Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/234

116 116 PROCEEDINGS OF THE GEOLOGICAL SOCIETY.

In the same way no argument can be satisfactorily applied to the formation of the Somme valley, without considering the relation of the chalk-valleys generally, which resemble each other in so many important respects, and are ahke in general features as well as in the mineral composition of their strata and their superficial contents.

On some other occasion, after the Society has heard my account of some other valleys, and seen my measured sections, I shall ven- ture to lay before them the views of the formation of the Somme valley which I hold. I shall now only remark that the bottom of the valley of Saveuse opens into the valley of the Cette, between Montiers and Eenancourt, at a height of 92 feet above the sea, and is 140 feet high above the sea near the Perme de Grace, and 187 feet above the sea near Saveuse, and that there are con- tinuous terraces of loess, varying in form and elevation, sometimes Tvith the configuration of the chalk-vaUey on which they repose from one end to the other, but evidently the consequence of water fiUing the Saveuse valley on its passage from Eerrieres to Montiers.

2. The Gravel and Loess. — From the fine or coarse character of the gravel, and from the thickness of the loess, we may infer some of the physical circumstances which occurred at the period of deposition. The loess is in some places sandy, and in others is a fine loam, but it varies little in coarseness. At the same height above the river

1 have observed great discrepancies in the thickness of the true loess : thus, at a pit 200 yards east of the line N Q, 15 feet of loess was well exposed in a new pit from which a good quality of brick- earth was being removed and carried a long distance to the brickfields at Montiers ; and I was informed there was a depth of 16 feet more before the gravel was reached. There was a gravel- pit on the same level, a little to the west. On the edge of the Saveuse valley, 400 yards south of 0, the loess was only from 1 to

2 feet thick. At St. Acheul it was only 5 or 6 feet thick ; but there were intermediate beds of marl and sand between the true loess and the true gravel there.

There seems to be a line of thicker brick-earth or loess run- ning south and north between the lines P and N Q. This would indicate that the water was more tranquil at that point. Such dif- ferences in currents are very apparent in rivers at the present time ; and the warp of our rivers approaches very nearly to the character of loess. The fossiliferous gravels at St. Acheul extend to a height of 70 feet above the river, or much higher than the correspond- ing fossiliferous gravels in the valley of the Thames. The shells are found in false-bedded fine sand, and not in clay, at St. Acheul, and in precisely the same condition as at Crayford.

The Cyrena shell-bed at Crayford, however, is only 38 feet above the sea ; but both the St.-Acheul and Crayford gravels extend up- wards and join the plateau beds, while they pass downward as far as the river in both cases.

The chalk is capped in some places with Tertiary sands at Cray^ ford ; but the gravel lies on the concavities of chalk and saud quite indifferently.