Recent Sewer Section at Park Hill Rise, Croydon. 209 
as its base was approached it became gradually more and more 
sandy and laminated, with partings of rusty-coloured sand. No 
fossils were observed in it, nor any selenite nor septaria, though 
septaria were found in making a small excavation in my garden 
at a spot about 150 ft. south-west of the junction of Park Hill 
Rise and Addiscombe Road, and some 20 ft. higher. 
As we proceed in a south-east direction up Park Hill Rise the 
London Clay thins out from below upwards, owing to its base 
rising more rapidly than the slope of the surface, and it ceases 
somewhere about 60 yards north-west of the Chepstow Road, 
though the exact boundary is difficult to ascertain, owing to the 
ground having been artificially raised with clay, probably dug 
from the foundations of neighbouring houses. The base of the 
London Clay rises from a level of 210 ft. O. D. at the Addis- 
combe Road to one of about 231 ft. 6 in. at its termination, or 
21 ft. 6 in. in about 580 ft., nearly 1 in 27. 
B. Pebble bed.—The base of the London Clay rests on a layer 
of pebble gravel; presumably belonging to the Blackheath (or 
Oldhaven) beds. At the junction of Park Hill Rise with the 
Addiscombe Road this pebble bed was only 4 in. thick, but it 
contained some very large pebbles. In ascending the Rise, 
however, the pebble bed increased in thickness; thus, at the 
point where the London Clay ended it was about 18 in. thick, 
at the Chepstow Road 2 ft. thick, and thence it increased more 
rapidly in thickness until about 300 ft. from the Chichester Road 
it attained a maximum exposed thickness of 11 ft., the bottom 
of the trench being still in the pebble bed. Beyond this point, 
however, the thickness of the pebble bed exposed in the trench 
became less, owing to its sinking beneath a capping of clay, to 
‘be afterwards referred to, and finally, about 25 yards from the 
Chichester Road, the pebble bed was abruptly cut off by a fault. 
(A similar pebble bed reappears, however, in the railway cutting 
further on, though I am not sure that it occupies the same 
geological position.) 
It will be observed that the steeper slope of the hill between 
the Chepstow and Chichester Roads coincides with the increased 
development and surface outcrop of the pebble gravel; so that 
Park Hill owes its elevation, partly at least, to its being, like 
Croham Hurst, Shirley Hills, Hayes Common, and other emi- 
nences in this neighbourhood; on a bank of pebbles accumulated 
near the shore of the Tertiary sea; the pebble capping forming 
a protection against denudation.* 
The pebbles contained in this gravel are black flints, all much 
rounded by the action of water. In the lower part of the Rise, 
* At the site of the reservoir on Shirley Hills the pebble bed has been 
proved to be at least 50 ft. thick. 9 
B 
