The Forest of Dean: An Historical and Descriptive Account Part 18

CHAPTER XVI.

_The Geology of the Forest, and its Minerals_--Their character in general--Description of the beds of conglomerate, mountain limestone, iron veins, millstone grit, and lower coal measures--"The Coleford High Delf"--Elevation of the Forest range of hills--The middle coal veins--The upper veins--Mr. Mushet's a.n.a.lysis of the Forest coal--Their fossils--The stone-quarries of the district.

The geological conditions of the Forest of Dean merit careful observation, not only as regards the mineral wealth comprised within its limits, but as explanatory of its undulations, and the means of maintenance for its inhabitants.

The strata of the Forest repose in a basin-like form, the greatest depression being near the centre; the longer axis extending from N. to S.

about eleven miles, and the transverse axis, in the widest part, ranging from E. to W. about seven miles. The general observer, if he takes his stand on the edge of hills by which this basin is bounded, will see the enclosing character of the ridge, as well as the less conspicuous circle of somewhat elevated land occupying the central portion of the field, and which is separated by a valley or plain from the surrounding ridge.

This outlying ridge marks in most places the outcrop of the Conglomerate, Mountain Limestone, Iron Veins, Millstone Grit, and Lower Coal-measures.

Mr. Maclauchlan's geological map of the district exhibits the course of the conglomerate bed, and the consequent disappearance of the old red sandstone formation under the Dean Forest basin. Occasionally this conglomerate, or hard grit, forms two distinct beds, very distant from one another, near Lydney for instance, and on the Kimin Hill and Buckstone, although it is sometimes cut off altogether by a "fault," as opposite Blackney. It varies in hardness as well as in the number of the pebbles, and not unfrequently presents an abrupt fall at its termination, as at "the Harkening Rock" in the Highmeadow Woods.

[Picture: General view of the centre of the Forest]

[Picture: Geological Map of the Forest]

The upper portion of the bed is soft, and acquires the character of the limestone clay, often throwing out springs, such as St. Anthony's Well, which have acc.u.mulated in the limestone rocks above. A very micaceous stone sometimes occurs in the upper parts, having the appearance of silver: hence the name of "Silver Stone" given to a spot near the Hawthorns, where it is found. The surface which the carboniferous limestone exposes is also represented in the map. The Forest coal-field is surrounded by this formation, with the exception of the line of fault between Lydney Park and Danby Lodge, a distance of four miles.

The princ.i.p.al iron-mine train of the district divides into a lower or more crystalline, and an upper or more argillaceous and sandy stratum.

Mr. Mushet thus describes this important metallic vein:--"The iron ores of the Forest of Dean, which have become intimately known to me, are found, like the ores of c.u.mberland and Lancas.h.i.+re, in churns or caverns formed in the upper beds of the mountain or carboniferous limestone. The leaner ores contain a great deal of calcareous matter in the shape of common limestone or spar, which reduces the percentage in the ore as low as between 15 and 25 per cent., and it seldom exceeds 25, except when mixed with fragments of what is called brush ore, which, when in quant.i.ty, raises the percentage to 40 or 45. Brush ore is a hydrate with protoxide of iron, and frequently, if not much mixed with calcareous earth, contains from 60 to 65 per cent. of iron. These ores are found in chambers, the walls of which are exceedingly hard limestone, crystallized in rhombs. This limestone is called the 'crease,' and is frequently found enveloped and covered with the iron ore. The miner has to cut his way through this crystallized limestone from chamber to chamber, a distance of from 20 to 100 yards, before he reaches the next of these deposits, which are sometimes found to contain 3,000 or 4,000 tons of ore. The princ.i.p.al part of the ore is then dug easily, somewhat like gravel; but the sides of the chambers are often covered with the stony ore before described, which requires gunpowder to detach it from the rock." These various ores were found by the same excellent authority to yield iron in the following proportions:--

Hydrates of Iron 57.5 per cent.

"Brush" Ore 64.5 ,, Red Calcareous Ore 9.7 per cent.

"Blake Ore" 22 ,,

The inhabitants of the Forest consider the ores obtained on the east side superior to those on the west. They likewise suppose, but probably without foundation, that the ore will be found to deteriorate in proportion as the workings descend. Red and yellow ochre of superior quality occur in the iron veins, and have at various times been in considerable request. They are now used in the neighbourhood for marking sheep, and tinting whitewash.

Reverting to the limestone beds of the district, the lower veins are locally called "blue stone," the middle "red stone," and the top vein the "white head," which is largely used as a flux in the smelting furnaces.

The researches of Mr. R. Gibbs, of Mitcheldean, have enabled him to furnish me with the following list of fossils discovered by himself in the Forest limestone formation:--

_Zoophyta_ Syringopora reticulata, Turbinolia fungites, Lithostrotion irregulare.

_Echinodermata_ Actinoerinus aculeatus, et ,, laevissimus, Platyerinus laevis et ,, rugosus.

,, Poteriocrinus cra.s.sus, et pentagonus.

Rhodocrinus costatus, et granulatus.

_Mollusca Dimyaria_. Pallastra complanata.

_Brachiopoda_. _Terebratula_ hastata.

,, Spirifer glaber, et rhomboideus.

,, Chonetes cornoides, et papilionacea.

,, _Leptoena_ a.n.a.loga.

,, _Productus_ cora, et longispinus, et martini, et pustulosus et cornoides.

_Lamellibranchiata_. Monomyaria.

Aviculopecten fallax.

Dimyaria.

Psammobia complanata.

Pisces.

Ctenacanthus tenuistriatus.

Cladodus conicus.

Psammodus porosus, et rugosus.

[Picture: Vertical section of the Plump Hill]

The millstone grit beds immediately succeed those of the carboniferous limestone just described, forming a similar belt round the Forest, and disappearing with it on the Blakeney side of the basin. Its chief interest consists in the circ.u.mstance that it has been employed from very early times as a material for building; for though it contains a vein of iron ore, little has been done in mining it. Most of the old buildings adjoining the parts where this grit crops out are formed of it, as several of the ancient neighbouring churches show, and likewise the oldest lodges in the Forest; now, however, this kind of stone is seldom used except for boundary walls, and such kind of rough work.

The rest of the outer circle of high land, on whose summit the observer has been supposed to be standing, and which so definitely marks the Forest coal-field, comprises the _lower_ coal measures, containing the lower and upper Trenchard veins, the Coleford High Delf, with the Whittington and Nag's Head seams, which together give about eleven feet of coal. Of these the Coleford High Delf, averaging a thickness of upwards of five feet, and extending over an area of 16,000 acres, is undoubtedly the chief, although in some places it has suffered from various disturbances, the princ.i.p.al of which occur in the neighbourhood of Coleford, extending in a line from Worcester Lodge to Berry Hill, and is marked on the surface by a succession of pools, named Howler's Well, Leech Pool, Crabtree Pool, Hooper's Pool, and Hall's Pool. Mr. Buddle describes the width as varying from 170 to 340 yards in the most defined part, called by the colliers the "Horse," and the dislocations adjoining, the "Lows." "It is not," he remarks, "what geologists term a _fault_, as there is no accompanying dislocation of the adjoining strata. In its underground character it is similar to those _washes_ or aqueous deposits in many coal districts, but it differs from them in not being under the bed of any river, nor in the bottom of a valley, nor does it show itself at the surface." And he adds, "On considering the various phaenomena presented by this fault, and the seam of coal on each side of it, we may infer that it occupies the site of a lake which existed at the period of the deposition of the High Delf seam, and that the carbonaceous matter which formed the seam was acc.u.mulated while the water was deep and tranquil. On the water being discharged from the lake, the 'Horse'

itself occupied the bed of the river, by which the complete drainage of the lake was effected, and which washed the coal entirely out."

The same scientific observer records an extraordinary depression about half a mile to the south-east, in the direction of the "Horse," and in the same seam of coal, amounting to about twenty feet in depth, and of an oval shape. Various other defects and disturbances in the Coleford High Delf are detected from time to time by the new workings, especially in those places where the surface is most uneven. Thus its outcrop at Lydney is very imperfectly defined, and at Oakwood Mill the vein is rendered worthless by a fault, whilst on each side of the Lydbrook valley there is a contortion, by which it is thrown down in one instance seventy yards, and in two others thirty yards each.

Such is the geological character of the conspicuous range of hills by which the Dean Forest coal-field is bounded, especially on its north and east sides. The following table gives their height in feet at certain places above the sea:--

Feet.

Symmond's Rock 540 Buck Stone 954 Knockholt 760 Clearwell Meand 727 Ruerdean Hill 991 High Beech 891 Coleford Meand 760 Berry Hill 750 Lea Bailey Hill 580 Mitcheldean Meand 870 Edge Hill 908 Stapledge 749 Putten Edge 664 Blaize Bailey 684 Blackney Hill 507

Nearly all these spots afford magnificent views of the surrounding country, reaching as far as the Coteswold, Sedgebarrow, Malvern, Herefords.h.i.+re, Welsh, and Monmouths.h.i.+re heights, relieved intermediately by the windings of the Severn, cultivated plains, and woodland. Several very striking ravines intersect this Forest range, particularly at Lydbrook, Blackpool Brook, and Ruspedge, such as would afford the artist many beautiful and interesting subjects for delineation. One of the hills, viz. that on which Mr. Colchester's house, called "the Wilderness," is situated, affords a prospect rarely equalled. The present residence dates from the year 1824, but it occupies a site which was built upon as early as 1710, if not before, for the accommodation of sporting parties in the days of Sir Duncombe Colchester, when its fine sycamores and trees of "the Beech Walk" were most likely planted.

Descending from the side of the hilly range on which the reader has been supposed to stand towards the middle of the Forest, a plain is reached varying in width from half a mile to little more than 100 yards, and forming a band round the somewhat elevated centre of the district. This circular valley or plain marks the outcrop of the middle series of coal seams, not less than ten in number, the princ.i.p.al ones being the Smith Coal, Lowery or Park End High Delf, Starkey, Rocky, and Upper and Lower Churchway. The combined thickness of these beds may be said to average 20 feet, and they are more argillaceous in character than the lower beds, which in general are harder in their nature, and hence they afford the larger portion of the fossiliferous remains observed and tabulated by Mr.

R. Gibbs, who has kindly furnished the writer with the following--

PLANTAE.

_Asterophyllites_ equisetiformis, et foliosus.

_Bothrodendron_ punctatum.

_Calamites_ approximatus, nodosus.

_Caulopteris_ primaeva.

_Lepidodendron_ dichotomum, et elegans, et Serlii, et Sternbergii, et majus.

_Neuropteris_ acutifolia, et angustifolia, et flexuosa, et macrophylla, et oblongata.

_Pecopteris_ abbreviata, et arborescens, et cristata, et dentata, et Serlii.

_Sigillaria_ contracta, et elongata, et mammillaris, et ornata, et reniformis.

_Sphenophyllum_ fimbriatum, et Schlotheimii, et truncatum.

_Sphenopteris_ Hibbertii, et macilenta.