File:The Quarterly journal of the Geological Society of London (12711373265).jpg

PRESTWICH — SAND- AND GRAVEL-PIPES.
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line " m, n " is perfectly straight. This section also corroborates the
inference drawn from fis-. 1 .
Fig. 2. — Section of the upper part of a Sand-pipe in the Chalk
at Grays.
Drawn on the same scale as Fig. 1 .
n Thanet Sands.
b Seam of clay and sand with green-coated flints.
c Gravel. d Chalk.
The general arrangement is the same in the gravel-pipes, only it is
rougher and less apparent * ; for as the gravel is usually spread over
the chalk in a large unstratified sheet of one rough homogeneous
structure throughout, it necessarily follows that, however slow its
subsidence at any point into a pipe penetrating the chalk may have
been, the absence of straight lines of bedding would prevent the
clear exhibition of any lines of flexure in the gravel of the pipes,
and would cause it to retain the same apparent want of structure
which characterises the mass of the gravel itself. But it sometimes
happens that the gravel is roughly stratified, or rather spread out in
layers of variable texture ; or at times a bed of sandy gravel full of
Tertiary flint-pebbles overlies another bed containing almost solely
subangular and unrolled flints. In cases where pipes have been
formed under such gravels, the curved or inverted conical ar-
rangement of the mass, and the descent of the central core from
the higher beds, generally become apparent, as in the sand-pipes.
Some good examples of these pipes are common on the Chalk downs,
especially in some pits above Westerham and Wrotham. The sides
of most of these pipes are there formed of an extension of the layer
of perfectly angular flint-rubble 1 to 4 feet wide, occurring at the
base of the drift, whilst the core consists of worn gravel, or often of
round flint-pebbles and sand derived from the Tertiary beds which
formerly overspread that area.
The size of the sand- and gravel-pipes is very variable, some being
only a few feet deep, and others reaching to a depth even of 100 feet
or more, with a diameter'of 20 to 40 feet. They are very common all
over the Kent and Surrey Chalk district ; also in Berkshire, Wilts,

• There are some very good instances of gravel-pipes in the Chalk-pits at Green-
  

hithe. The neighbourhood of Watford, Henley, and the Downs a few miles N.E. of
Maidstone also offer convenient localities for studying this phenomenon ; there
are, in fact, few places in the Chalk district where it may not be observed to a