Pleistocene Climate Even though glaciation of the Northern Hemisphere began earlier, we currently recognize 1.6 million years ago as the start of the Pleistocene epoch, corresponding to the southward advance of large ice sheets in North America and Europe. The final retreat (melting and backward movement) of these glaciers northward began roughly 13,000 years ago and didn’t actually end until the ice margins reached their present position about 8,000 years ago. According to paragraph 1, what happened around the start of the Pleistocene epoch? •A.Glaciers retreated from North America and Europe.•B.Glaciation began in the Northern Hemisphere.•C.Ice margins reached their present location.•D.Glaciers moved southward into North America and Europe.Geologists studying Pleistocene glacial deposits (rocks and soil left behind by melting glaciers) recognized long ago that ice coverage was not continuous during the entire epoch. Soils containing plant debris are found between the layers of glacial sediments in many areas, recording times when the climate warmed and the glaciers retreated. Correlations (relationships) between different regions of glacial deposits and soils allowed geologists to develop an approximate chronology of glaciation. Initially, four principal stages of glacial advance were recognized; these were named (from oldest to youngest) Nebraskan, Kansan, Illinoian, and Wisconsin after the southernmost states of the United States in which the deposits of the advances were found. The warmer interglacial stages were named Aftonian, Yarmouth, and Sangamon after locations in which soils that formed following glacial retreats are particularly well developed. The chronicle of advances and retreats developed to describe the history of glaciation in Europe corresponds approximately to that of North America. According to paragraph 2, what does plant debris found between layers of glacial sediment indicate? •A.The climate was warmer during the entire epoch than previouslythought.•B.Glaciers must have advanced in some areas and retreated in otherareas at the same time.•C.Glaciers must have retreated at some points during the Pleistocene.•D.Plants were able to form and develop underneath the glaciers.But correlating and dating glacial deposits is a notoriously tricky business: glaciers tend to destroy older deposits during advances, and the deposits themselves tend to be very disorganized. Deposits older than the age limit of radiocarbon dating (about 70,000 years) cannot be dated directly, although analysis of the pollen deposited in the past and found now in surface layers can be useful to infer a record of past climate. Additionally, glacial advances did not always occur simultaneously in different regions. For example, geologists now recognize that the ice margin in the Great Lakes region experienced several mini advances and retreats during the late Wi...
