If we suppose that the Pre-Chellean flint workers arrived in Europe not earlier than Third Interglacial times, we can explain all the gradations in the evolution of their implements in connection with the changes of climate and of animal life which the geologic and fossil deposits reveal, especially in the valleys of the Somme and of the Thames. If, on the other hand, the Pre-Chellean is dated in Second Interglacial times,[U] it carries this culture back another hundred thousand years and involves our prehistory in great difficulties. First, there is no proof whatever that the Pre-Chellean and Chellean flint workers lived during the period of the formation of the 'high river terraces' of the third glaciation, for no Palæolithic flints have ever been found buried in the sands or gravels of the 'high terraces.' The occurrence of archaic flints on the 'high terraces' of the Somme and of the Seine is in superficial gravel beds which were deposited long after these 'terraces' had been cut by river action; this is best seen in the Somme, where archaic flints occur alike in the gravels deposited upon the 'low,' 'middle,' and 'high terraces.' Second, there is no proof that the Pre-Chellean and Chellean flint workers passed through the cold climatic period of the third glaciation; nowhere in Europe have any records been found of their camps or stations in association with the cold fauna or flora of Third Glacial times. Third, the geographical evidence is equally at variance with the theory that the Pre-Chellean flint workers entered Europe during the Second Interglacial Stage, for we know positively that in many of the great river-valleys of Europe, especially those surrounding the Alps, the rivers were at much higher levels than at present and that they were transporting the materials out of which the 'high terraces' were being formed or cutting these 'terraces' down by erosion. In other words, the geography of Europe in First and Second Interglacial times was very different from what it is at present; most of the river-valleys were broader and less deep; some of them had been eroded to a point below their present levels and had begun to silt up in alluvial deposits. In Third Interglacial times the river geography of Europe was substantially as it is to-day, although the coast-lines were still very different.