Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/215

Rh gravel or loess now to be seen there, and in this respect corresponded with all other valleys in which Quaternary deposits of this character are met with.

Second, that the whole of the Amiens-valley gravel is of one formation and of similar mineral character, and contains nearly similar organic contents, the La Neuville, Montiers, and St.-Acheul gravels being of the same age, and capped with a covering of loess also of one age and mineral character, the whole deposit being of a date not much antecedent to the Historical period.

Third, that the gravel in the valley of the Somme at Amiens is partly derived from débris brought down by the River Somme and by the two rivers the Celle and the Arve, and partly consists of material from the adjoining higher grounds, washed in by land-floods,—the immense quantity of chalk present in the gravel having been derived from the latter source. It is where the surface of the chalk is concave that the gravel is thickest.

Fourth, that the quaternary gravels of the Somme are not separated into two divisions by an escarpment of chalk parallel to the river as has been stated. They would have formed an exception to other river-gravels if this had been the case. The St.-Acheul gravels thin out gradually as they slope from the high land down to the Somme, and they pass away into the Loess formation,—and so also at Montiers.

The Loess deposit, on the contrary, forms a distinct escarpment for many miles along the Somme; and this, I believe, is the bank of the ancient river whose floods produced the St.-Acheul and Montiers gravels.

Fifth, that the existence of river-floods, extending to a height of at least eighty feet above the present level of the Somme, is perfectly proved by the gradual slope and continuity of the gravels deposited by those floods upon the sloping sides of the valley towards the Somme, and also by the Loess or warp, of similar mineral composition and colour, extending continuously over the whole series of gravels, and finishing with a well-defined bank near the present stream.

Beds of gravel, brick-earth, and loess, having an even sloping surface from the escarpment of the sides of the valley down to the terrace near the river-bank, are often to be observed near other rivers whose channels bear the same proportion to their valleys that the Somme river bears to its valley, and where gravel- and loess-deposits reach to a height of 100 feet above the present river-levels.

Sixth, that many of the Quaternary deposits in all countries, clearly posterior to the formation of the valleys in which they lie, are of such great dimensions and elevation that they must have been formed under physical conditions very different from our own. They indicate a Pluvial period, just as clearly as the northern drift indicates a Glacial period. This Pluvial period must have immediately preceded the true Historical period.

Since June 1866 I have visited Amiens several times, and compared the gravels as accurately as I could, both as to situation and