Ice Age

the spreading of ice as glaciers over areas that have, before then, been ice-free. Most is known about the last ice age during the Pleistocene geological epoch 2 million years ago. However, geological studies show evidence of earlier ice ages although less is known about these.

Taken from Dictionary of Science

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The general term "ice age" or, more precisely, "glacial age" denotes a geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in an expansion of continental ice sheets, polar ice sheets and alpine glaciers. An ice age is a natural system. Within a long-term ice age, individual pulses of extra cold climate are termed "glaciations". Glaciologically, ice age implies the presence of extensive ice sheets in the northern and southern hemispheres; by this definition we are still in an ice age (because the Greenland and Antarctic ice sheets still exist).

Taken from Wikipedia