File:The Quarterly journal of the Geological Society of London (12711376765).jpg

PRESTWICH — SAND- AND GRAVEL-PIPES. /3
This phsenomenon is not confined to these eocene sands, but is
equally common in connexion with one of the most recent of our
drift-gravels, and in beds of various other ages. In the instance
of the drift-period, this is shown in the line of section (No. 1,
PL VI.) extending from the neighbourhood of Rickmansworth
to the hills above Wycombe, where the gravel caps a succession
of Chalk hills and exhibits a considerable number of very illus-
trative gravel-pipes, including the fine case mentioned by Dr.
Buckland between Beaconsfield and Wycombe. Both this section
and the other (No. 2), referring to the sand-pipes, are actual
sections, in which all the pipes found on the lines of section are in-
troduced and distinguished from those supposed to exist where the
chalk is not exposed. The relative position of the pipes and their
distance from the main masses are maintained, the height of the
ground at the various points Where they occur having been approxi-
mately determined by the aneroid barometer.
Some of the most remarkable instances of these detached gravel-
pipes, and instances affording a test to a much-debated period of
denudation, occur in the slopes of the North Downs. A thick drift
of ferruginous clay, sand, and gravel extends generally to the very
edge of this escarpment, and is there abruptly truncated, whilst the
slopes of the hills present a bare and clean chalk surface ; but dotted
a considerable way down these slopes may often be found portions of
detached and isolated gravel-pipes, — the termination of pipes descend-
ing from the main mass of the gravel when prolonged above them.
Such tail-pieces may readily be observed above Charing, Wrotham,
and Westerham. The annexed sketch gives a section of the hill above
the latter town (see fig. 3) . This fact is important, as it affords a
Fig. 3. — Section of the hill-side above Westerham, showing the pipes
on the top of the hill filled with drift, and the terminations
only of similar pipes some way down the slope of the hill.
Drawn on scale. The height from the dotted line to the base line is about 300 feet.

• Pipes of which the ends still exist on the slope of the hill. Descent by the London road,
  

ffl Chalk.
b Ferruginous clay, sand, and gravel filling pipes on the surface of the chalk,
c Outlined continuation of former surface of the chalk with similar pipes.
strong proof that the final excavation of the valley of Holmesdale,
including, I believe, that of the Weald, was not effected until after this
comparatively recent drift period. This, however, is a point I had
not intended to have entered upon at present, and the further con-
sideration of which I must reserve until I have occasion to treat of