Two hypotheses attempt to explain the formation of desert in central Asia. One explanation holds that roughly 22 million years ago, at the onset of the Miocene, the Tibetan Plateau started to lift upward. The rising land curbed the flow of moist air from the south, sparking the onset of central Asian desertification. An alternative explanation is that the supposedly arid region to the northeast of the Tibetan Plateau actually harbored shallow lakes or wetlands until as recently as 8 million years ago, at which point the historical desertification was initiated by some other mechanism. The current debate between these two proposals—either a 22- or an 8-millionyear- old onset of desertification—hinges, to a sizable degree, on the history of the fine sediments of the Tianshui Basin in central China.