WITH ESPECIAL REFERENCE TO THOSE OF ESSEX.
27
mation supplied by well-sinkers the thickness of the Boulder Clay on
the higher ground about Thaxted would be as under :—Elms Farm,
two miles N.N.W. of the town, 100 ft.; at Bardfield Green, a mile
E. of the town, 120 ft.; whilst at Tindon End and at Cutler's Green
it is 140 and 80 ft. respectively." The advantages due to the existence
of these glacial deposits above the London Clay are unquestionable.
Here and there the glacial sand and gravel offers a much better site
for a farm or village than the London Clay could afford, while the
existence of that impermeable formation beneath the sand and gravel,
provides a water supply that would otherwise be wanting, the water
percolating through the gravel being held up by the clay, and easily
made available by means of wells of no great depth. On the other
hand, a well sunk in London Clay must pierce the whole of the clay,
and the water supply be obtained from some permeable stratum, such
as sand or chalk, beneath it. Turning to the Boulder Clay we find
that it offers certain agricultural advantages. It varies a good deal in
composition, and, though on the whole nearly impervious, is by no
means so thoroughly impermeable as the London Clay. In compo-
sition it is highly calcareous or chalky, alike in Norfolk, Suffolk, and
Essex, as a general rule. And, in those counties, it has been
much dug into in order to obtain top-dressing for the land, the value
of chalk or marl in improving heavy clays having long been recog-
nised. We learn, however, that it has fallen into disuse for that
purpose since the establishment of railways, "chalk being found to
be more efficacious, as five loads to the acre serve the purpose of
twenty of the Boulder Clay. The inferiority of the Boulder Clay is
due partly to the clayey matrix enveloping the chalk-pebbles, but
chiefly to the greater hardness of those pebbles, which enables them
to resist the dissolving power of water in the same measure that they
resisted the grinding of the glacial action" (W. H. D Memoir on
Sheet 47, pp. 5 9-60.) Still in many parts of the eastern counties, where
there is a difficulty in getting chalk either by railway or from the
Thames by sea, it affords a means not to be despised of improving
the productive powers of the London Clay; and the numerous marl
pits of Norfolk, Suffolk, and Essex testify to the high repute of Boulder
Clay as manure in former times.
In addition to sand, gravel, and boulder clay, there are con-
siderable deposits of loam belonging to the Glacial Period.
These attain their greatest development—as regards Essex—in
the neighbourhood of Colchester, mainly on the north and east