240 STRATA CORRESPOND ON OPPOSITE SIDES OF VALLEYS. 
them, are within the limits of the north and north-west escarpment 
of the green sand formation; and in their continuation southward 
they cut down into the oolite, lias, or red marl, according as this or 
that formation constitutes the substratum over which the green sand 
originally extended. There is usually an exact correspondence in 
the structure of the hills inclosing each valley; so that, whatever 
stratum is found on one side, the same is discoverable on the other 
side upon the prolongation of its plane: whenever there is a want of 
correspondence in the strata on the opposite sides of a valley, it is 
referable to a change in the substrata upon which the excavating 
waters had to exert their force. 
The section of the hills in this district usually presents an 
insulated cap of chalk, or a bed of angular and unrolled chalk-flints, 
reposing on a broader bed of green sand; and this again resting on 
a still broader base of oolite, lias, or red marl (see Plates XXV. and 
XXVI.) With the exception of the very local depression of the 
chalk, and subjacent green sand, and red marl on the west of the 
Axe, at Beer Cliffs, the position of the strata is regular and very 
slightly inclined; nor have any subterraneous disturbances operated 
to an important degree to affect the form of the valleys. 
If we examine the valleys that fall into the bay of Charmouth 
from Burton on the east to Exmouth on the west, viz. that of the 
Bredy, the Brit, the Char, the Axe, the Sid, and the Otter, we shall 
find them all to be valleys of diluvian excavation; their flanks are 
similarly constructed of parallel and respectively identical beds ; and 
the commencements of them all originate within the area and on the 
south side of the escarpment of the green sand. 
The valley of the Sid, as it is coloured in the annexed map, may 
