Editing Ice age (section)

==Evidence==
There are three main types of evidence for ice ages: geological, chemical, and paleontological.

''Geological'' evidence for ice ages comes in various forms, including rock scouring and scratching, [[glacial moraine]]s, [[drumlin]]s, valley cutting, and the deposition of [[till]] or tillites and [[glacial erratic]]s. Successive glaciations tend to distort and erase the geological evidence for earlier glaciations, making it difficult to interpret. Furthermore, this evidence was difficult to date exactly; early theories assumed that the glacials were short compared to the long interglacials. The advent of sediment and ice cores revealed the true situation: glacials are long, interglacials short. It took some time for the current theory to be worked out.

The ''chemical'' evidence mainly consists of variations in the ratios of [[isotope]]s in fossils present in sediments and [[sedimentary rock]]s and [[ocean sediment]] cores. For the most recent glacial periods, [[ice core]]s provide climate [[Proxy (climate)|proxies]], both from the ice itself and from atmospheric samples provided by included bubbles of air. Because water containing lighter isotopes has a lower [[heat of evaporation]], its proportion decreases with warmer conditions.<ref>{{cite journal |title=How are past temperatures determined from an ice core? |journal=Scientific American |date=2004-09-20 |url=http://www.scientificamerican.com/article.cfm?id=how-are-past-temperatures |access-date=2011-04-04 |archive-date=2013-05-20 |archive-url=https://web.archive.org/web/20130520182757/http://www.scientificamerican.com/article.cfm?id=how-are-past-temperatures |url-status=live }}</ref> This allows a temperature record to be constructed. This evidence can be confounded, however, by other factors recorded by isotope ratios.

The ''paleontological'' evidence consists of changes in the geographical distribution of fossils. During a glacial period, cold-adapted organisms spread into lower latitudes, and organisms that prefer warmer conditions become extinct or retreat into lower latitudes. This evidence is also difficult to interpret because it requires:
#sequences of sediments covering a long period of time, over a wide range of latitudes and which are easily correlated;
#ancient organisms which survive for several million years without change and whose temperature preferences are easily diagnosed; and
#the finding of the relevant fossils.

Despite the difficulties, analysis of ice core and ocean sediment cores<ref>{{cite journal |title=Glacier advance in southern middle-latitudes during the Antarctic Cold Reversal |first1=Aaron E. |last1=Putnam |first2=George H. |last2=Denton |first3=Joerg M. |last3=Schaefer |first4=David J. A. |last4=Barrell |first5=Bjørn G. |last5=Andersen |first6=Robert C. |last6=Finkel |first7=Roseanne |last7=Schwartz |first8=Alice M. |last8=Doughty |first9=Michael R. |last9=Kaplan |first10=Christian |last10=Schlüchter |year=2010 |journal=[[Nature Geoscience]] |volume=3 |pages=700–704 |issue=10 |doi=10.1038/ngeo962|bibcode=2010NatGe...3..700P }}</ref> has provided a credible record of glacials and interglacials over the past few million years. These also confirm the linkage between ice ages and continental crust phenomena such as glacial moraines, drumlins, and glacial erratics. Hence the continental crust phenomena are accepted as good evidence of earlier ice ages when they are found in layers created much earlier than the time range for which ice cores and ocean sediment cores are available.