The existence of past ice ages was discovered by several 19th century geologists from scratch marks on rocks, erratic boulders, moraines, and other physical observations. As early as 1920, Chamberlain provided a map of the North American and Greenland ice sheets at the last glacial maximum that remain quite accurate even today. Two massive ice sheets dominated the northern hemisphere. Nearly a quarter of the earth’s surface lay under the weight of a mountain of ice. The Laurentide ice sheet is believed to have reached a height of 12,500 ft. Ice covered nearly 5 million square miles of North America. As the glaciers grew, they drew so much water that the ocean levels dropped more than 100 m. The expansion of the glaciers dramatically affected the distribution and composition of vegetation. Global flora was impacted, by both CO2 starvation than cold. Deserts expanded and wind-blown dust became prevalent at the last glacial maximum.