Gravels of Croydon and its Neighbourhood. 231 
the plateau and valley slopes, or from the clay-with-flints, and 
they have been blunted and rounded in the transport down the 
valleys to their present position in the gravels. (2). Flints of 
a similar irregular form to those just mentioned, but of a 
brownish umber tint on the exterior, and brownish or grey 
within. These brown flints are considered to have been derived 
from high-level plateau gravels, where they have been long ex- 
posed to meteoric influences. They can be recognized in nearly 
all the valley gravels. (8). Flint fossils. For the most part 
they are silicified casts of sponges and sea-urchins which have 
been washed out of the chalk. (4). Flint pebbles. These are 
smooth, well-rounded, dark or brownish, the greater number 
about an inch in diameter, but a few reach to 4 or 5 in. across- 
They haye been derived from the Lower Tertiary Blackheath 
beds, and are the products of an extensive erosion of the chalk, 
probably in the present Wealden area, during early Tertiary 
times. They are very abundant in the gravels. The con- 
glomerate boulders which have been mentioned are from the 
same beds. (5). Green-coated flints. These are irregularly- 
shaped flint nodules, with a thick surface rind of a dark sage- 
green tint. They come from a stratum between the base of the 
Tertiary Thanet sand and the Chalk. 
The quartz-sand with the loamy and clayey materials which 
form the matrix of the valley gravels are probably derived from 
the erosion of Lower Tertiary beds; some of the clay and loam, 
however, may be an insoluble residue of the chalk. 
Of the foreign materials in the valley gravels, i.e. derived from 
rocks not occurring in our drainage area, may be mentioned :— 
(A). Light brownish flattened pebbles of quartzite from 8 to 
7 in. across. I have traced these as pebbles to the Tertiary 
Blackheath beds, but the rocks from which they come originally 
are not at present known. They are of rare occurrence both in 
the Tertiary beds and in the gravels. 
(B). Flattened and rounded fragments of iron sandstone. 
These are of coarse or fine quartz-sand cemented by iron. Their 
origin is not fully certain. They have been attributed to Tertiary 
_beds,* of which the softer portions have altogether disappeared, 
and some of the fragments are evidently post-cretaceous, as they 
contain pebbles of flint cemented with the sand. The iron 
sandstone of the Folkestone division of the Lower Greensand is 
also indistinguishable from these fragments. The pieces of iron 
sandstone are freely distributed in the valley gravels, and they 
are likewise very abundant on the surface of the chalk plateau 
in the clay-with-flints, and it is from this deposit that they have 
found their way into the valley gravels. Some of the fragments 
ey 
* Prestwich, ‘Quart. Journ. Geol. Soc.,' vol. xiv., 1858, p. 523. 
