76 Mr. Fohn Flower. 
drainage of the Wandle basin does not, in some places, follow 
the same course as the surface drainage. Possibly this may 
be due to the levels to which the valleys have been lowered in 
the manner above mentioned. The height of the Merstham 
valley at the Red Lion Inn, Smitham Bottom, is about 250 feet 
above the sea. The height of the corresponding point in the 
valley of the Hog’s Mill river, which is just three-quarters of a 
mile south of Epsom, and is about six miles due west of the 
Red Lion, is 200 feet only, whilst the valley of the Mole at 
Leatherhead, which is about three miles south-west of this 
point, is 100 feet only. At their southern extremities the dif- 
ference between the heights of the Merstham and Mole valleys 
is still more striking, the Merstham valley being, as we have 
seen, about 450 feet, whilst the Mole valley at Burford Bridge, 
distant rather over seven miles, is 130 feet only. The levels 
of these valleys therefore are lower as we go westward, and the 
natural effect of this would seem to be that the water, instead 
of going northward, as one would naturally expect it to do, 
would soak into the porous chalk, and would drain away to the 
westward. 
I have only one more thing to call attention to. We 
have seen that the Catchment Basin of the Wandle extends 
from the chalk hills to the Thames. Its dimensions are 
roughly about 10 miles from east to west, and about 14 from 
north to south. The southern half of this basin is chalk, 
the northern half, on the other hand, is clay. Now there is 
not, so far as I am aware, a single stream running above 
ground throughout the length and breadth of the chalk part of 
the basin, and the same remark applies equally to the chalk part 
of the basin of the Hog’s Mill river. The Bourne, of course, 
being an exceptional and periodical stream, I do not reckon. 
The reason for this is as follows:—The Chalk which is so 
common in our district consists of a thick bed, which slopes 
gradually down from the chalk hills, and bending under 
London comes up again on the other side, where it forms in 
Hertfordshire another district somewhat similar to our own. 
The curve in the Chalk is filled up with gravels and sands, on 
the top of which is the London Clay. Below the Chalk again 
is the Gault. The rain which falls on the Chalk sinks down 
into it and drains away underground in a direction towards the 
Thames. The Gault, which is impervious to water, does not 
allow the water to pass downward out of the Chalk. When 
this water therefore reaches the edge of the London Clay it 
reaches a point where the Chalk, lying in a curve between the 
Gault and the London Clay, is necessarily saturated with water, 
and the rain which falls on the Chalk to the south of the area 
which is covered with the London Clay, not being able to make 
