Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/225

Rh C H is near the Imperial Road, and is parallel to that road, to the railway and to the River Somme.

Section G H.—The length of G H is 1400 feet. See Pate IV. fig. 5. The highest point is 157 feet above the sea, 79 feet above the River Arve, 3 feet over the highest part of the Imperial road, (31 feet above the rails, and 84 feet above the River Somme at Neuville. The gradient, commencing at point H, 129 feet above the sea. Rue de Cagny, rises to the east 1 in 30, then 1 in 33, 1 in 35, 1 in 61, and 1 in 100, reaching the well-known section of St.-Acheul Pit, with Roman graves, fossiliferous sand, and wavy marls, at a height of 152$1⁄2$ feet above the sea. A portion of this is shown in Plate IV. fig. 12.

The loess in this section is four feet at the highest and most easterly point, G, gradually thinning to the west, and ceasing when it reaches H.

The gravel is sixteen feet thick at its most easterly point, G, thinning out as it passes to the west a little before the loess disappears.

The surface of the chalk is 133 feet above the sea at G, and 128 feet at H. The surface falls 1 in 280 to the west.

Section E F.—The surface gradient commences at F, at a height of 156 feet above the sea, and it passes the Cemetery Road on the level, and rises at a gradient to the east of only 1 in 700, then falls to the east at 1 in 165 and 1 in 701, reaching the point E at a height of 154 feet above the sea (Plate IV. fig. 4).

The loess is 4 feet at F, thickening to 5 feet at the summit-level of the whole section C H, and then thinning out to 4 feet at E. The regularity of the loess is a very important fact.

The gravel is 17 feet thick at F, and 15 feet at E. The surface of the chalk is 133 feet above the sea at both E and F, showing a perfectly horizontal line, while there is only a variation in the level of the surface-loess of 3 feet in this section, which is 1586 feet long.

Section C D (Plate IV. fig. 3).—Section C D commences at D with an elevation of 153$3⁄4$ feet above the sea; and the gradient falls east at 1 in 157, then rises to the east 1 in 80, then falls to the east 1 in 40 and 1 in 300. Here the tramway (Plate IV. fig. 1) crosses the Imperial Road, and some very extensive gravel-pits are now being worked for ballast.

The gradient continues falling east 1 in 88, 1 in 180, 1 in 160, 1 in 41, 1 in 33, and rising 1 in 200 to the east, where it reaches the escarpment at a height of 142$1⁄4$ feet above the sea. The loess is here 5 feet thick, and the gravel 2 feet, according to M. Guillom's survey; but I found only two or three feet where I observed it. The loess is 5 feet thick near the tramway, and 4 feet at the point D. The gravel is 13 feet thick at D, and 10 feet thick at the tramway, thinning out as it approaches the escarpment on the east, as it did on the west. The surface of chalk is horizontal throughout this section also up to the escarpment.

At the escarpment the chalk falls to the east 52$1⁄2$ feet in a distance of 106, or at an angle of 45° and gradient of 1 in 2 nearly. The line