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. [Illustration: FIG. 58. Four typical forms of the Chellean _coup de poing_, or 'hand-stone,' from the ancient quarries of St. Acheul. About one-half actual size. _a._ Disc-shaped--upper left. _b._ Oval--upper right. _c._ Poniard-shaped--lower left. _d._ Almond-shaped--lower right. In the collection of the American Museum of Natural History.] [Illustration: FIG. 59. Section of the ancient river terraces on the south bank of the River Somme at St. Acheul-Amiens, showing stations on the low, middle, and high terraces where flints were worked from the very beginning to the very end of the Old Stone Age. After Commont, 1908, 1909--modified and redrawn. The section shown runs northwest and southeast, in a gentle slope nearly 1-1/2 miles in length, from the summit, 70 meters (about 230 feet) above sea-level, down to the river 47.3 meters (155 feet) below. Since Rigollot's excavations in 1851 sixteen flint-working stations have been discovered here, chiefly through building operations, most of them being on the middle and high terraces. This gives some idea of the vast extent of these ancient encampments, which cover the entire period of the Old Stone Age--perhaps 125,000 years.] PREHISTORY OF ST. ACHEUL _NEOLITHIC._ _Campignian_, recent earth and loam. _UPPER PALÆOLITHIC._ _Solutrean._ _Upper Aurignacian_, loam. _Middle Aurignacian_, 'newer loess' and gravel. _LOWER PALÆOLITHIC._ _Late Mousterian_, gravel and 'newer loess.' _Early Mousterian_, base of 'newer loess' (_l'ergeron_). _Middle Acheulean_, 'older loess' and drift. _Early Acheulean_, gravels below 'older loess' (_E. antiquus_). _Late Chellean_, fluviatile sands and mollusk fauna. _Early Chellean_, first coups de poing; old 'white sands' (_E. antiquus_). _Pre-Chellean_, prototypes of coup de poing; old 'lower gravels' (_E. antiquus_).