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When did large ice sheets first form in the Northern Hemisphere?
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Tertiary Period
Gypsum cones, which resulted from the evaporation of the Mediterranean Sea during the Messinian Salinity Crisis, in the Sorbas basin, Spain.
Large ice sheets first formed in the Northern Hemisphere approximately 3.5 million years ago, during the latter part of the Tertiary Period. Following this initial formation, a major expansion of these ice sheets occurred about 2.5 million years ago. This period marked the beginning of an interval of clustered glacial periods, often referred to as ice ages, which have continued to shape the Earth's climate and surface.1