On the Weald. 71 
how the Catchment Basin of our own river, the Wandle, has 
been formed. 
Before proceeding to examine this in detail it will be as 
well perhaps to explain what is meant by a Catchment 
Basin. It must not be supposed that the area which is 
drained by any particular river is merely the area upon 
which it is seen onan ordinary map. Fora variety of reasons 
it very seldom happens that this is the case. The whole of 
the area which a river drains is termed its Catchment Basin, 
because it is bounded on all sides by a ridge of high ground, 
termed its watershed, which separates it from the basins of 
adjoining rivers, and within this circle of high ground is caught 
all the rain, which, by the natural process of drainage, forms 
the river. The above definition, however, has reference 
only to the surface drainage. The underground flow of 
water follows, as a rule, the same course as the surface 
water, but, as will be explained hereafter, this is not by 
any means always the case. The distance between the 
streams which are actually flowing and the edge of the basin 
differs very considerably in different river basins, and is regu- 
lated entirely by the nature of the soil, and the natural struc- 
ture of the district. Thus, on clay soils, which are impervious 
to water, the rain which fallsis obliged to flow over the surface 
of the ground, and therefore if the Catchment Basin of a river 
is bounded by beds of clay, as is the case, in some places, 
with the basins of the Mole and Medway, the streams will run 
right up to the boundary of the basin. But if, on the other 
hand, the edges of the basin are on a porous soil, like Chalk, 
the rain which falls sinks in at once, and does not come to the 
surface again until some natural peculiarity of the ground 
compels it to do so. 
The Hog’s Mill river seems to afford a very good illus- 
tration of what I have just said. This river appears in 
the map as a very insignificant stream, which falls into 
the Thames at Kingston, having one branch which origin- 
ates near Epsom, and another which originates near Ewell, 
and apparently it has no connection whatever with the Weald. 
The Catchment Basin of this river, however, seems to prove 
that at one time the Hog’s Mill river was a much larger and 
more important stream than it is now. Its Catchment Basin 
extends right up to the chalk hills, narrowing rapidly at its 
southern extremity, and ultimately runs into that very remark- 
able deep valley, cut out of the southern slope of the chalk, 
which is to be seen immediately to the north of Betchworth 
station, and up which passes the high road to Walton-on-the- 
Hill. This valley may now represent the opening in the 
chalk through which the ancient Hog’s Mill river passed out 
