The geologic rock-record shows that, rather than the continuous presence of polar ice-sheets on Earth throughout time, there have been five major, well defined periods of glaciation during Earth history. These periods include a number of prominent events in early- and late-Proterozoic times, the Ordovician, the Permo-Carboniferous, and more recently during the late-Cenozoic, each of which lasted from tens to hundreds of millions of years.
These glacial events were often cyclical, with individual cycles lasting for millions of years, interspersed with relatively warm interglacial periods. The Earth is currently passing through an ice-age, known as the Quaternary glaciation, and is presently in the warmer Holocene interglacial period.
These events are shown on the main time period index at left and are indicated by a blue coloured panel and an ice cube symbol
Evidence for ancient glacial occurrences in the rock-record comes from various sources. These include the presence of striated rock surfaces and rock formations created by the passing of a glacier, large foreign rocks embedded within sedimentary strata, and the accumulation of characteristic rock debris. This rock debris accumulates where a glacier or ice-sheet melts and includes deposits that accumulate adjacent to streams flowing from a melting glacier, through to debris rafted well out to sea by icebergs where melting later deposits the debris on the seafloor.
Because of the wide variation in types of glacial debris, the data of Hambry and Harland are plotted on each of the small Earth models as a common red dot symbol. In this context, the plotted data simply represents rock, formations, or rock debris that have a glacial origin and includes debris that was deposited both on the lands and on the sea floors.
The major events include the:
Early-Proterozoic Era.
Late-Proterozoic Era.
Ordovician Period (no evidence for North Pole glaciation due to the presence of an ancient continental sea covering the polar region).
Permo-Carboniferous Periods.
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Late-Cenozoic Era to present-day.
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Note: glacial debris is more prevalent away from major ice sheets and generally prominent within the more temperate regions.
in the right hand age column. On each model corresponding to these time periods the north and south Polar Regions are also shaded to indicate the likely extent or presence of an ice-sheet.