246 ON A RECENT SUBSIDENCE AT MUCKING, ESSEX.
Of course, in classifying variable sands, gravels and clays,
much depends on the nature of the information attainable by the
geologist recording sections in them. However, in the present
case the one important point is the depth to the Chalk from the
base of the London Clay, which is nearly the same at each
place. From the same volume of Mr. Whitaker's memoir we
may learn that at Corringham a section showed that there were
Tertiary beds 137ft. in thickness between the London Clay and
the Chalk ; at Laindon 136ft., and at Vange 127ft. 6in. Thus,
remembering that the uppermost 11 or 12ft. in the recent
Mucking subsidence are the lowest beds of the London Clay, and
that the older subsidences westward have some old Thames
gravel at the surface in addition, it is very improbable that the
Chalk is at a less depth than 150ft. beneath any of them.
This depth makes it in the highest degree improbable that
these Mucking subsidences owe their existence to a natural
cause, such as the sudden filling in of a pipe in the Chalk. On
the other hand this thickness of the intervening beds also makes
it very improbable that they are the result of the collapse of
a denehole ending in the Chalk. And the vertical sides and great
breadth of the recent hole in themselves suggest the collapse of
a chamber or chambers at no great depth Then, while a
single pit of the denehole class might be a rare possibility (as in
the case of that at Eltham), yet it is in the highest degree
unlikely that a group of them would be made in a district where
the total depth would be from 160 to 170ft., that is, double the
depth attained by the deneholes of Hangman's Wood. At
Mucking, too, besides the mere depth, there would be water
difficulties, arising from the presence of clayey bands in the
Woolwich series, from which the Hangman's Wood pits
are free.
In short, a single pit ending in the Chalk, where these
Mucking subsidences occur, might be a bare possibility, just as
a well of that or greater depth might be sunk for the supply of
some public institution, brewery or factory situated in a village.
But the existence of a group of pits of this depth and special
difficulty of construction is as unlikely as that of a group of very
deep wells for the supply of the cottagers in the village street.
In this district, too, where deneholes are known to abound
wherever the Chalk is at the surface, or not more than 60 feet