Page 64 - Earth's Climate Past and Future

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4 Ice sheets
present

Past glaciations and continental positions. During Earth’s 4.55-
billion-year history, intervals when large continental ice sheets were
present alternated with times when they were not (left). The earliest
history of these changes is poorly defined because few ancient
records are preserved. The movements of continents in relation to
ocean basins are well known only for the last several hundred million
years (right). (Globes adapted from D. Merritts et al., Environmental
Geology, © 1997 by W. H. Freeman and Company.)