The most recent ice age can be considered the last million years of geologic time—the Cenozoic era, the latter part of which comprises the Pleistocene and Holocene epochs, which include the most recent ice age. There is evidence, however, that other ice ages occurred much earlier in time, although their existence and extent are more difficult to determine. Evidence of ancient climate change, which covers a range in temperatures from tropic to arctic, is mostly found with fossil evidence. For example, a reef complex and marine invertebrates tend to indicate a tropical area, while mastodon fossils show a cold environment. Thus, knowledge about the ice ages depends to some extent on the study of fossils.
At present, we know of six ice ages. The oldest two known occurred during the Precambrian era, more than 570 million years ago; one of these may have extended into the early Cambrian era. The next oldest ice age occurred during the Permian period and had a time span of about 55 million years, beginning about 280 million years ago. The next ice age came in the early Cretaceous period and another in late Cretaceous, but both were rather limited in extent. The span of time between the early and late Cretaceous ice ages is about 71 million years, beginning about 136 million years before the present (BP). The present ice age began about 1 million years ago, and because glaciers still exist but are receding, it can be considered an interglacial stage, presuming another glacial stage is forthcoming.
The ice ages of the Cryptozoic eon might be difficult to imagine, but glacial till or tillite with a thickness of more than 500 feet has been identified, as have areas with grooved, striated, and faceted boulders, dating from middle Huronian time during the Proterozoic era in Canada, north of the present Great Lakes, extending more than 1000 miles in diameter. Also, ancient tillites have been found in Manitoba, eastern Greenland, and northern Utah, where layers of tillite with other formations have a thickness greater than 12,000 feet and could be a part of the Cambrian. This type of ice age evidence is also found in southwest Africa, as well as the Transvaal, the Katanga, Griqualand, and South Africa. In Australia, the Flinders Range shows tillite more than 600 feet thick. Ancient tillites are also found in northeast China, and northwestern and eastern India, dating from the end of the Proterozoic. With only tillite to show the location of the glaciation, and without fossil evidence from the Precambrian, it is impossible to estimate the time periods for these occurrences, except that these ice ages, with perhaps others not yet known, occurred approximately from 500 million to 2 billion years BP. So, at least two ice ages are presumed to have occurred in the Precambrian eras.
The ice age of the Permian period was mostly in the southern hemisphere. Primarily consisting of ice sheets, the glaciers covered a large part of southern Africa, portions of Nigeria, Uganda, and the southern tip of Madagascar. Three small portions in India, three areas in Australia, and six sections of South America were also covered with glacial ice. These areas show striated, grooved, and polished rock, with adjacent tillite formations sometimes greater than 100 feet in thickness. These locations appear to have been glaciated repeatedly, all moving in a northerly direction lying within 20° to 35° of the equator. It is likely that these land masses were much farther south than they are at the present time, giving rise to the acknowledgment of continental drift. That is, ice sheets in areas of India and Nigeria that are now north of the equator were well south of the equator in Permian times. Fossils formed during the Permian have been found near the glaciated areas. Especially noted were the small, hardy tongue ferns, Glossopteris and Gangamopteris, surviving the increasingly harsh climate. These fossils tend to confirm the specific time of the Permian ice age.
The ice age during the Cretaceous period, again, was mostly in the southern hemisphere during Aptian time or in the Late Cretaceous. This is when the dinosaurs ceased to exist, during the Laramide disturbance. Thus, the end of the Mesozoic era and the beginning of the Cenozoic era, about 70 million years BP, is adequately marked. It is noted that the plateau of eastern Australia was ice-capped, with glaciers flowing toward the west into the sea. Icebergs also calved into an inland sea to the east.
During the Cenozoic era, the most recent and well-known ice age occurred. The Pleistocene epoch had four ice ages, or in some accounts, one great ice age with four separate ice advances separated by interglacial stages. Using North American terminology, beginning with the Nebraskan stage about 1 million years ago and lasting about 100,000 years, global warming, or the Aftonian interglacial stage, was about 200,000 years in length. Then, the Kansan stage of glaciation began and lasted for about 100,000 years, followed by the Yarmouth interglacial stage, which lasted for 310,000 years. The Illinoian stage was only about 100,000 years in length, but it appears to have the greatest ice coverage of the four advances during the Pleistocene. The Sangamon interglacial stage was about 135,000 years long; this was the shortest of the three interglacial stages. The Wisconsin stage began about 55,000 years ago and ended a mere 11,000 years ago with the advent of the present interglacial stage, which is known as the Holocene epoch. It is known that the Wisconsin glacial stage had four distinct ice advances and recessions, making a complex landscape across the Midwest of the United States with the same being true in Europe and Siberia.
It is generally accepted that during the last million years or so, crustal movement has been poleward in the northern hemisphere, and that the continents whose shapes and location we know today were different long ago. These are all, of course, geological concerns involving time. Also to be considered are meteorological concerns, which involve the cyclical changes in temperature of land, sea, and atmosphere. At present there is a warming trend that began an estimated 11,000 years ago. It is questioned whether we are at the end of the ice ages or merely between two glacial ice advances. If the global atmospheric temperature decreased a mere 5° C, another ice advance would be likely to begin. How long is this warming trend to last? How has humankind affected this warming trend? What effect will it have if we are still in the Pleistocene ice age or at the end of it? There are more questions than answers. Perhaps significant, however, is that the occurrences of the six known ice ages are spaced closer in time toward the present.
Approximately 4,500 to 5,000 years ago, ice covered all of Minnesota; it had been receding from Iowa since about 2,800 years earlier, although ice had reached as far south as northern Missouri. Today the last vestige of this ice is the arctic polar ice cap, which is continuing to recede. As the ice receded, vegetation and wildlife followed, with aborigines migrating northward from the southern climes. So it is today with nations looking for environmental resources where the ice is melting. Since the shortest known interglacial stage is the Sangamon, which lasted 135,000 years and ended 55,000 years ago, it can be expected that a phase of global warming is just beginning and will be here for tens of thousand of years more, whether affected or not by the activities of humankind.
Richard A. Stephenson
See also Cretaceous; Erosion; Geology; Glaciers; Pangea;
Ausich, W., & Lane, N. G. (1999). Life of the past
(4th ed.). Upper Saddle River, NJ: Prentice Hall. Chernicoff, S., & Whitney, D. (2007). Geology (4th ed.).
Upper Saddle River, NJ: Pearson Prentice Hall. Martini, I. P., Brookfield, M. E., & Sadura, S. (2001).
Principles of glacial geomorphology and geology.
Upper Saddle River, NJ: Prentice Hall.
Weiner, J. (1986). Planet earth. Toronto, ON, Canada: Bantam Books.