Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/232

114 114 PROCEEDINGS OF THE GEOLOGICAL SOCIETY.

a stream which must have fallen with great velocity, as the slope of the bottom is 1 in 40, nearly equal to the slope of the chalk itself at St. Acheul, which is 1 in 33. This river is now dry.

I ask particular attention to the position and level of this dry valley, which is similar in character to those occurring on all chalk- downs and plateaux.

The chalk surface at St. Acheul is also hollowed out into a valley situated north of the Imperial Eoad, widening out as it approaches the Somme, after the ordinary manner of valleys.

Ey the sections through the St. -Acheul pits, we know this valley did not extend south of the Imperial Road ; but the eastern boun- dary of this small vaUey, only 400 or 500 yards long, is well seen at the La jS'euviUe Eastern Bridge, where the chalk is well exposed in the railway-cutting, at a height of 20 feet above the rails, and slopes westwards, passing under the rails near the point C in the map, between the lines of section I K and L M.

The surface of chalk is shown in a very clear section on the railway here, covered with 20 feet of loess (fig. 11). The chalk is nowhere naturally exposed. The force of water from St. Acheul originally hollowed out this small valley in the chalk, which has been partly filled up with gravel and loess ; and the surface -drainage of St. Acheul flows to the Eiver Somme down this valley, over a bed of gravel and loess of some thickness.

There is a very small lateral valley in the chalk, running from St. Acheul into the now dry valley at the western escarpment, also covered with loess and gravel. The slope of the side of this valley is as much as 6°.

Crossing over from the east of Amiens to the west, we come to the section NOP, which gives us a correct view of the surface of the chalk at Montiers, where fossiliferous gravels were discovered by Mr. Prestwich. (Plate lY. fig. 7.)

The gradients have been already described. Between N and the surface of the chalk is slightly convex ; but between 0, a point 120 feet above the Somme, and the Somme itself, the surface of the chalk is concave.

In an elevation of 60 feet, between and P, the concavity of the chalk is as much as 20 feet, or one-third of the total heigh c. It is in this basin of the chalk that the great gravel-beds of Mon- tiers may be seen, in which 30 feet of gravel and loess is well ex- posed, south of and close to the Imperial Boad. The fossiliferous gravel extends above the railway ; and Mr. Prestwich found shells in a pit which appears to be about 50 feet above the river at Montiers.

The chalk is nearly horizontal beneath the rails for a distance of 1077 yards between the line of the section NOP and N Q ; at least it is 15 feet below the rails on the line NOP, and 9 feet below the rails at N Q (fig. 12, p. 123). As at St. Acheul, the slope of the surface follows the chalk to some extent, and falls towards the river. The average gradient is 2^°, or 1 in 43, along the line N Q, against a gradient of 2|°, or 1 in 33, at St. Acheul, along the line L M ; but