Transactions of the Geological Society, 1st series, vol. 3/On the Wolds of Lincolnshire

From Wikisource
Jump to navigation Jump to search

XI. A Sketch of the Geology of the Lincolnshire Wolds.

By Mr. EDWARD BOGG, Land Surveyor.
Communicated by H. Warburton, Esq. Secretary of the Geological Society.

[Read January 19th, 1816.]

Being resident in a district which has been but little noticed as to its geological structure, I have drawn up the following remarks on its stratification; and though omissions may occur, yet I trust that an account of the Wolds of Lincolnshire and the adjacent country, may not prove uninteresting to the members of the Geological Society.

In conformity to the request of the Rev. Mr. Buckland, Professor of Mineralogy in the University of Oxford, I have endeavoured to trace on the annexed map, Pl. 24. fig. 4. the denudation's or bassets of the chalk, the oolite limestone, and the sand strata, together with the alluvial earth and hills of the same formation, with as much accuracy as my personal knowledge and the nature of the country will admit. I have also added a sketch of a section, Pl. 24. fig. 3. on the same scale as the map with respect to horizontal distance (the elevation being merely imaginary) intended to elucidate the order of the stratification.

It is obvious that as the general dip of the strata of this district forms but a very small angle with the horizon, the superficial extent of the different beds occupies a greater stretch of country than from their thicknesses might at first be apprehended: in the section indeed, these measures are represented at considerable angles, yet by keeping the circumstance above mentioned in view, I hope it will be found to answer the purpose for which it is intended.

The marshes betwixt Louth and the sea, No. 1 of the section, consist principally of unstratified clay, with mixtures of sand and various marine depositions, which tend to prove that this mass of earth has been left by the ocean. A further confirmation of this may be derived from tracing the sites of the different old banks which once evidently formed the boundaries of the sea, which has now retired a considerable distance from them.

Proceeding westward we come to the basset of the chalk No. 2, being the highest stratum in this district. It forms the principal part of the Wold hills, extending in this section from Louth to the highest hills in Donington, a distance of about six miles: this stratum dips under the marshes; for on boring in them for water, the chalk is always found. On arriving at this, the workmen never fail to come at springs, and generally of sufficient strength to flow up higher than the surface, thereby justifying the conclusion that this bed is the uninterrupted continuation of the same chalk, the rough elevated basset of which forms the Wolds; and by the pressure of the water in which, the fountain springs obtained by boring in the marshes, are at once accounted for.

In the neighbourhood of Tetney, a village situated on the coast, are natural outlets of water called blow-wells; their depths have never yet been ascertained, but we cannot entertain a doubt of their communicating with the chalk. These wells overflow with a greater flux at the time of high water, and particularly at spring tides; showing that the water in the chalk communicates with the sea, a circumstance which would lead to a supposition that the sea rests immediately on the chalk at a certain depth: if so, we might possibly expect that the agitation of the water would frequently throw up chalk on the coast, which I have never yet observed.

The chalk consists of two colours, red and white, each lying in regular strata, the red being generally undermost: in the white, seams of flint are frequently met with from two to six inches thick.

The stratum, No. 3, immediately below the chalk, is a coarse brown pebbly sand without organic remains, consisting of quartz and oxyd of iron. This bed is, I apprehend, of irregular thickness; I suppose it may vary from six to ten yards, but its appearance at its basseting is very uncertain; for, being of a very loose texture and the chalk which reposes upon it being of a more compact nature, it is evident that when these parts were exposed to the action of water, the sand being less capable of resisting the washing of that element than the chalk, this latter would be left in many instances forming projecting cliffs; which time, and the well known action of the atmosphere, would crumble down over the sand, and form those declivities which now in such variety are exhibited to our view.

The next bed, No. 4, contains in nearly equal proportions oolite limestone and calcareous clay of a lightish grey color. In certain parts of this bed the clay divides the seams of stone into regular strata; in others the stone is found to occur in the clay in large detached pieces. This bed never extends to any great distance beyond the chalk, forming in general a sort of step at the foot of its basset. Lumps of pyrites are frequently met with in the limestone. The thickness of the entire bed of stone and clay may be about twelve or fourteen yards: it forms a close measure, and I apprehend supports the water collected by the open strata which repose upon it, for water is always found either in the chalk or in the ferruginous sand immediately below it, the well sinkers never having occasion in this district to go deeper.

The next in order, No. 5 of the section, is a stratum of grains of quartz, which for the most part are conglomerated into sandstone of different shades of colour, from a dark brown to a light grey, while in some places loose sand predominates. Marine shells are found in this stone, each species appearing to be restricted to certain laminæ of the stratum: in the sandy portions I have never observed any organic remains. I consider this measure as considerably thicker than either of the two incumbent ones, Nos. 3 and 4.

I now come to the lowest visible stratum in our district, No. 6 of the section, which I term the shale stratum. It generally makes its appearance in valleys, and its thickness I cannot estimate, for I do not know where it exhibits any thing like a den undated termination. My brother (Mr. Thomas Bogg of Louth,) and myself have bored in this stratum to the depth of a hundred yards near the village of Donington, on the west side of the river Bain, and found it to consist of the following varieties.

yds feet inch.
1 A clay soil 1 ─── ───
2 Dark coloured clay 3 ─── ───
3 Soft grey slate with marine impressions ─── 1 ───
4 Blue argillaceous stone ─── ─── 5
5 Dark coloured clay ─── ─── 1
6 Soft grey slate same as No. 3 ─── 1 ───
7 Laminated clay slightly indurated 7 2 ───
8 Soft grey slate sightly inflammable 1 2 3
9 Same as No. 8. but darker coloured 2 2 3
10 Indurated clay with white marine organic remains 12 1 6
11 Same as the last but harder and blacker 2 1 3
12 Dark coloured bituminous inflammable schist 2 ─── ───
13 A dark blue coloured ironstone ─── ─── 3
14 Laminated indurated clay with white marine organic remains 11 ─── ───
15 Same as No. 14. but harder, with marine impressions consisting of thin leafy pyrites 3 1 4
16 Dark blue argillaceous stone ─── ─── 4
17 Hard indurated, laminated clay; with impressions consisting of thin leafy pyrites 6 ─── 4
18 Laminated bituminous schist, with white marine organic remains and inflammable ─── 1 10
19 Dark blue ironstone ─── ─── 2
20 Laminated bituminous schist, same as No. 18 3 2 ───
21 Dark blue ironstone ─── ─── 1
22 Laminated bituminous schist, same as No's. 18 & 20 6 ─── 10
23 Dark indurated clay, with some white marine organic remains 1 ─── 6
24 Laminated bituminous schist, same as Nos. 18, 20 & 22 3 ─── ───
25 Dark indurated clay, same as No. 23 2 2 ───
26 Laminated bituminous schist, same as Nos. 18, 20, 22 & 24 1 1 6
27 Dark dry indurated clay, same as Nos. 23 & 25, intermixed with thin seams of laminated bituminous schist 10 ─── 3
28 Grit ─── ─── 2
29 Brown laminated schist ─── ─── 2
30 Hardstone bind or argillaceous stone ─── 2 10
31 Hard laminated, bituminous schist ─── 1 2
32 Hardstone bind, same as No. 30 ─── 2 ───
33 Hard laminated bituminous inflammable schist ─── 2 4
34 Inflammable slaty bind 1 ─── ───
35 Hard laminated bituminous schist, very inflammable 1 ─── 7
36 Hard dark blue bind inter laid with thin strata of bituminous schist 4 1 9
37 Very inflammable schist ─── ─── 2
38 Hard dark blue bind, same as No. 36 1 ─── 8
39 Argillaceous stone ─── 1 ───
40 Same as No. 39, but not so hard ─── 1 ───
41 Hard dark blue bind, same as Nos. 36 & 38 in which the boring was discontinued 7 1 10
103 ─── ───

Every variety of this stratum agrees in the two following properties, the presence of calcareous matter, which is manifested by a brisk effervescence when any part of it is submitted to the action of acids; and secondly, a more or less abundant mixture of pyrites; all the bituminous slates when exposed to the action of fire burn with a very strong offensive smell; but those found below the depth of eighty yards were not so disagreeable, in that respect, as those which were higher in the stratum. Varieties 33 and 35 were remarkable for their inflammability, and buened with a thick bituminous flame, appearing nearly equal in this respect to common coal; but after the bitumen was exhausted, the remainder was left undiminished in size. The organic remains which I have observed in this stratum as its general characteristics, are impressions of the cornua ammonis and some small bivalve shells.

The last division which I shall here notice, No. 7 of the Section, represents the alluvial collection of earth which appears to consist almost entirely of the spoils of the neighbouring strata. When these parts were deluged by water, the current evidently appears to have set in from east to west; for this collection of matter is found to consist principally of a mixture of the different strata which occur to the east of it in a state of decomposition; and in this mixture which is by no means regular, detached pieces of the beds before alluded to are found unaltered. Of these beds the chalk would expose the greatest surface to the erosion of the water, and next to this the shale would be most extensively acted on; accordingly we find that the alluvial deposition is composed of white marle and of blue, the former of which being of the least specific gravity, has overspread the higher parts and the summits of the hills, while the lower parts and the vallies are occupied chiefly by the latter. In the blue marle I have frequently found a variety of fossil shells, and fossil wood, &c. together with specimens of all the earths and stones, more or less worn by trituration, which the district produces; I apprehend also that fragments of strata are deposited here which have come from a great depth, for pieces of very pure laminated coal are frequently to be met with which burn with a very bright light, and leave but a small residue of ash. Other pieces of jetty coal often occur breaking with a conchoidal fracture, but these agree in every respect with a thin stratum of this fossil which has been discovered at superficial depths in some situations in the shale stratum.

On the Lincolnshire coast, a considerable quantity of coal agreeing in properties and appearance with the latter of these varieties, is washed up by the sea, and is often collected by the poorer sort of the inhabitants for fuel; and along with this particular species of coal, slates similar to those in the shale measures often occur, a fact which gives us reason to suppose that the seam of coal from which these specimens have been displaced extends under the incumbent strata already noticed, into the sea, where the coal seam is denudated and from whence these washings proceed.

As the discovery of coal would be a circumstance of much importance to this country, it might be worth while to have it ascertained whether a seam of this jetty coal (called by the country people gromel) does not exist in sufficient thickness, and at such a depth, as would render it worth the miner's pursuit; the expense of the boring requisite for this purpose might be defrayed by a subscription among the different land proprietors in the district.