Gravels of Croydon and its Neighbourhood. 227 
at present shown in a field by the north side of the Sanderstead 
Road, now being laid out for building, at a level of about 210 ft. 
The gravel is in places 15 ft. in thickness, and it rests on an 
uneven surface of chalk. It is reddish, coarse, quite un- 
stratified, of unworn and subangular flints, Tertiary pebbles, a 
few fragments of iron sandstone, and, rarely, pieces of chert and 
flattened ovoid quartzite pebbles from the Blackheath beds, with 
occasionally, boulders of pebble conglomerate, which are tightly 
packed in a matrix of sandy loam. In the upper portion of the 
section there is a bed of brown loam from 1 to 2 ft. in thickness, 
and here and there some thin layers of fine chalk rubble. This 
gravel closely resembles that of the bottom of the valley, and is 
probably continuous with this latter. 
The bouldersor blocks of conglomerate found in this Sanderstead 
Road pit, and more numerously in the gravels of the Brighton Road 
immediately below, deserve some further notice, since in this dis- 
trict masses of stone larger than the detached flint nodules from 
the chalk, from 6 to 12 in. in diameter, are of rare occurrence in 
gravels, and they do not appear to have been mentioned hitherto. 
The boulders are portions of the Blackheath Tertiary beds, in 
which the flint-pebbles are cemented either by a ferruginous or 
siliceous cement; and they occur as. fairly well-rounded blocks 
up to 12 in. in diameter, or as irregular masses with the angles 
rounded off and their surfaces smooth and even, the rounded 
flint-pebbles having been worn level with the cement by the 
attrition to which they have been subjected. One of the larger 
boulders, which’ I have myself seen in the gravel-pit, measured 
20x17 x12in., but blocks of much larger size, in all probability 
derived from the gravels, may be noticed against water-troughs 
in the Brighton Road. The lighter coloured boulders, in which 
the cement is siliceous, resemble in character the Hertfordshire 
conglomerate. I do not know of any beds of similarly cemented 
rock in the Blackheath deposits yet remaining in our area. 
They have evidently travelled some distance to produce their 
worn smooth surfaces, and it must have required a current of 
water of considerable volume and force to transport them down 
valleys of such slight fall as those in which they now occur. 
So far we have been considering the gravels in the dry and 
comparatively narrow valleys in the chalk district to the south 
of Croydon; at the south end of the town the Chalk dips beneath 
the surface, and is succeeded northwards by the Lower Tertiary 
beds and the thick mass of the London Clay. This change is 
markedly reflected in the configuration of the surface. Instead 
of the narrow valleys, which result from the resistant nature of 
the chalk to subaerial erosion, the more readily denuded Tertiary 
beds have been excavated so as to form a wide shallow de- 
pression, in which a large part of Croydon is built, and which ~ 
