Gravels of Croydon and its Neighbourhood. 221 
chalk, for there are on it in places small patches of sand, loam, 
and pebble beds of the Lower Tertiary series, the fragmentary 
remains of strata which formerly extended quite over the chalk 
in our area. The Tertiary pebble beds, known as the Blackheath 
or Oldhaven beds of Whitaker, are of considerable importance 
in relation to the valley gravels, to which they have largely 
contributed. These Tertiary sand and pebble beds occur prin- 
cipally on the higher portions of the plateau south of Caterham, 
at Worms Heath, also at Croham Hurst, Addington Hills, and 
in places on the western line of watershed, near Walton-on-the- 
Hill, and Burgh Heath. 
Even where not covered by Tertiary beds the surface of the 
chalk plateau in our area is overlaid generally by a comparatively 
thin layer or crust of the Clay-with-flints of Whitaker. This 
consists of a matrix of red clay (brown, near the surface) in 
which unworn chalk flints, Tertiary pebbles, and very frequently 
pieces of iron sandstone and chert, are embedded. From: the 
erosion of this deposit the valley gravels have derived much of 
their flint constituents, and, most probably, all the chert and 
ironstone fragments which they contain. 
The most striking physical feature which the chalk plateau of 
our area exhibits in common with the rest of the North Downs 
is the system of comparatively narrow dry valleys with which 
the plateau is now deeply furrowed, and nowhere along the 
whole course of the North Downs are these peculiar valleys and 
the gravels of their lower portions better shown than in the 
district south of Croydon. A short account of these valleys by 
the late Mr. John Flower has already appeared in the ‘ Transac- 
tions’ of the Club (1878-1881, p. 72). Most of them begin 
near to the summit of the chalk plateau on the south, and some 
extend in the form of a pass or gap quite through the escarpment 
and look down into the transverse valley of the Gault below, 
others take their rise on the high level plateau of the western 
line of the watershed. In our district these dry valleys converge 
in a generally northerly course towards Croydon, and unite in 
the main valley of Smitham Bottom, which is continued through 
Croydon, and opens into the wide flat area of Beddington, 
Waddon, and Mitcham, on the western side of which is the 
channel of the present stream of the Wandle. It is along 
Smitham Bottom that the great mass of the gravels brought 
down the main valleys from the erosion of the chalk plateau has 
been carried and spread out over the area of what is now 
Croydon and the flat tracts to the north of it. 
Leaving out of account the Bourne, which at irregular inter- 
vals, several years apart, flows down the lower part of the 
Caterham valley and Smitham Bottom for short periods, the 
valleys in the chalk area to the south of Croydon are all dry; 
