The earliest known glaciation occurred about 2.3 billion years ago and is recognized in Ontario, Canada, from older tills that have lithified into a rock called
tillite. Tillites contain angular, unsorted rock fragments, many of which are polished, striated, or faceted. Another major period of glaciation occurred about 600 million years ago. Tillites from about 300 million years ago indicate that the ice sheet flowed over a supercontinent that later split apart to form Africa, Australia, Antarctica, India, and South America.
Glaciation has occurred more frequently in the last 20 million years. Episodic continental glaciations span back to about 3 million years ago. A number of different glacial periods occurred during the Pleistocene epoch, more commonly known as the “Ice Age.” The last period of glaciation that covered a large portion of North America and Europe peaked about 18,000 years ago. Evidence for earlier glaciation is less complete because of weathering in the interglacial warm periods and subsequent glaciations. The estimated worldwide temperature difference between the Pleistocene and today is only about 5 degrees centigrade.
Geologists do not understand why glaciers form, advance, and retreat. Possible causes include variations in the earth's orbit and inclination to the sun, atmospheric changes, volcanic eruptions, changes in continental positions, changes in ocean currents, or movements in the Antarctic ice sheet. Serbian mathematician Milutin Milankovitch has shown that climatic variations in the past 100,000 years coincide with periodic variations in the amount of solar energy received by the earth.