THE ESSEX FIELD CLUB.
265
At the present time all the chalk used in the brick-making is obtained from
underneath the brickfield by means of the mine, which was first opened about 50
years ago, and has been in use almost ever since.
NOTES ON THE "CHISLEHURST CAVES."
By T. V. HOLMES, F.G.S., F.R.Anthtop.Inst.
THESE Caves appear to me to be simply workings for Chalk, their form
resulting from the local geological conditions. For in the valley in which
the railway runs at and north of Chislehurst Railway Station we have a space
about a mile in length, in which a slight anticlinal fold brings up the Chalk
so that it appears at, and slightly above, the bottom of the valley. Around, on
all sides, the Chalk is covered by Tertiary beds, the nearest places at which
Chalk is seen at the surface being St. Mary Cray and Orpington.
Similar caverns in the Chalk exist, in the same valley, a few hundred yards
northward, at the foot of Camden Park. They were visited by the Geologists'
Association in the years 1872 and 1876. The "Chislehurst Caves," now so well
known, were visited by the Association on April 26th, 1902, the directors of the
exclusion being Messrs. T. V. Holmes and C. W. Osman.1 In their report of
the excursion the directors thus speak of the Caves :—
" They consist of passages driven into the Chalk and connected by others
ranging in directions nearly at right angles to the first. Their height varies
from about 7 to 10 or 11ft., apparently with the height of the flint band or hard
and compact bed of Chalk suitable for a roof. The reason for the adoption,
here of this method of excavating the Chalk arises from the fact that the slight
anticlinal fold, which brings the Chalk to the surface in the valley north
of Chislehurst station, allows it to appear only at the bottom. Consequently an
open pit for Chalk would involve the removal of very large quantities of the
overlying sand clay and gravel in order to obtain a comparatively small amount
of chalk."
The directors then note the positions of the nearest exposures of Chalk at the
surface, and the fact that the excavation of the Chalk at the Chislehurst Caves
does not interfere with the use of the land above the workings for other
purposes.
Though I have never seen any reason to alter the views as to the nature and
purpose of these excavations expressed in 1902, it becomes necessary to notice
certain features exhibited by them which have been brought into prominence
in more discussions as to their character.
Thus, an apparent complexity in the passages is frequently produced by
downfalls of Thanet Sand (resulting from an insufficient thickness of Chalk
roof) and the turning aside of the excavators in consequence. Many passages
end in a conical heap of Thanet Sand extending from roof to floor. Where
these heaps occur some writers suppose denehole shafts to exist, but, in
themselves, these heaps furnish no evidence whatever of the existence of denehole
shafts. It has also been stated that the positions of seventy denehole shafts are
shown on the surface of the ground above the workings. But any hollows
giving that impression must either be diggings for surface gravel, or must mark
1 "Excursion to S.E.R., Main Line widening at Elmstead Cutting, and to Chislehurst
Caves." Proc. Geol. Assoc. Vol. 17, p. 368.