The unit of glacial measurement chosen by Penck is the time which has elapsed since the close of the fourth and last great glaciation; this is known as the Würm in the Alpine region and as the Wisconsin in America. While more limited than the ice-caps of the second glaciation, those of the fourth glaciation were still of vast extent in Europe and in this country, so that an estimate of 20,000 to 34,000 years for the unit of the entire Postglacial stage is not extreme. Estimating this unit at 25,000 years and accepting Reeds's[(54)] estimate of the relative length of time occupied by each of the preceding glacial and interglacial stages, we reach the following results (compare Fig. 14, p, 41):
| Relative Duration | Grand Totals | Descent of Alpine Snow-Line | |||
| Postglacial Time. | Units | Years | Years | Meters | |
| (Period of Upper Palæolithic culture, Crô-Magnon and Brünn races) | 1 | 25,000 | 25,000 | ||
| IV. | Glacial Stage (= Würm, Wisconsin) | ||||
| (Close of Lower Palæolithic culture, Neanderthal race) | 1 | 25,000 | 50,000 | 1,200 | |
| 3d. Interglacial Stage. | |||||
| (Opening period of Lower Palæolithic culture, Piltdown and pre-Neanderthaloid races) | 4 | 100,000 | 150,000 | ||
| III. | Glacial Stage (= Riss, Illinoian). | 1 | 25,000 | 175,000 | 1,250 |
| 2d. Interglacial Stage | |||||
| (= Mindel-Riss, Yarmouth) (Period of Heidelberg race.) | 8 | 200,000 | 375,000 | ||
| II. | Glacial Stage (= Mindel, Kansan) | 1 | 25,000 | 400,000 | 1,300 |
| 1st. Interglacial Stage | |||||
| (= Günz-Mindel, Aftonian) (Period of Pithecanthropus or Trinil race.) | 3 | 75,000 | 475,000 | ||
| I. | Glacial Stage (= Günz, Nebraskan) | 1 | 25,000 | 500,000 | 1,200 |
The Postglacial time divisions are dated by three successive advances of the ice-caps, which broadly correspond with Geikie's fifth and sixth glaciations; they are known in the Alpine region as the Bühl, Gschnitz, and Daun. These three waves of cold and humid climate, each accompanied by glacial advances, finally terminated with the retreat of the snow and ice in the Alpine region, the same conditions prevailing as with the present climate. The minimum time estimates of these Postglacial stages and the corresponding periods of human culture, as calculated by Heim,[(50)] Nüesch,[(55)] Penck,[(52)] and many others, are summarized in the Upper Palæolithic (p. 281).
Geologic and Human Chronology
There are four ways in which the lesser divisions and sequence of human chronology may be dated through geologic or earth-forming events. First, through the age of the culture stations or human remains, as indicated by the 'river-drifts' and 'river terraces' in or upon which they occur; second, through the age of the open 'loess' stations which are found both on the 'older terraces' and on the plateaus between the river valleys; third, through the age of the shelters and caverns in which skeletal and cultural remains occur; fourth, through the age of the 'loam' deposits, which have drifted down on the 'terraces' from the surrounding meadows and hills. The men of the Old Stone Age were attracted to these natural camps and dwelling-places both by the abundance of the raw flint materials from which the palæoliths were fashioned and by the presence of game.
In more than ninety years of exploration only three skeletal relics of man have been found in the ancient 'river-drifts'; these are the 'Trinil,' the 'Heidelberg,' and the 'Piltdown'; in each instance the human remains were buried accidentally with those of extinct animals, after drifting for some distance in the river or stream beds. It is only in late Acheulean times that human burial rites or interments begin and that skeletal remains are found. Owing to the less perishable nature of flint, relics of the quarries and stations are infinitely more common; they are found both in the river sands and gravels, in the 'river terraces,' and in the 'loess' stations of the plateaus and uplands. Thus prehistoric chronology is based on observations of the geologist, who in turn is greatly aided by the archæologist, because the evolution stages of each type of implement are practically the same all over western Europe, with the exception of unimportant local inventions and variations. In brief, the large divisions of time are determined by the amount of work done by geologic agencies; the comparative age of the various camp sites is determined by their geologic succession, by the mammals and plants which occur in them, and finally by the cultural type of any industrial remains that may be found.
Times of the 'High' and 'Low' River 'Terraces'
The so-called 'terrace' chronology is to be used by the prehistorian with caution, for it is obvious that the 'terraces' in the different river-valleys of western Europe were not all formed at the same time; thus the testimony of the 'terraces' is always to be checked off by other evidence.