Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/227

Rh TYLOK AMIENS GKAVEL. 109

gradient of 1 in 4 for some distance, and then becomes horizontal again at tlie lliver Somme.

Th« loess is clearly seen in the railway-cutting and at one cellar (Plate III. C*) in Nciiville (fig. 1), resting on chalk without any in- termediate gravel, C on the plan; but I have left the junction between the loess and gravel undefined in the lower part of the section I K, as I could not put the junction in accurately.

If a straight line be dra^vn from point I on the Rue de Cagny to K on the Somme, it will pass 32 feet below the top of the railway -cutting along the line I K, and it will pass over the chalk at the Imperial Road at a height of 17 feet, showing that the surface of the chalk between those two points is concave.

Section L M. — (Plate lY. fig. 7.) — This section commences at the point beyond the Rue de Cagny, at a height of 187 feet above the sea ; the gradient dips northwards towards the river, and falls 1 in 37, until it passes the Rue de Cagny at a height of 1 60 feet above the sea. It passes through a part of the great St.-Acheul pit, with a gradient of 1 in 15, 1 in 40, 1 in 70, 1 in 130. Here it crosses the Imperial Road and falls to the north, with a gradient of 1 in 600, 1 in 300, 1 in 40, 1 in 688, 1 in 43, to the La-Neuville Railway ballast-pit close to the railway workshops, where it reaches a height of 132 feet above the sea. The ground is quarried out ; but the surface apparently dipped north at a gradient of 1 in 12, then rose 1 in 33, then fell 1 in 7, horizontally crossing the railway- cutting at a height of 107 feet, 11 J feet above the rails.

The ground then faUs north, at a gradient of 1 in 21, forming the top of the loess terrace, at 95 feet above the sea. The escarpment of the loess terrace is here at a very steep angle, falling 12 feet in a horizontal distance of 14 feet, then at a gradient of 1 in 71 to the first brook, then level to the Somme.

The loess is 4 feet thick at the Rue de Cagny, in the section L M, and the same thickness at the Imperial Road. At the escarpment of the chalk it is only 2 feet thick, and seems to appear again at the terrace in considerable thickness, at a height of 95 feet above the sea.

The gravel is 13 feet thick at the Rue de Cagny, nearly 20 feet thick in the St.-Acheul pit, and runs out to 6 feet at the Railway- works ballast- and chalk-pit at the escarpment. I have no section of gravel further north than this pit.

If a straight line be drawn from the Rue de Cagny to the Somme, along the line L M, it passes the Imperial Road on the level, and is 15 feet below the surface at the Railway- works ballast-pit ; so the surface is convex at that point.

The convexity of the chalk on the same line is 14 feet at the ballast-pit.

Section N P. — (Plate lY. fig. 8.) — This section commences at the Perme de Grace, point N, at a height of 201 feet above the sea, and goes along the road to Montiers by the line N P as far as 0.

Longueau on the same Plate III.
 * The letter C in La Neuville must be distinguished from the letter C in