Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/240

122 122 PK0CEEDI2fGS OF THE GEOLOGICAL SOCIETY.

chalk marked by Mr. Prestwich. But, instead of chalk, there are nine feet of good gravel and loess exposed in this cutting. The La ]^eu-ille ballast-pit, exposing ten feet of gravel and loess, is also on the supposed outcrop of bare chalk (Plate lY. fig. 2; E S on the Plan).

The outline of the sections C D and I K vrould at first sight seem to confirm the opinion advanced by Mr. Prestwich, that the gravel at the 140 -feet level might be of a diiferent age to that 50 feet below it.

The loess, also, at Longueau, at the 90-feet level, near C, can be traced to La Xeuville, and then up to the St.-Acheul pits con- tinuously. The railway -cutting in La Xeuville for half a mile is in loess, with veins of gravel (fig. 12) ; and this is seen to be con- tinuous with the St.-Acheul gravel to the north, by a number of old pits, and in the tramway. The surface of the chalk is concave in this part of the La Neuville valley, between E S and I K, so that gravel and loess would be*retained on it ; while along the lines C D and I K there is a very steep escarpment, on which no gravel would lie. This escarpment would be swept by the stream of the river Arve and the Somme, flowing at its base, when swollen by large floods, such as must have happened in the gravel-period.

The only inference, I believe, that can be faiily drawn from the sections on Plate lY. is, that all the gravel and loess of St. Acheul and La Neuville is of one period, and that it remained spread over the vaUey of the Somme where the ground was concave enough to retain it. The absence of gravel on the steep escarpments and near the river- channels is a proof of great floods and rapid currents during the Quaternary period.

I saw the railway-cutting at Montiers, soon after Mr. Prestwich's visit ; it was sloped and very much in the state it is at present. (Fig.

12-)

By M. Guillom's section the depth of the chalk below the rails

has been accui'ately determined at two points, where the sections

N Q, and NOP cross the railway (see figs. 8 and 9, Plate lY.).

In fig. 8, M. Guillom found the chalk 8 feet below the rails; in

fig. 9, 14 feet below the rails. Mr. Prestwich has represented

this railway- cutting as on one of his bands of chalk, dividing the

valley- gravels into two horizons ; and, in consequence, I had the

section NOP taken, as nearly as I could, on the same line as Mr.

Prestwich, because I had always remarked gravel in the Montiers

railway-cutting, and not chalk. I also give a section along this

railway (see fig. 12, page 123). By the French sui'vey the chalk

is 14 feet below the rails. In Mr. Prestwich's section of the

same place, it is 20 feet above the rails. This difference of

34 feet added to the error in the height of rails, before mentioned,

of 31 feet, makes a total diff'erence of 65 feet in the height of

the chalk between Mr. Prestwich and M. Guillom, supposing I am

correct in placing Mr. Prestwich's section on the line NOP.

Mr. Prestwich has recently informed me that he considers his

section was intermediate between the lines NOP and N Q. As the

railway -cutting ceases at the baUast-pit (fig. 12), and there is an em-