I.

Introduction

Pleistocene Epoch, third division of the Neogene Period of the geologic time scale (see Geologic Time). The Pleistocene Epoch (1.8 million years to 11,500 years before present) followed the Pliocene Epoch and is the epoch just previous to our current epoch, the Holocene Epoch (11,500 years before present to the present). Huge sheets of ice covered much of Earth’s surface during the Pleistocene Epoch. The ice changed the surface of the planet and helped provide a suitable climate for huge ice-age mammals. Animals such as the mastodon and saber-toothed tiger thrived during the Pleistocene, but became extinct at the end of the epoch as the final ice sheets retreated. The first humans appeared in the Pleistocene Epoch.

British geologist Charles Lyell named the Pleistocene Epoch in 1839. The word Pleistocene comes from the Greek words pleistos “most” and kainos “recent”. In the 1820s Lyell studied fossils of the shells of snails and clams that were embedded in layers of sediment in Europe. By studying how these shells had changed over time, he was able to divide geologic history into pieces. The Pleistocene Epoch became one of these pieces. Another British geologist, Edward Forbes, helped popularize the name Pleistocene in the 1840s.

Swiss geologist Ignaz Venetz-Sitten and Norwegian geologist Jens Esmark were the first to recognize that certain deposits in Earth’s surface from the Pleistocene Epoch had probably been created by glacial ice (see Glacier). Other geologists, including Swiss-born American scientist Louis Agassiz, expanded on these discoveries. Agassiz was the first to theorize that the Pleistocene was a time of massive glaciation. He believed that Earth had gone through an ice age. Modern geologists believe that glaciers occupied only small areas of the planet at any one time, and that they melted at different times and in different places, so the term ice age is somewhat misleading.

Agassiz used a layer of rock in a tiny town called Vrica, in Italy, to designate the beginning of the Pleistocene. That layer is about 1.80 million years old. Agassiz thought that the layer of rock corresponded to the beginning of a worldwide ice age, but further research showed that glaciers actually began advancing about 2.58 million years before present. In the 1990s many geologists began suggesting that the designation of the beginning of the Pleistocene should be pushed back to that date. The end of the Pleistocene Epoch is more straightforward: Geologists designate the boundary between the Pleistocene and the current epoch, the Holocene, as 11,500 calendar years before present. No rock layer or other geological feature marks this date. Instead, scientists use absolute dating methods such as radiocarbon dating to determine whether a rock layer falls within the Holocene Epoch.