72 Mr. Fohn Flower. 
of the Weald, for there seems good reason to believe that this 
river once drained a part of the area to the south of the chalk 
hills, until, like the other streams on the north of the Weald, 
its supply of water was cut off, as explained above, by the 
deepening of the valley of the Gault. The reasons why this 
river has shrunk as it were so far from the edges of its basin 
will be better explained a little later on, in connection with the 
Wandle basin, with which it has much in common. 
In dealing with the Wandle it will be convenient first to 
describe the chief physical features of its river basin, and then 
to try and see how, in all probability, they have been brought 
into existence. I have here a large tracing, taken from the 
six-inch Ordnance map, of that part of the area drained by the 
river Wandle which lies between the chalk hills, which are the 
southern boundary of its Catchment Basin, and a line drawn 
east and west through the southern part of Croham Hurst. 
This tract of country is about seven miles across from north to 
south. I hoped to have been able to add to this a similar 
tracing of the remainder of the Wandle basin, but the time at 
my disposal has not allowed of this. This tracing shows all 
the contour lines on the Ordnance six-inch map, and shows 
therefore, and to any one who is accustomed to contour maps 
at a glance, the exact position and size and shape of every 
valley in this, which is, for our purpose, the most impor- 
tant part of the area which is drained by the Wandle. The 
contour lines to the south. show the shape and heights of the 
chalk hills. The broad blue line on the east side shows the 
line of the watershed between the Wandle and Ravensbourne 
river basins, and the corresponding broad blue line on the west 
side indicates the watershed between the basin of the Wandle 
and that of the Hog’s Mill river. These two watershed lines 
at their southern extremity are eleven miles apart, but as they 
go northward they rapidly converge, the eastern one passes 
close to Sanderstead Church, and the western one close to 
Banstead Church, and at this part, which is about six miles 
from the southern edge of the Wandle basin, they are only about 
five miles apart. 
If we examine in detail the district which is comprised in 
this tracing we shall see that in the centre of it is the narrow 
continuous valley which runs from Croydon to Merstham. 
The highest point in this valley is about three-quarters of 
a mile north of Merstham Church, at which point the valley 
is a little less than 450 feet above the sea level. From there 
to Beddington, a distance of about nine miles, the valley 
slopes gradually down towards the Thames, and at Beddington 
it is only 100 feet above the sea level. There seems reason to 
believe that before the Wandle was cut off from the Weald 
