The history of the climatic changes in the ancient valley of the Somme is most clearly written in these successive deposits, 15 feet in thickness, above the 'lower gravels' at St. Acheul. Along with the Pre-Chellean and Chellean flints in the 'old gravels' and 'white sands' we find records of the moist warm temperate climate which then prevailed in northern France and which undoubtedly was most favorable to the hippopotami, rhinoceroses, and elephants of those times. The river mollusks found with the late Chellean flints are another indication of the temperate forest climate which continued through early Acheulean times. In the middle Acheulean are found the earliest deposits of 'older loess' which indicate a climate still temperate but arid, belonging to the middle of the Third Interglacial Stage. In Mousterian times we find heavy deposits of gravels corresponding to the moist cold climate of the Fourth Glacial Stage, followed in middle Aurignacian times by fresh layers of 'newer loess,' indicating the return of a dry climate. Finally, the layers of loam which were washed down over the sides of the valley, and in which the remains of Solutrean and Aurignacian camps are found, indicate the renewal of moist and probably forested conditions. Thus, two dry loess periods are indicated in this valley, the first or 'older loess' belonging to Third Interglacial times, and the second or 'newer loess' to Postglacial times; and we clearly perceive that in the culture layers here there is no evidence whatever of more than one glacial stage preceded by a dry climatic period and deposits of loess. If the Pre-Chellean flint workers had arrived in this river-valley as early as Second Interglacial times, we should find proofs of three periods of arid climate and loess deposition and of two glaciations. Beginning with middle Acheulean times the flints are found in deposits of gravels, loams, brick-earths, and 'older loess,' which all belong to a succeeding geologic stage and are of more recent date than the lower gravels and sands on the terraces which they overlap and conceal. Deposits of this kind have also been drifted down from the highest levels toward the bottom of the valley, and Commont distinguishes three different depositions or layers of 'loess loam,' the lowest or oldest of which contain Acheulean flints, while the middle loams contain Mousterian implements.