Many people are aware that during the last couple of million years the Earth experienced a series of ice ages, when ice sheets expanded into regions now occupied by major cities such as Chicago, New York, Birmingham, Oslo, Stockholm, Berlin, Moscow and Zürich. However, this is only part of the story because the geological record shows that for over 3000 million years (m.y.), Earth's history has been dominated by warm ‘greenhouse’ conditions, punctuated by a series of cold phases or ‘ice-house’ conditions when extensive glaciers and ice sheets developed.
For the last cold phase on the Earth, the Cenozoic Era, the evidence for extensive ice cover is usually in the form of a wide variety of landforms (as described in Chapter 10), and soft poorly sorted sediment called till, while associated sand and gravel are indicative of deposition from glacial meltwaters. Evidence for glaciations in the more distant past is sketchier, but is represented mostly in the form of ancient glacial deposits, commonly referred to as tillites (till which has hardened into rock), and associated sediments and erosional phenomena.
The purpose of this chapter is to outline how ideas concerning ice ages developed, how geologists recognize the evidence for glaciation in the rock record, provide a summary of when and where the main periods of glaciation took place, and to make some observations on the causes of glaciation.