The plants in the highest levels of the same tufa, however, indicate the advent of a colder climate and also connect this with the Acheulean culture stage through the presence of Acheulean flints. The deposit of tufa is covered by a sheet of 'loess' corresponding with the return of an arid period in late Acheulean times, in the very heart of northern France. Thus we have a record in the region near the present city of Paris of three climatic phases, which are also more or less completely indicated in deposits to the north along the River Somme and in the valley of the ancient Thames.

In western France we again interpret the fossil flora of Lorraine as belonging to the cooler closing period of Third Interglacial times and to the advent of the fourth glaciation, for here the most northern varieties of the larch (Larix) and of the mountain-pine (Pinus lambertiana) predominate.

The clearest view of the contemporary alpine forests is found near Zürich in the lignitic deposits of Dürnten and of Utznach, which are so characteristic of the temperate period of the Third Interglacial Stage that Geikie has proposed to call this stage the Dürntenian.[(8)] It was, we recall, at Dürnten that Morlot[(9)] found the first proofs of a warm or temperate interglacial flora, between the deposits of a retreating glacier and those of an advancing glacier; for Dürnten is well within the region which was covered by the vast ice-fields both of the third and fourth glaciations. The forests which flourished there in Third Interglacial times were similar to those now found in the same region, consisting of the spruce, fir, mountain-pine, larch, beech, yew, and sycamore, with undergrowth of hazel. With this hardy flora are associated the remains of the straight-tusked elephant, of Merck's rhinoceros, of wild cattle, and of the stag; another evidence for our opinion that all these Asiatic mammals had become habituated to the cool temperate climate of the north.

Life on the River Somme from Pre-Chellean to Neolithic Times

The borders of the River Somme at St. Acheul give us a vista of the whole story of the succession of geologic events; the great changes of climate, the procession of animal life, the sequence of human races and cultures. Here Commont[(10)] has found the key to the history of this entire country and enabled us to parallel events here with those occurring far away in Taubach, on the borders of the Thuringian forest, and at Krems in Lower Austria, as studied by Obermaier. This is because the 'older' and 'newer' loess periods, the succession of climates and of mammals, and the development of human cultures were all not local but continental events. The purely local events are found in the kinds of gravels and soils which washed down over the terraces.

It is very important first to clearly picture in our minds and understand the geography of the Somme at the time of the arrival of the Pre-Chellean flint workers. It appears certain that all three of the old river terraces composed of limestone had been cut long before and that the river had already reached the bottom level of the underlying chalk rock.[(11)] The higher terrace, then as now, was 100 feet above the Somme, the middle terrace about 70 feet, and the lowest terrace extended from a height of about 40 feet down underneath the present river level (see Fig. 59).

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Fig. 57. Three ancient river terraces (I, II, III), on the west bank of the Connecticut River in Vermont, believed to be of Postglacial age. The terraces are respectively 140, 60, and 20 feet above the river, and thus show a profile similar to that of the terraces on the Somme in Pre-Chellean times previous to the accumulation of the deposits bearing Palæolithic flints. Photograph by H. H. H. Langill.

Since the most primitive Pre-Chellean flints occur in the coarse gravels which lie on the floors of these terraces immediately above the chalk, they prove that the entire excavation of the valley had been completed when the Pre-Chellean workers arrived there. Commont believes that this was the actual topography of the valley during the Third Interglacial Stage. The occurrence of Chellean flints in the white sands overlying the coarse gravels of the middle and upper terraces does not indicate that the flint workers were encamped here while these terraces were being cut out by the River Somme but rather that they sought these convenient bluffs for their quarries during the time that these sands and gravels were washing down from the sides of the valleys and from the plateaus above.