MARKING THE BOUNDARY between the Holocene and Pleistocene epochs, the Younger Dryas, a period of glacial conditions between 12,900 and 11,500 years ago, is named for Dryas octopetala, a flower that is adapted to the cold. Dryas pollen is found in abundance in strata of this age. Dryas pollen is also found in older strata, necessitating the term Younger Dryas to distinguish this time from older periods in which Dryas pollen is abundant. Locked in an ice age, earth had finally warmed and the glaciers had begun to retreat 15,000 years ago. Counteracting this warming trend, the Younger Dryas reduced temperatures 50 degrees F in only a decade. Glaciers once more advanced in North America and Europe. Rainfall diminished, and frigid winds carried dust from ...

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