4
THE SUBSIDENCE AT LEXDEN.
While I think that Mr. Rutley's remarks on the chalk cavity are
decisive against that explanation—apart from other reasons against it
—I cannot but consider Mr. Fisher right in rejecting, among other
things, the notion of a cavity in the gravel. For it is extremely
difficult to understand in what way a cavity—especially one of any
size—could come into existence in this gravel. Of course, where there
is a broken drain, or an old cesspool, into which gravel may be
washed by rain, a large quantity of material may find its way into
them in time. And if, at the same time, the upper beds of the gravel
are very much harder and more coherent than the lower ones, a sub-
terranean cavity may gradually be formed, and suddenly reveal itself.3
But we have no reason in this case to suspect the presence of any
artificial workings, and, apart from them, I do not see how a cavity
could originate, for directly below the gravel, we have, not chalk, but
London clay, for one hundred feet or more, and no cavity could be
formed (as Mr. Fisher remarks) in the clay. Finally, even if we
grant the abstract possibility of a cavity in the gravel, it is extremely
unlikely that the gravel, in this case, is thick enough to allow of the
formation of a subterranean cavity in it of the size implied by that of
the sinking at the surface.
We may now consider the possibility of the existence of a denehole
as the cause of this sudden subsidence; deneholes being known to
have a way of disclosing themselves in this somewhat startling
fashion. The section at the Lexden sinking is probably almost
identical with that at Colchester Water Works, which is thus given by
Mr. Fisher:—
Well at Colchester Water Works.
ft.
Soil and low-level gravel .......................................... 12
London Clay........................................................... 105
Woolwich and Reading Beds...................................... 25
Chalk, bored ....................................................... 152
Plenty of good water at ........................................... 294 ft.
Other Colchester wells, being sunk in higher ground, show a greater
thickness of beds lying above the chalk. It is evident, therefore, that
the chalk would hardly be reached at Lexden at a depth less than
3A cavity was thus formed at the back of my own house lately. It stands on Thames Valley
gravel. On having some paving-stones taken up last February, in order that their surface might
be made even, it was discovered that below them was a hollow, about six feet by two feet, and
two feet six inches deep. The gravel removed had been washed down into an old cesspool that
had not been filled up, the existence of which was unsuspected when these same paving-stones
were taken up four or five years ago. The cesspool was a few feet away from the cavity, and the
paving-stones had acted just as a hard upper band in the gravel might have done.