As with other older geologic periods, the rock beds that define the start of the Pleistocene are well identified, but the exact dates of the start and end of the period are slightly uncertain. The Pleistocene was originally intended to cover the recent period of repeated glaciations, however, the start was set too late and some early cooling and glaciation are now set in the Pliocene. Some would prefer a start date of around 2.5 million years BP.
Continents were at essentially their modern positions during the Pleistocene probably moving no more than 100km at most. The Pleistocene climate was characterized by repeated glacial cycles where continental glaciers pushed to the 40th parallel in places. Four major glacial events have been identified as well as many minor intervening events. The four major identified glacial excursions were the Nebraskan-Gunz, Kansan-Mindel, Illinoian-Riss, and Wisconsin-Wurm. There may have been as many as 14 additional unnamed advances whose results have been largely erased by the later glaciers. Each glacial advance tied up huge volumes of water in continental ice sheets 1500-3000 meters thick resulting in temporary sea level drops of 100 meters or more. Antarctica was ice-bound throughout the Pleistocene as well as the preceding Pliocene Period.
Both marine and continental faunas were essentially modern. Humans evolved into modern man during the Pleistocene. Major extinctions of large mammals including mammoths, mastodons, sabertoothed cats and giant sloths started late in the Pleistocene and continued into the Holocene. The extinctions were especially severe in North America where native horses and camels were eliminated.
Pleistocene continental deposits are found primarily in lakebeds and caves as well as in the large amounts of material moved about by glaciers. Pleistocene marine deposits are found primarily in areas within a few tens of kilometers of the modern shoreline. In a few geologically active areas such as the Southern California Coast, Pleistocene marine deposits may be found at elevations of several hundred meters.