Glacial experts call a period of unusual and widespread rains a pluvial. They believe that the growth of the glaciers was accompanied by pluvials, and some maintain that pluvials also marked the melting of the ice. Antevs lists a great pluvial—the Bonneville—which formed Lake Bonneville and Lake Lahontan during the first maximum of the last, or Wisconsin, glaciation, 65,000 years ago. He places another period of great moisture, the Provo Pluvial, at the last glacial maximum, about 12,000 years ago, and believes it was all but spent 7,000 to 9,000 years ago.[29] Thus he dates Lake Mohave and its early artifacts about 9,000 years ago. His point is that annual precipitation is about the same throughout the globe, but that the pluvials shift at various times. “It is essentially the location of the rainfall that changes.”[30]
All authorities do not agree that annual rainfall is constant, and some deny Antev’s late date for the last pluvial. Sauer writes, “I know of no climatologic basis for postulating a postglacial pluvial period,” either in New Mexico or the Lake Mohave and Pinto Basin area of southern California.[31] Some European authorities believe that pluvials are entirely glacial. M. C. Burkitt, for example, cites the fact that the same type of tools is found in Africa during pluvials as in Europe during glacials.[32] According to Simpson’s theory of the formation of the glaciers, rainfall increased enormously during two periods of the Great Ice Age; the last pluvial was at its height during the building up of the Würm-Wisconsin ice sheets. (See page 58.) The pluvials make a rather good case for early man in the New World during the last glaciation.
Ten years ago, we knew that men in America had once killed, skinned, and eaten animals now extinct, for we had found their weapons and a few of their bones mingled with the fossils of mammals long extinct. We could not question the association, because it often involved the remains of campfires; spear points and bones might be moved about in the course of time, but not fragile heaps of charcoal. We knew, too, that the bones and tools of man had been found sealed away by the chemistry of the ages. But when we tried to date man securely by animals that were dead and gone, or by the earths that lay above him, we began to guess. The guesses of conservative glacialists gave him at least 15,000 years in the New World. Other speculations—supported by rather plausible evidence as well as not implausible theory—placed him still earlier. Now, at last, through the miraculous time clock of radiocarbon, we know that man was here before the chill of the last Ice Age settled upon the land.