What caused the ice ages? Why were the Siberian mammoths frozen alive, or how did great forests grow in Antarctica? The theory propounded by Maurice Ewing and William Donn, and described by Mr. Andrist in his article, is only one of the most rigorous and ingenious of many attempts to account for the often-contradictory evidence left behind by the glaciers. As recently as 1953 a geologist called this “one of the greatest riddles in geological history.”

Some investigators have argued for a change in the amount of heat received from the sun, either by a shift in the earth’s position or the interference of some cosmic dust cloud. And some, with much data to support them, have suggested that the poles may not always have been located where we find them now. Studies of ancient rocks have shown that many (those older than sixty million years) are permanently magnetized in a very different direction from the present poles, reinforcing the theory that the land masses on the earth’s surface—slowly, during millions of yearsmay change their positions.

One of the most unorthodox proposals is that of Charles H. Hapgood, who maintains in his book Earth’s Shifting Crust (1958), that the surface may have moved even in more recent times, shifting the poles, causing the last icecap over North America to melt, and bringing down the freezing cold on Siberia. What the globe would have looked like earlier—if the pole had until then been situated in the region of Hudson Bay, as Mr. Hapgood believes—is shown in the sketch below. His theory is by no means accepted scientifically, but his vigorous defense of it serves to remind us how many puzzles are buried in the past, and are yet to be solved.

— The Editors