113
THE UNDULATIONS OF THE CHALK IN
ESSEX.
By W. H. DALTON, F.G.S, late of H.M. Geological Survey.
[Read May 17th, 1890.]
WITH MAP, PLATE III .
AS the surface of the greater part of Essex consists of clay, the
water-supply is almost everywhere derived from wells, and a
considerable proportion of these have been carried down to the
Chalk, and derive their value from the copious stores of water yielded
by that formation. There are, of course, hundreds of shallow wells
in the gravel areas, and a great many artesian borings that go no
further than the sands in or under the London Clay. But the water
in gravel is always liable to contamination by infiltration of impurities
from the surface, whilst the yield from the Tertiary beds, besides
being often charged with an objectionable amount of mineral matter
in solution,1 is very apt to be diminished, if not altogether stopped,
by the influx of sand carried up by the water into the bore-hole. In
fact, the utility of any such wells is but a question of time, and in
view of their cost, it is often found that the larger primary expenditure
is eventually the more economical procedure. Accordingly, scarcely
a month passes but we hear of some new boring being made, or an
old one deepened, to the Chalk.
This being the case, it becomes not infrequently a matter of much
importance to know the depth at which the Chalk lies from point to
point, so as to estimate the approximate cost of getting water from
that source in spots hitherto supplied from higher beds.
If the surface of Chalk were a uniform plane the determination
of its position with regard to sea-level of any desired point would be
one of the most simple geometrical problems—scarcely more than a
rule-of-three calculation, but the case is very much otherwise. Instead
of a plane we have an elaborately-puckered surface, which I have
tried to illustrate by the accompanying map, a reproduction, with
geological additions, of a part of the index of the old Ordnance
Survey. The scale is ten miles to an inch. Photography does not
admit correction of names misspelt in the original. The curved
lines with figures annexed indicate approximately where the surface
1 See Dr. J. C. Thresh's Report on the Water Supplies of the Chelmsford and Maldon Rural
Sanitary Districts, 8vo, Chelmsford [1891].