to have retreated into the higher mountains. During some part of the Post-pliocene ages, the large pachyderms and accompanying beasts of prey, now extinct, wandered from the Continent to England; but whether the junction of France and any part of the British Isles was as late as the period of the gravels of St. Acheul, or the era of those engulfed rivers which, in the basin of the Meuse, near Liége, swept into many a rent and cavern the bones of Man and of the mammoth and cave-bear, is still doubtful. There have been vast geographical revolutions since the times alluded to, and oscillations of land, during which the English Channel, which can be shown by the Pagham erratics, and the old Brighton beach (p. 280), to be of very ancient origin, may have been more than once laid dry and again submerged since it originated. During some one of these phases, Man may have crossed over, whether by land or in canoes, or even on the ice of a frozen sea (as Mr. Prestwich has hinted), for the winters of the period of the higher level gravels of the valley of the Somme were intensely cold.
The primitive people, who coexisted with the elephant and rhinoceros in the valley of the Ouse at Bedford, and who made use of flint tools of the Amiens type, certainly inhabited part of England which had already emerged from the waters of the glacial sea, and the fabricators of the flint tools of Hoxne, in Suffolk, were also, as we have seen, post-glacial. We may likewise presume, that the people of post-pliocene date, who have left their memorials in the valley of the Thames, were of corresponding antiquity, posterior to the boulder clay, but anterior to the time when the rivers of that region had settled into their present channels.
The vast distance of time which separated the origin of the higher and lower level gravels of the valley of the Somme, both of them rich in flint implements of similar shape (although those of oval form predominate in the newer gravels),
