The story becomes clearer and the life forms become more and more familiar to us during the 65 million years of the Cenozoic, the present era. In the first half of the Cenozoic, subtropical vegetation grew in the southern United States and temperate vegetation grew north to the Arctic. As these plants would indicate by their ability to grow here, this was a time of warm or mild climates throughout the Northern Hemisphere. Land bridges between North America and Eurasia, by way of the Bering Strait and perhaps Greenland, allowed the spread of a remarkably homogeneous flora throughout the then-temperate parts of these two continents. The Great Smokies, with their feet in the South and, as it were, their head climatically in the North, must have had both subtropical and temperate vegetation early in the Cenozoic era. During the second half of the Cenozoic, a cooling trend set in. The widespread “Arctotertiary” vegetation of the northern latitudes moved southwards through North America and Eurasia. By the end of the Tertiary, which includes all but the past two to three million years of the Cenozoic, the vegetation zones of North America were probably very similar to those of the present. In the Smokies the trees probably ranged from southern types, such as sweetgum, at low elevations through the great mixture of cove forests and possibly to spruce and fir at the highest elevations. After a long period of gradual change in climate, the stage was set for the drastic events of the Pleistocene. It is hard for us to imagine what an ice age must have been like in our country. Perhaps the only way to imagine it is to visit the Antarctic or one of the great glaciers in Alaska, and to watch giant slabs of ice fall from those towering walls. Then ... mentally transport the scene to the Hudson River valley or to the flatlands of Illinois, while magnifying the thickness of those glaciers several times over. Then imagine the surface of that great ice sheet stretching all the way to northern Canada. If you had stood near the front of that massive ice sheet, you would have felt the cold air flowing off it. How far south that cold, dense air flowed and to what extent it affected temperatures in the southern states are unanswered questions. But undoubtedly temperatures were lowered throughout North America and perhaps farther south. Some scientists postulate a drop of 5.5 degrees Celsius (10 degrees Fahrenheit) in mean annual temperatures in southern United States. The high pressure that developed over the ice sheet would have pushed storm tracks southward, increasing precipitation in the South.