The story of the ice age

"For thousands of years it never occurred to anyone that there might have been an Ice Age. Then about 150 years ago a Swiss scientist named Louis Agassiz starting asking questions. Why did the huge wooly mammoth disappear? How did boulders from Scandinavia end up at the bottom of Swiss glaciers? From the answers, Agassiz drew a startling conclusion: tens of thousands of years ago there had been an Ice Age. Great sheets of ice once covered the land where famous cities now stand. How did the Ice Age start? Why did it end? Can it happen again?