Glacial Periods and Eras

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1. What are "glacial periods" and in what geologic eras and periods do they occur? What explanations help to explain these glacial periods?

DEFINITION OF GLACIAL PERIODS

Glacial periods are times during which large portions of the earth's surface were covered with thick glacial ice sheets.

PERIODS OF GLACIATIONS

In the Pleistocene epoch, in the Carboniferous and Permian periods of the Paleozoic era, and in Huronian time of the Precambrian era, the earth experienced an overall cooling of the climate, resulting in great ice sheets covering great portions of the oceans and continents. More or less extensive continental glaciations, or glacial advances, may have occurred at other times. ...