ALTHOUGH the Soke of Horncastle does not possess much variety in its
geological structure, yet a considerable diversity is to be found in the
formation of the adjacent elevated country, called the wolds. On the
annexed map the denudations of the various strata in this district are
traced out, and distinguished by different colours: the order of
stratification is also exemplified by a section, of imaginary elevation,
but on the same scale as the map with respect to horizontal distance.
The section too shows a greater extent than the map to the east, where,
on account of the dip of the strata towards that quarter, it is necessary
to commence the description; although in point of distance it cannot be
properly considered within the prescribed limits of this work.
[Picture: Geological Map]
The tract of marsh land between the sea and the wolds, (No. 1.) consists
principally of unstratified clay, with admixtures of sand, and various
marine depositions. These circumstances, together with the old sea
banks, evidence that this mass of earth has been left by the gradual
receding of the ocean.
On the west of these marshes is the rough elevated denudation of chalk,
(No. 2.) which forms the highest stratum of the wolds, and gently dips
underneath the marshes; for in boring in them for water the chalk is
always found. The chalk is of two colours, white and red, each lying in
regular strata, which alternate frequently, the red bearing but a small
proportion to the white. In the white, compact seams of flint, of a
light grey color, from two to six inches thick, are often met with. In
the chalk several extraneous fossils are found.
The stratum immediately below the chalk, (No. 3.) is a coarse, brown,
pebbly sand, consisting of quartz and oxyd of iron, varying in thickness
from six to ten yards, and of uncertain appearance at its denudation.
While the different strata in these parts were exposed to the action of
the water, the sand would naturally be more affected by that element than
the superincumbent stratum of chalk; and the latter being thus left
projecting, would from the decomposing effects of the atmosphere, crumble
down, and form those various declivities which now present themselves to
view.
Beneath the sand is a bed, about fourteen yards in thickness, (No. 4.)
containing equal proportions of oolite limestone and calcareous clay of a
light grey colour. In some parts the stone is divided by the clay into
regular strata, whilst in others it occurs in large detached pieces
imbedded in the clay. Fossil shells, and lumps of pyrites, or sulphuret
of iron, are frequently met with in the stone.
The next stratum (No. 5.) consists of grains of quartz, for the most part
agglutinated into sandstone of different degrees of induration, and
varying in colour from a light grey to a dark brown, whilst in some parts
loose sand predominates. In the grey variety of this stone, marine
shells of different kinds, are found in great abundance: in the brown
they occur very seldom, and not at all in the sand. This stratum is
considerably thicker than the two incumbent ones.
Beneath this is the shale, {98} (No. 6.) which generally makes its
appearance in vallies; but it no where exhibits a denudated termination.
On sinking a shaft in the parish of Woodhall, about six miles south-west
of Horncastle, it was found to be one hundred and fifty yards in
thickness. A great variety of organic remains are also found in this
stratum.
Under the shale is a stratum of forest marble, which denudates about
sixteen miles westward of Horncastle.
The last division to be considered is an alluvial collection of earth,
almost entirely consisting of the spoils of the neighbouring strata in a
state of decomposition, wherein however detached pieces of the different
strata are to be found unaltered. From this it appears that when these
parts were deluged by the water, the current set in from east to west.
As the chalk and the shale would present the largest surfaces to the
erosion of the water, they would be more extensively acted upon than the
other strata: hence the greatest part of the alluvial deposition consists
of white marle and blue; the former (No. 7.) being of the least specific
gravity, has covered the hills, while the latter with sand and gravel
(No. 8.) occupies the lower parts.