At the start of the Quaternary period, approximately 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 1.20). 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, reaching their last maximum extent 25,000–18,000 years ago. The Holocene epoch is the most recent (and current) period of retreat, and is referred to as an interglacial interval. The beginning of the Holocene is considered to be 11,700 years ago, or about 9700 BCE.
The Pleistocene ice sheets did not extend into the Southwest—at their greatest extent in the western US, continental ice sheets reached into Washington, Idaho, Montana, and the Dakotas. However, local alpine glaciers covered the highest parts of the Rocky Mountains as far south as northern New Mexico (Figure 1.21). These glaciers carved U-shaped valleys with steep headwalls (cirques) and sharp drainage divides (arêtes). They also left unsorted deposits of glacial till in lateral moraines along the sides of valley walls, and in terminal and recessional moraines marking both the farthest extent of the ice and the places where the ice front paused during retreat. Glacial lakes formed in low areas between or in front of glaciers, and also during times between glacial advances. One such lake, Lake Bonneville in Utah, was the precursor of today’s Great Salt Lake (Figure 1.22)
See Chapter 9: Climate to learn more about how climate change affects the environment.
The Quaternary period has been dominated by the advance and retreat of continental ice sheets and alpine glaciers; glacial periods alternate with warm interglacial periods. While the “ice age” continues today, the Earth is in an interglacial stage, since the ice sheets have retreated for now. The glacial-interglacial cycling of ice ages indicates that the world will return to a glacial stage in the future, unless the impacts of human-induced climate change radically shift these natural cycles.
Figure 1.20: Extent of glaciation over North America during the Quaternary.
Figure 1.21: Pleistocene alpine glaciers of the Southwest.
Figure 1.22: Lake Bonneville’s maximal extent during the Pleistocene. Inset: Graph of the lake’s changing level.
Why was there an ice age?
What led to the formation of large continental glaciers in the Northern Hemisphere between 3.5 and 2.5 million years ago? Movement of the Earth’s tectonic 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, and surprisingly, the formation of the Central American Isthmus, which connects North and South America in what is now Panama, likely had a major effect on climate. Ocean currents that had once flowed east to west through the Central American Seaway were now diverted northward into the Gulf of Mexico and ultimately into the Gulf Stream in the western Atlantic. The strengthened Gulf Stream now transported more moisture to high northern latitudes, causing more snow, which eventually formed glaciers.
