Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/237

Rh TYLOR AMIENS GRAVEL. llU

Somme, with escarpments as distinct and well marked as those drawn of tlie Saveuse valley.

Fig. 10. — Section three-quarters of a mile south of M. DaiWs house {valley of the Arve), Loess Tein-ace.

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These were steps cut in the brick-earth of the iSaveuse valley by the peasants, to enable them to get up the steep sides ; but that was the only information I was enabled to get as to the structure of parts of these terraces, except at Longueau, where a pit was open and good brick-earth visible; so I do not know their relation to the chalk. At Camilla Lucy House, West Humble, near Dorking, I saw a terrace cut into, sloping to the valley at 25°. The gravel was 5 feet thick on the face of chalk, and 7 feet thick 30 yards from the escarpment.

These terraces are of great importance to any one investigating the geology of the Somme, but are not noticed by any other writer, as far as I am aware.

Y. CoifCLUSION-.

"What the sections described in this paper plainly tell us is, that the chalk vaUey of the Somme was excavated exactly to its present form prior to the deposition of any of the gravel now lying in it. Perhaps many layers of gravel may have been deposited and removed again in this ancient chalk valley before the present gravel was deposited ; but of this we cannot be certain ; so that we must take the first layer of gravel covering the chalk from the higher part of the section to the lower as the oldest in the section, and infer that the remainder of the gravel-series was deposited in regular sequence. The most delicate shells are fossilized in the river-sand of St, Acheul and Montiers, just as they have been in that of Cray ford and Erith.

This is a proof of the peaceful character of the deposition of some part of the Amiens beds, just as the large flints and blocks of Ores, which are so abundant among the gravel, are a proof of the power of the floods which brought the coarse gravel from the pla- teau, or down the rivers. If the sections near Amiens show the valley-gravel continuous from a height of 200 feet, at St. Acheul and the Ferme de Grace, to the Eiver Somme (coated over by a nearly uniform warp of loess), and laid at a low gradient not exactly par- allel to the surface of the chalk, but rather in its concavities, then we must necessarily admit that the water of the Somme has at times flowed over the whole surface in question from top to bottom in one flood. This is not an exceptional case at all, as I should have been able to demonstrate, had I been able to bring forward mj^ sec- tions of other river-gravels this evening. We are all agreed that a state of meteorological phenomena existed dimng the glacial period