Climates of the past reveal that climatic changes are never ending. They occur on all scales with a succession of variations of decreasing magnitude and duration. In the past billion years there have been numerous cold periods, those of the highest magnitude are called glacial ages. These ages are not continuously cold, they are composed of cold glaciation periods which are separated by relatively warmer interglacial periods.
At present we are living in an interglacial period which peaked about 10000 years ago. The last glacial maximum occurred approximately 25000 to 15000 years ago during the Wisconsinan Full Glacial, at which time the Laurentide and Cordilleran ice sheets covered half of the North American continent.
North America 18000 years ago North America 13000 years ago
North America 10000 years ago North America 7000 years ago