INTERGLACIAL STAGES, a series of comparatively warm and mild intervals, each lasting several thousand years, between the equally prolonged times of extreme cold in the Pleistocene glacial period (q.v.). For the stages of extreme cold the geologist Albrecht Penck chose names from repre sentative districts in the Alps : Mindel, Riss and Wurm; of these, the Gunz stage is the earliest, the Wurm stage is the most recent. Penck chose these names because their order in the alphabet corresponds to the chronological order of the four stages, while space is left between them for additional names, should additional stages of cold be defined.
The same names are conveniently used to indicate the inter vening warmer stages. Thus the first mild interglacial stage is called the Giinz-Mindel stage; the second, the Mindel-Riss stage; the third, the Riss-Wiirm. This cyclic alternation of climate through the Pleistocene period is accompanied by alternating Arctic and temperate fauna and flora, while there is a further change, particularly of the fauna, due to the steady evolution of forms throughout the Pleistocene period. Thus in Europe during the colder stages the woolly mammoth and reindeer reached the south of France, while one of the interglacial stages was so warm that the lion and hippopotamus flourished in England.
In North America there was a similar cyclic alternation of climatic conditions throughout the Pleistocene glacial period, but there are difficulties in the way of drawing definite parallels be tween the Old World and the American stages. It is likely, how ever, that the earliest glacial stage in North America, which includes the pre-Kansan, Nebraskan and Albertan drifts, and which is called sub-Aftonian by American geologists, coincided with the Giinz of Penck. The earliest, or Aftonian, interglacial stage of North America would thus correspond with Penck's Giinz-Mindel interval. In North America, the Aftonian inter glacial stage was marked by the presence of mastodons, three species of elephants, six species of horses, and the sabre-toothed tiger.
The second, or Kansan glacial stage in North America, cor responding to the Mindel of Europe, saw the extinction of some American camels and horses. It was followed by the Yarmouth interglacial stage, parallel to Penck's Mindel-Riss, when the fauna included mastodons, mammoths, horses, tapirs, bison, deer and sabre-toothed tigers.
The third, or I1linoian, glacial stage in North America, cor responds to Penck's Riss. It was followed by the Sangamon interglacial stage; this was in its turn fourth, or Iowan, glacial stage, the record of which is not well defined. The fourth, or Peorian, interglacial stage followed; it presents similar difficulties. The fifth, or Wisconsin, glacial stage appears to correspond to the European Wiirm. It was followed by the post-glacial stage, which gradually merges into the present time. Elephants, mastodons, horses and the great sloths disappeared, the exact cause of their disappearance being unknown.
In the earlier or pre-glacial stage, Pleistocene vegetation was characterized by a commingling of warm temperate and cool temperate forms. During the alternating interglacial periods the record of plant life in the more northern regions indicates a suc cession of forest, barren ground, steppe, tundra and arctic types with each increase of glaciation. Upon the retreat of the glaciers this succession of plant types recurred but in reversed order. In some interglacial periods the climate was milder for the same latitudes than at the present time, so that numerous trees, for example, extended some hundreds of miles north of their present range. In the post-glacial or late Pleistocene stage, which blends into the Recent, plant life assumed substantially the present status and distribution. Many arguments may be advanced sup porting the view that the post-glacial and recent constitute a part of an interglacial stage, as the time that has elapsed since the disappearance of the last great ice-sheets from Europe and north eastern North America is less than the duration of some of the former interglacial stages.
The effect of glaciation upon the vegetation of Europe and North America, especially the former, was profound, not only with reference to the present distribution of the flora, but also as regards the extinction of old and the evolution of new types. In Europe the high mountain chains, which extend in an east and-west direction, together with the Mediterranean, Black and Caspian seas, prevented the migration of the flora southward. In consequence an immense number of plants formerly components of the European flora, particularly that north of the Alps, was obliterated by glaciation. Numerous types of trees now found only in North America and Asia were abundant in Europe in pre-Pleistocene and part of Pleistocene times. Among these were such important trees as the black walnut, butternut, hickory, magnolia, bald cypress and sassafras (qq.v.).
In North America the back-and-forth movement of plant life was not impeded, as in Europe, by high mountains and inland seas. The presence of the larch (Larix laricina) in Georgia, of the black spruce (Picea mariana) in Kansas and of the white spruce (Picea glauca) in Iowa, all far south of their present range, has been demonstrated. Likewise, the presence during a mild interglacial stage of a fig in fruit in British Columbia, and of the redbud (Cercis), the Osage orange (Madura), the papaw (Asimina triloba) and other warm temperate types as far north as Toronto, Ont., has been proved.