1.14: As dense glacial ice piles up, a glacier is formed. The ice begins to move under its own weight and pressure.

Figure 1.14: As dense glacial ice piles up, a glacier is formed. The ice begins to move under its own weight and pressure.

The landscape of the Midwest has been heavily influenced and shaped by the advance and retreat of these glaciers, with the surface of every state shaped by the forces of the moving ice that scraped loose rock, gouged the bedrock beneath, and deposited sediment and water as the ice advanced and retreated (Figure 1.15).

Figure 1.15: Glacial features.

Figure 1.15: Glacial features.

Figure 1.16: The formation of the Great Lakes.

Figure 1.16: The formation of the Great Lakes.

The Great Lakes of the Midwest and Northeast were formed during the last great glacial advance some 18,000 years ago. The broad, deep basins of the Great Lakes were flooded as the glaciers receded (Figure 1.16). Glacial meltwater poured into these basins, and the ice blocked the drainage that would eventually flow to the northeast via the St. Lawrence River. Once this path was available, the lakes gradually dropped to their current levels.

The ice age continues today, but the Earth is in an interglacial stage, since the ice sheets have retreated for now. The glacial-interglacial cycling of ice ages predicts 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.

With the onset of the Industrial Revolution, significant amounts of greenhouse gases have been released into the atmosphere that contribute to global warming.

See Chapter 9: Climate for more details.