patients i 
A. Tylor on the Amiens Gravel. 315 
of 1077 yards between the line of the section NO P, and N Q; 
at least it is 15 feet below the rails on the line N O P, and 9 
feet below the rails at N Q (fig. 12, p. 123). Asat St. Acheul, 
the slope of the surface follows the chalk to some extent, and 
falls toward the river. The average gradient is 23°, or 1 in 
43, along the line NQ, against a gradient of 23°, or lin 
33, at St.-Acheul, along the line LM; but the chalk is convex on 
the average at L M, and concave on the average at NQ. This, 
however, requires more explanation. 
The surface of the chalk between the line of 200 feet and of 
120 feet above the sea is convex on the line NO P, at the max- 
imum to the extent of 14 feet out of 80 feet perpendicular 
height; and we see very little gravel reposing on the convex sur- 
face. On the contrary, in LM, between the 200-feet and 135-feet 
levels, the maximum concavity is 15 feet; and the great mass 
of the St.-Acheul gravel is deposited in this hollow. 
But when we examine the surface of the chalk between O and 
P, between the 120-feet and 60-fect levels, we find the chalk 
‘face is concave to the extent of a maximum of 29 feet, out 
of a total of 60 feet; and, singularly enough, this 29 feet is 
almost exactly the maximum thickness of gravel and loess in 
the great pit at Montiers, where a section several hundred yards 
long is exposed, 
n the contrary, between the 130-feet and 76-feet levels on 
the lines LM and IK, where the surface of the chalk is convex, 
Te 1sno gravel of any importance. 
In the section (Plate IV, fig. 1), between L M and IK the 
chalk ig nearly a straight line, falling 23° between the 130-feet 
and 90-feet levels ; we have 9 feet of gravel and loess exposed 
in this favorable position for its accumulation. ; 
hen we see the gradual slope of the surface from the point 
O to the river Somme at P, we are indeed surprised to find the 
sudden change in gradients in passing southward from O to Re- 
ng by Saveuse) to Amiens, a distance of four or five 
These escarpments commence near Ferriéres, and in- 
“ease as the bottom of the valley falls in a northeasterly di- 
—" toward the river Cette, JI measured a section near the 
erme de Grice, where the side of the dry valley slopes at an 
= Point O, toward Renancourt, and giving gradients from 
to 50° . peri ; 
ie definition that they look like the work of the last win- 
ig. 7.) 
