Ice Ages

The most recent ice age can be considered the last million years of geologic timethe 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.

Figure: The Scandinavian Peninsula is a landscape that was largely shaped by glaciers over the last ice age. The moderate resolution imaging spectroradiometer (MODIS) instrument aboard NASA's Terra satellite captured this image of the Scandinavian Peninsula on February 19,2003. Along the left side of the peninsula one can see the jagged inlets (fjords) lining Norway's coast. Many of these fjords are well over 2,000 feet (610 meters) deep and were carved out by extremely heavy, thick glaciers that formed during the last ice age

Source: Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC.

[Page 698]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.

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