Historical and descriptive sketches of the town and soke of Horncastle [1822] / in the county of Lincoln and several places adjacent

by George Weir

GEOLOGY.

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.

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.