The Pleistocene Epoch ranges from 1,800,000 to 10,000 years ago and is the oldest division in the Quaternary Period. The boundary between the Pleistocene and the Holocene is defined loosely and is based on the start of the warm interval. The boundary is delineated by the melting of the continental ice sheets and the simultaneous rise of sea level and is placed closest to the midpoint in sea-level rise and ocean warming. The Pleistocene was first described by Charles Lyell in 1839 from strata in southern Italy previously lumped into the Pliocene. His basis for separating the two epochs was his recognition, in 1833, of the occurrence of modern North Sea mollusk species in the Mediterranean in the higher beds. The Pleistocene was described as a glacial epoch by Edward Forbes in 1846 without realizing that glaciation began at different latitudes at different times.
During the Pleistocene, glaciers advanced and retreated many times in the Northern Hemisphere, sea level rose and fell through a vertical range of 100 m, weathering rates and soil-forming (pedogenic) processes varied with temperature and precipitation changes, and habitats contracted and expanded, forcing species to migrate and adapt to environmental changes. It was during this time period that our own species evolved and rose to dom inance. Today, we recognize that the advance and retreat of glaciers was complex and occurred as many as eighteen times, as recorded by glacial moraines, lake-bottom varve deposits, glacial ice sheets on continents, and oxygen isotope ratios in carbonates preserved in deep-water sediments.
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