Although published as a paperback, I regard this as an important contribution to the study of past catastrophes and their relation to earth changes. I have a physical copy of this book, though it is not to hand as I write and I am relying on memory for this description. My recollection is that the authors suggest that ice ages occur quickly. That ice builds on land masses very rapidly, augmented by the albedo effect (more ice reflecting sunlight results in colder temperatures and more ice formation). The consequence is that sea levels fall, dramatically and as a result of the climate change the loci of human habitations changes as people occupy newly exposed land and flee from inhospitable regions. The thesis explains the land bridges that once connected Europe to the Americas (they suggest that at one time Europe was separated from the Americas by a very shallow sea, and Australasia to Asia.