Besides the blocked seaways of the present ice age, most of the northern hemisphere is land, which generates a continental climate, whereas the southern hemisphere is encircled by a continuous seaway that provides far more maritime climates. If the land and sea were uniform in both hemispheres, the precession effect would be canceled out. Some researchers propose that other large-scale influences, including feedback from changes in ocean currents, are equally important. 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.