Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/231

Rh TYLOR AMIENS GRAVEL.

113

The highest point of the chalk on the line C H is only three feet higher than the lowest point on that line.

There is a steep escarpment, 50 feet high, -at Longucau, of bare chalk facing the east, and an escarpment, 30 feet high, near the Hue de Cagny, of bare chalk facing the west. The outcrop of chalk is marked on the plan, in order to be well seen.

The contrary is the case with the sections from south to north. The escarpment of the chalk facing the River Somme is not so steep,

Fig. 5. — Section of decomposed Chalk exposed in a quarry in the escarpynent near C, ivith bank of Loess at the hase.

N.

Loess.

Talus.

Fig. 6. — Section showing the escarpment of Loess between the Quarry (fig. 5) and the Imperial JRoad.

E. ^ W.

.^i

and is therefore nowhere bare, being covered with a Quaternary deposit.

The slope of the chalk from south to north is considerable when compared with the almost perfect horizontality of the chalk in an east and west direction.

Thus we have a slope or gradient of 1 in 33, or of 2|°, beginning at the point L on the line L M, 175 feet above the sea, to M on the Somme, at a height of 76 feet above the sea. The distance is 3342 feet between L and M. These escarp- ments are evidently the sides of lateral valleys, and are not due to the action of the E-iver Somme, but to that of smaller lateral and and more rapid streams running into the Somme. The Eiver Arve still approaches closely to the eastern escarpment of St. Acheul. The western escarpment of St. Acheul is the side of a valley now dry, but which evidently contained a rapidly flowing stream when the western escarpment was formed.

The gradient of the River Arve is much steeper than that of the Somme ; but the valley at the west of St. Acheul formerly contained