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In 1839 Charles Lyell introduced the geological system of classification. The Pliocene (Newer, from the Greek pleíon" "more," and kainos, "new", Latinized as cænus) was follwed by the Pleistocene (Newest, from pleīstos, "most," and kainos), which was followed (now believed to be in ca. 9700 BCE) by the Holocene (Entirely New, from holos, "whole," and kainos). The Pleistocene is popularly called the "Ice Age" and it ended with a climatic period known as the Younger Dryas; it was followed by the Boreal ("[far] northern" in Latin and Greek) , a liminal period that was followed by thewarmer and moister Atlantic period. Lyell had based his classification on his study of molluscan fauna in Sicilian strata; the paleontological classidication on sequences of Danish peat bogs. The peat bog layers were first noticed by Heinrich Dau in 1829, and explained by the Norwegian botanist and geologist Axel Blytt in his "Essay on the Immigration of the Norwegian Flora during Alternating Rainy and Dry Periods" in 1876, hypothesizing that the darker layers were deposited in drier times, the lighter ones in moister times. In 1908 the Swedish botanist, geologist, and archaeologist Rutger Sernander defined subboreal and subatlantic periods; he was also one of the founders of palynology, the "study of dust" or "strewn particles," which the Swedish naturalist and geologist Lennart von Post further developed and incorporated the Blytt-Sernander classification into a sequence of pollen zones.
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