A DENEHOLE AT PURFLEET, ESSEX 271
A Denehole at Purfleet, Essex
By Kenneth Marshall
During the night of 18/ 19th November 1963 a subsidence took
place on the northern approach road to the then recently com-
pleted Dartford-Purfleet road tunnel. The main contractors,
Messrs Marples, Ridgeway and Partners were good enough to
permit a detailed examination of the site. It is quite clear that the
subsidence was entirely due to the collapse of the greater part of
the roof of a denehole only a short distance below road level.
The location is at map reference TQ 576788, a short distance
south of the roundabout by which the tunnel approach roads con-
nect with the A.13 trunk road. It is beside the eastern edge of
the southbound carriageway and partly beneath the cycle track.
It seems likely that the roof structure had been weakened by the
passage of heavy machinery during road construction, although it
is probable that persistent heavy rain during the preceding two
days was the immediate cause of the collapse.
Geologically the site is to be found near the northern extremity
of the low arch of chalk, cut through by the Thames, which gives
rise to the slight ridge between the West Thurrock marshes and
the valley of the Mar Dyke. The height above Ordnance Datum is
about 12 M. Unlike the locality of a group of deneholes at Hang-
mans Wood (TQ 631793) some 5.5 Km to the east, the tunnel
approach road area only shows a shallow deposit above the chalk.
It is estimated that before the commencement of road works, the
depth of the chalk below the surface at this point was about 2 M.
There is some doubt as to the identification of the overburden,
but the existence of a considerable number of flints in a loamy
matrix immediately above the chalk suggests the presence of the
so-called "Bullhead bed", the basal bed of the Thanet Sand.
Section AB demonstrates this in the face exposed below the cycle
track. The "Top bed" of flint is found in the section, 1.25 M below
the upper surface of the chalk. This is a poor, shattered flint bed
but a number of good, isolated nodules occur a short distance
above.
The form of the denehole, both in plan and section, can be seen
in the attached figures. It has many points of comparison with
previously recorded specimens. (Holmes, 1882, 1883, 1898, 1908;
Christy, 1895.) The north central chamber (I) was completely
exposed and a S.W. (II) chamber was just visible under the cycle
track, but this was heavily choked with collapsed material and
access was impossible. The eastern, trilobate chamber (III) pro-
vided most interest. The floor in this was at a slightly higher level
than that below the debris mound, and the walls of each were
extensively marked by tools. At no point did the comparatively
low roof of the chambers show any signs of blackening as found at
Grimes Graves, Norfolk. From the traces left by both glancing