Glaciation in the Pleistocene Era

In 1847, the renowned Swiss naturalist Louis Agassiz argued that geologic formations such as large boulders, polished and striated bedrock, U-shaped valleys, and deposits of sand and gravel found in parts of Europe resulted from huge glaciers moving over the land. Although the idea initially met with considerable resistance, scientists finally came to realize that Agassiz was correct and concurred with the idea that an Ice Age had taken place in the recent geologic past.

We know today that the Pleistocene—commonly called the Ice Age—began 1.6 million years ago and ended about 10,000 years ago. During this time, several intervals of widespread continental glaciation took place, especially on the Northern Hemisphere continents, each separated by warmer interglacial periods. In addition, valley glaciers were more common at lower elevations and latitudes, and many extended much farther than their shrunken remnants do today. Unfortunately, scientists do not know whether we are still in an interglacial period or entering another cooler glacial interval.

As one would expect, the climatic effects responsible for Pleistocene glaciers were worldwide. Nevertheless, Earth was not completely frozen, nor was the onset of the climatic conditions leading to glaciation very rapid. Indeed, evidence from several types of investigations indicates that the climate gradually cooled from the Eocene (54-38 million years ago) through the Pleistocene.Furthermore, recently collected data show that 20 major warm-cold cycles have occurred during the last two million years.

Glaciers moving over Earth's surface have produced distinctive landscapes in much of Canada, the northern United States, and the mountains of the western United States.Also sea levels have fallen and risen with the formation and melting of glaciers. These changes in turn have affected the margins of continents. Glaciers have also altered the world's climate, causing cooler and wetter conditions in some areas that are arid to semiarid today.

Enormous quantities of snow and ice blanketed the continents during the maximum glacial coverage of the Pleistocene. The storage of ocean waters in glaciers lowered sea level 130 meters and exposed large areas of the present-day continental shelves (the submerged borders of continents), which were soon covered by vegetation. Lowering of sea level also affected the base level of rivers and streams. When sea level dropped, streams eroded downward as they sought to adjust to a lower base level. At their greatest extent Pleistocene glaciers covered about three times as much of Earth's surface as they do now. Like the vast ice sheets now present in Greenland and Antarctica, they were probably 3 kilometers thick. Geologists have identified four major glacial episodes that took place in North America.

Stream channels in coastal areas were extended and deepened along the emergent continental shelves. When sea level rose with the melting of the glaciers, the lower ends of stream valleys along the east coast of North America were flooded. These are now important harbors, whereas just off the west coast they form impressive submarine canyons. Great amounts of sediment eroded by the glaciers were transported by streams to the sea and thus contributed to the growth of submarine fans along the base of the continental slope.

As the Pleistocene ice sheets formed and increased in size, the weight of the ice caused Earth's outermost layer, the crust, to slowly sink deeper into the mantle (the layer of Earth between the crust and the core). In some places, Earth's surface was depressed as much as 300 meters below the preglacial elevations. As the ice sheets retreated by melting, the depressed areas gradually returned to their former positions. During the last major glacial stage, many large lakes existed in what are now dry basins in the southwestern United States. The lakes formed as a result of increased precipitation and overall cooler temperature (especially during the summer), which lowered the evaporation rate. At the same time, increased precipitation and the movement of land water into oceans—runoff—helped maintain high water levels. Lakes that formed during these times are known as pluvial lakes, which were lakes formed during a period of exceptionally heavy rainfall, in a period of glacial advance and are now either extinct or only a remnant.