Evidence of earlier ice ages also exists. For more than 90 percent of that time the earth has been free of ice ages, and no large glaciers have existed except in high mountains. Ice ages occur about every 150 million years, and last a few million years. The longest of the ancient ice ages was probably the Permo-Carboniferous, which began about 300 million years ago and affected all southern hemisphere lands. Still earlier, about 435 million years ago, another giant ice sheet extended from Brazil to North Africa and all the way across to Yemen and Saudi Arabia.
Paleomagnetic measurements indicate that the South Pole then lay in West Africa.
About 600 million years ago, yet another great glacial age occurred. Evidence of these ancient ice ages is provided by layers of tillite, a rock consisting of hardened glacial drift. Between Ice Ages . How a Global Catastrophe Allowed Humans to Evolve . Explaining the Ice Ages .