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266 present a succession of salient and retiring angles." (Lyell, Principles, p. 206.) During these wanderings from one side of the valley to the other, the river continually undermines, and removes the gravels which at an earlier period it had deposited. Thus the upper-level gravels are now only to be found here and there, as it were in patches, while in many parts they have altogether disappeared, as, for instance, on the right side of the valley between Amiens and Pont Rémy, where hardly a trace of the high level gravels is to be seen.

At length the excavation of the valley was completed; the climate must have approached what it is now, and whether from this change, or whether perhaps yielding to the irresistible power of man, the great Pachydermata had become extinct. Under new conditions, the river, unable to carry out to sea the finer particles brought down from the higher levels, deposited them in the valley, and thus raised somewhat its general level, checking the velocity of the stream, and producing extensive marshes, in which a thick deposit of peat was gradually formed. We have, unfortunately, no reliable estimate as to the rate of formation of this substance, but on any supposition the production of a mass more than 20 feet in thickness must have acquired a very considerable period. Yet it is in these beds that we find the remains of the stone period. From the tombs at St. Acheul, from the Roman remains found in the peat near the surface of the ground, at about the present level of the river, we know that fifteen hundred years have produced scarcely any change in the configuration of the valley. In the peat, and at a depth of about 15 feet in the alluvium at Abbeville, are the remains of the stone period, which we know from the researches in Denmark and Switzerland to be of an age so great that it can only be expressed in thousands of years. Yet all these are subsequent to the excavation of the valley; what antiquity then are we to ascribe to the men who lived when the Somme was but beginning its great task? No one can properly appreciate the time required who has not stood on the heights of Liercourt, Picquigny, or on one of the other points overlooking the valley: nor, I am sure, could any geologist return from such a visit without an overpowering sense of the change which has taken place, and the enormous time which must have elapsed since the first appearance of man in Western Europe.