Glaciers have been an important feature of the earth’s landscape during the present Cenozoic era, which spans about 65 to 70 million years. Cooling near the end of the era began in the Pliocene epoch, and mountain glaciation in the western United States occurred somewhere between 1 and 2 million years ago. The Cenozoic era is divided into the Tertiary and Quaternary periods, with the most recent glaciation occurring during the Quaternary period, which covers approximately one million years before the present (BP). This period is divided into the Pleistocene and Holocene or Recent epochs. During the Pleistocene, four major glacial advances and three interglacial stages occurred. The Holocene or Recent epoch spans a period of approximately 11,000 years BP. During this time, the fourth interglacial stage or a temperature increase has been occurring, including melting ice and a rise in sea level. This may last for tens of thousands of years, and may be followed by another future period of cooling and the growth of glaciers.
Prior to this present age of Pleistocene glaciers, there were at least five other known glacier periods, dating back to two occurrences of glaciation in Precambrian times. Toward the end of the Permian period glaciers appeared again, and they were rather widespread. During the early and late portions of the Cretaceous period glaciers appeared once more. The occurrence of glaciers is evidently a cyclical phenomenon, but the precise cause of their appearance is not known.
During the Pleistocene glaciers were widespread, covering both polar regions and extending outward a considerable distance, covering approximately 30% of the earth’s total land area. In the present Holocene epoch, the earth is witnessing a global warming as well as a global dimming: Both of these appear to be antagonized by increased human interference. All glaciers are receding, and have been for thousands of years, but at seemingly a much faster rate now than when they were flowing forth. Consequently, the sea level rose at an average rate of about 6 inches per 100 years during the Holocene. A noticeable increase has been noted during the last several hundred years, however, and this is expected to continue.
In the 19th century, Louis Agassiz, like James Hutton and other scientists in the century before, asserted that the large boulders in the valleys were not placed there by Noah’s flood, but were once plucked by erosion out of the advancing ice, sometimes a mile or more deep, and then deposited by a receding ice flow. Agassiz’s views were initially met with skepticism and hostility by most of the audience at a meeting of the Swiss Society of Natural Sciences; widespread acceptance came only gradually, later on.
Glaciers are commonly defined as either continental or mountain types. Continental glaciers begin in an area with polar temperatures and extremely large accumulations of snow. There were three areas in the northern hemisphere where this most strikingly occurred during the Pleistocene; in Europe over the northern Baltic Sea and much of Norway, in North America over Hudson Bay and north central Canada, and over northern Siberia. Not to be overlooked are the general expanse of the Arctic icecap including Greenland and other parts of the northern hemisphere, as well as the massive Antarctic continent in the southern hemisphere. These continental glaciers appear to be located on or very near shields consisting of Precambrian igneous and metamorphic rock. With regard to mountain glaciers, the Alps, Andes, Rockies, and the vast Himalayan chain are all fine examples of mountainous glaciated terrains. Also, snow and ice occur on isolated mountain peaks in Hawaii, Japan, and Kenya, to mention a few.
Continental glaciers begin with a decrease in temperature and an accumulation of snow, over a long period, tens of thousands of years. The snow is compacted by its own weight. Eventually, the snow at the deepest level begins to form ice. Glacial or moving ice is subjected to immense pressure and is transformed molecularly into an extremely hard metamorphic rock. Subsequent snows provide added weight and pressure, and the deepest ice begins to move outward by the force of gravity into lobes that move more rapidly along lines of least resistance. The lobes are guided by original terrain features such as mountains, hills, and plains, and the relative resistance of the various types of earth materials and rock structures. The magnitude of accumulation at the glacier’s source is believed to be more than 10,000 feet in some cases, and the outward movement is estimated at more than a thousand miles, perhaps at speeds of 30 miles per year or even more.
Continental glaciers sculpt the land by erosion, carving out a variety of landforms. The moving ice deposits the carried sedimentary materials as depositional features upon melting. Some of the eroded features that can be seen today are due to abrasion by rocks carried along by the ice, leaving behind striations, grooves, and polished surfaces. Also left behind are gouged-out valleys that were deeper and wider areas of softer rock, forming depressions such as the Great Lakes, cuestas or edges of land surfaces formed by resistant rock strata such as that of the Niagara Cuesta including Niagara Falls, and smaller hills known as roche moutonee formed by resistant rock. The materials are deposited in recessional and terminal moraines, or in ridge-like terrains that are accumulated during stand-still periods. As the ice recedes in a sometimes erratic manner, it leaves behind landforms such as ground moraines consisting of glacial till, also known as tillite, along with such features as kames and kettles, drumlins and eskers, resulting in a hummocky topography. Braided streams also result from retreating glaciers, as well as “misfit” valleys where the valley width is much narrower or wider than it should be for the width of the stream currently flowing through it. For example, the upper Mississippi, the Des Moines, and some of the other streams in the midwestern United States are considered misfit streams. In general, the landscape where continental glaciation has taken place is quite depressed compared to unglaciated areas, and exceedingly complex as compared to stream, eolian, or coastal landscapes. This is, in part, due to a yet-to-be-integrated drainage pattern and the periglacial landscape of outwash materials.
Mountain glaciers begin with a cold temperature and an accumulation of precipitation at the highest elevations. Over a long period of time, as with continental glaciers, snow is compacted, eventually forming ice. The glacial ice begins to move downward by gravity along lines of least resistance, forming tongues, usually in former stream valleys. In many cases these tongues of ice find their way to sea level, and when the ice recedes the fjords remain behind as remnants.
Mountain glaciers sculpt valleys by eroding various types and hardnesses of rock. Then the melting ice leaves the sculpted materials in depositional features. Eroded features include cirques or bowl-shaped indentations near the peaks, or matterhorns where the glaciers begin, the knife-like ridges between the valleys called aretes, and low places in the ridges called cols. Valleys that were originally V-shaped due to stream action become U-shaped due to ice action, and some have depressions called tarns. Retreating glaciers deposit terminal and recessional moraines during stillstands, as well as lateral moraines along their edges as they retreat and medial moraines where two lateral moraines have merged. Glacial till carried from the eroded areas is dropped by the melting ice to form braided streams and outwash plains. In general, mountainous terrain that has been glaciated appears much more concise, complex, and abrupt when compared to unglaciated mountains. For example, the Matterhorn and its descending glaciers in the Swiss Alps are much more sharply unique in appearance as compared to Mount LeConte and other unglaciated peaks and their descending streams in the Great Smoky Mountains of the United States.
Time, distance, and speed play their part in sculpting the earth. Glaciers, like wind, streams, waves, and currents, flow. How fast does the ice flow? How far does the ice flow? How long does the ice take to flow? Winds move much faster than ice and travel the farthest, across land and sea, while streams are restricted to the land, and waves and currents are restricted to the sea. Glaciers move slowly across the land and out into the sea. If they didn’t, there would be no icebergs today!
Glacial Movement and Climate Change
Glaciers, at their maximum extent during the Pleistocene, were widespread, covering more than an estimated 45 million square kilometers. Glacial ice moves much like a stream, in that the fastest motion is found in the center of the lobe or tongue. That is, the slippage and flowage is considerably slower closer to the rock formations where plucking and erosion by the ice adds friction. This is tempered by variations in rock hardness and degree of slope. Consequently, there is a huge variation in the mean rate of ice advance. Some of the intermittent advances of mountain glaciers with short duration vary between 2.4 and 32.3 meters per day, with advances between 606 and 4,850 meters per year. For long-term advances of mountain glaciers, the mean rate varies between 80 and 128 meters per year over a period of 100 years.
Continental glacier movement is much more difficult if not impossible to measure, as continental glaciers either no longer exist or are in a recessional mode. It is known that four major ice advances and accompanying recessions occurred during the Pleistocene, with their nomenclature varying depending on location and language. But these major advances included minor advances and recessions. For example, seven substages of the Wisconsin glacial stage in the midwestern United States have been observed. In the western hemisphere, numerous lobes descended from the Laurentide of Canada across the plains of the midwestern United States, forming a complex landscape. From radiocarbon dating it is known that ice receded from northcentral Iowa less than 3,000 years before the present, and that near the bottom of the Des Moines lobe the oldest drift is older than 40,000 years before present. This is presumed to be during the Wisconsin, which began about 70,000 years before present. The maximum glacial extent during the Wisconsin occurred about 14,000 years before present, and a considerable amount of glacial activity occurring before and after, including loess deposition and the development of the present day paleosols, all occurred during the interglacial stages. It can be assumed that similar circumstances occurred in Europe and Siberia.
Climate fluctuations during the Pleistocene and Holocene, coupled with periglacial effects, are felt around the world, especially eustasy—worldwide change in sea level. The outwash from glaciers is the most obvious periglacial activity, along with the less obvious permafrost. Also, glacial rebound is evident where the deepest ice has melted away, such as Hudson Bay. Then too, there is the migration of vegetation poleward as temperature and moisture changed. Humankind soon followed the retreating glaciers.
Today, scientists worldwide are monitoring climate change, including rising sea levels, something that has been happening since the late 17th century. Ancient shorelines around the world show that when glaciers were near or at their minimum extent, sea level was about 400 feet higher than it is today. If the sea level continues to rise, then it may reach that stage again; how soon no one knows for sure, but it will continue to rise if global warming and global dimming continue. On the other hand, when and if the present interglacial stage ends, the sea level may fall. During the Illinoisan glacial stage when ice was at its maximum, the sea level is estimated to have been 600 feet below the present level, a condition that is difficult for many people even to imagine.
Although glaciers worldwide are receding at the present time, within the next 90,000 years or so they will again expand across large portions of the earth’s landmasses, especially those located in the higher latitudes and altitudes.
Richard A. Stephenson
See also Erosion; Geologic Timescale; Global Warming; Ice Ages; Nuclear Winter
Flint, R. F. (1957). Glacial and Pleistocene geology. New York: Wiley.
Martini, I. P., et al. (2001). Principles of glacial geomorphology and geology. Upper Saddle River, NJ: Prentice Hall.
Weiner, J. (1986). Planet Earth. Toronto, ON, Canada: Bantam Books.
Wright, H. E., Jr., & Frey, D. G. (Eds.). (1965). The quaternary history of the United States. Princeton, NJ: Princeton University Press