The Pleistocene was a time of ice ages: cold glacial periods with shorter, warmer, interglacials. Ice ages are when the world becomes much colder for a long time. In much of the 20th century geologists counted four "major" ice ages. Nowadays several more are called "major". During these glaciations, much of the world we know was covered by ice: North America down to and past the Great Lakes; all of northern Russia, and Europe; England down to the Thames.