ScienceDaily: Hidden below the surface of California's Central Valley are pollen grains from the Pleistocene that are providing scientists with clues to the severity of droughts that struck the region between glacial periods. The Pleistocene -- the age of mammoths and mastodons -- occurred between 1.8 million and 11,500 years ago. For this new study, scientists dug up Pleistocene sediment samples containing buried pollen from the Central Valley. They found that pollen samples dated from interglacial periods --...