During evaporation, lighter isotopes are concentrated in the water vapor in the air. The snow accumulates to form glaciers. Cold, dense polar water flowed across ocean bottom. Climates cooler, drier, and more seasonal. Late Pliocene and Pleistocene had strong, rapid, climatic fluctuations.
Ice age is characterized by many glacial expansions separated by warmer interglacial intervals.
Click here to see US relief map from Johns Hopkins University Applied Physics Lab. Click here for Ice Age exhibit from Illinois State Museum . Includes a video of retreating glaciers and fossil photographs. A large continental mass became centered over a pole. Polar areas were cut off from warming by tropical waters.