242 REPORT ON THE DENEHOLE EXPLORATION
feet; and are so worked that when the required amount of chalk for
top-dressing has been obtained they fall in so as to leave a "dell" or
hollow on the surface, so even and shallow as not to interfere with
ploughing. The advantage of a chalk-well, as compared with a
shallow chalk-pit, is that the surface of the chalk being often full of
"pipes" containing sand or clay, or covered by a deposit of clay
with flints of variable thickness, a dell resulting from a shallow open
chalk-pit would often imply the removal of a considerable quantity
of other material than chalk instead of the nearly pure chalk insured
by the chalk-well. Mr. Bennett mentions that chalk-wells to obtain
chalk for lime are often as much as sixty feet deep, and that when
the chalk wanted for the time has been extracted they are sometimes
bricked over at the top for safety. He also remarks that the method
for obtaining chalk by means of chalk-wells appears to be more
modern than that of making open pits, "perhaps only dating from
the middle of the last century," though he has not been able to
ascertain its exact age, and a correspondent of his thinks it only sixty
or seventy years old in Berkshire. But the practice of using marl
or chalk as a top-dressing is, as Mr. Bennett remarks, of very
ancient date, and may go back to prehistoric times. The construc-
tion of chalk-wells, as compared with deneholes, is well illustrated
in Mr. Bennett's diagrams.
A class of ancient pits in chalk which deserves mention in any
discussion on the affinities of deneholes is that of which the ancient
flint workings at Grimes Graves and Cissbury are the best known
examples. A visit to Grimes Graves, near Brandon, on the borders
of Suffolk and Norfolk, discloses a number of depressions at the
surface like those surrounding the closed shafts in Hangman's Wood,
but even closer together. Canon Greenwell states that they are
about 254 in number, and generally about 25 ft. apart. In their case
the object sought was a flint band 39 ft. from the surface, singularly
well suited to the manufacture of flint implements. The shafts to
this band vary in diameter from 20 ft. to 65 ft., and the flint forming
their floor having been removed, galleries about 3 ft. high and from
4 ft. to 7 ft. wide, were driven in various directions, and the flint in
them extracted, till the shafts were connected together by their
means. The material taken from a new shaft went to fill up an old
and exhausted one. The general arrangements at Cissbury appear
to have been of a similar character. Above the chalk at Grimes
Graves is a thickness of about 13 ft. of sand, at Cissbury but a few