[From Knowledge pp277/8 11th May 1883]

[This article is included as it shows an attempt to explain the geology based on the limited understanding of the time]

THE GEOLOGY OF THE ISLE OF MAN.

By W. Jerome Harrison, F.G.S.

THE main feature of the surface of the Isle of Man is a ridge running from Maughold Head (S.E. of Ramsey) to the Calf of Man, a distance of thirty miles. The highest point of this rocky backbone just exceeds 2,000 ft. at Snaefell, but Greebah and South Barrule both approach 1,600 ft From the line of hills so formed, the surface slopes down on either side to the sea, the width of the Isle (at right angles to its mountain axis) being from ten to twelve miles A low flat plain, called the Curragh, occupies about fifty square miles north of a line drawn from Ramsey through Bishop's Court to Peel.

Fig. 1.—Geological Sketch-Map of the Isle of Man.

In studying the geology of this isolated tract, we have a great advantage in the fine sections of the rocks displayed in the cliffs all round the coast. Inland, indeed, good exposures are not common , the strata are so overlaid by beds of sand, clay, and peat of recent origin, and the hill slopes are so well covered by grass, that the hard rocks which lie beneath are only to be examined at wide intervals.

If a geologist has only two or three days to spend on the island, he should make at once for Castletown, as the neighbourhood of that town affords by far the most interesting and varied display of the strata. Within a fortnight or three weeks it is, however, quite possible to examine the entire rocky structure of the island, so far as it is visible.

The Government Geological Survey has not yet visited Manxland, so that Sheet 100 of the Ordnance Survey Map cannot be purchased "geologically coloured." The late Rev. J. G. Cumming did good and careful work among the rocks, and published his results as part of his excellent book on "The Isle of Man" (1848), and also in the "Journal of the Geological Society of London" (vol. II, p. 317). Of recent papers, those by Mr. Jno. Horne in 1874 ("Trans. Edinburgh Geol. Soc.," Vol. II.), and the late Rev. J. C. Ward in 1880 (Geological Magazine, p. 1), are well worth reading.

Silurian Formation.—Three-quarters of the surface of Mona is formed of rocks of Lower Silurian age. The fine sections exposed in the cliffs show hard rocks of varying texture, all of which were deposited as mud on a sea-floor, but which have since been so hardened, that the sandy mud is now a rough stone called grit ; while the fine mud has formed hard shales, which are sometimes cleaved, then becoming a true slate. There must be a total thickness of perhaps five or six thousand feet of these Silurian rocks in the Isle of Man, and yet such appears to have been the paucity of life in the seas in which they were originally deposited, that this enormous thickness of strata has only yielded a single species of fossil, viz., Palaeochorda major, once considered to be a fucoid (seaweed), but now known to represent the meandering burrow of a species of marine worm.

This fossil, and the general similarity of the strata, help us to identify the Lower Silurian rocks of the Isle of Man with the Skiddaw Slates of Cumberland, and again, through them, with the Arenig beds of Wales. A hard band of grit is very noticeable at the base of the Arenig slates in North Wales ; it occurs in the Lake district, and a some- what similar bed, which crops out at Carn Gerjoil (S.E. of Snaefell), and on St. Ann's Head, may represent it in the Isle of Man. The hills forming the central ridge of the island are composed of hard grey slaty shales and flags; The quarries at South Barrule show true slates of a black hue, upon which, to the west, rest soft black shales.

Owing to slow and long-continued movements of the earth's crust in bygone ages, these old slaty rocks have been bent into a double curve. A line drawn from south of Ramsey to Crosby (west of Douglas), and thence to Port Erin will indicate an anticlinal axis or line, from which the strata did in opposite directions ; those on the Douglas side inclining at a high angle to the south-east, and those on the Peel side dipping towards the north-west. Beyond (or east of) Douglas, however, from the Dock to the Head, the strata rise up again and the dip here is westerly. At Spanish Head, in the extreme south of the island, bluish flaggy slates lie horizontally, and are traversed by narrow vertical cracks of great depth, called the "Chasms." The strata here probably form the crown of an anticlinal or arch, a position in which they would be under a great strain, and consequent liability to fracture (Fig. 2)

Fig. 2.—Strata forming an anticlinal curve or saddle.
The strain has caused the beds to fracture, so forming the fissures C, D, E.

It is very desirable that these Silurian rocks should be more thoroughly searched for fossils ; graptolites ought to occur in them, for forty species are known in the Skiddaw Slates.

Granite bursts through the Silurians at two points—viz., at Doon, near Laxey, and near Foxdale, about two miles north-east of South Barrule. In each case the intrusive rock has shattered the Silurian shales of the neighbourhood, and heated water, traversing the fissures so formed, has filled them with various minerals, including much lead-ore (galena), zinc-ore (blende), and a little copper-ore. The lead-ore is richly impregnated with silver. In the year 1879, the Laxey mines produced 7,200 tons of zinc-ore, worth £27,000, and 1,200 tons of lead-ore (containing 40,000 oz. of silver), value £20,000. In the same year, the Foxdale mines produced 2,750 tons of lead-ore (contain- ing 55,000 oz. of silver), value £37,000.

Fig. 3.—Section at Langness, showing the conglomerate resting unconformably on the Skiddaw Slates.

B. Skiddaw Slates. A. Conglomerate forming base of Carboniferous Limestone. x Fault.

Lower Carbonirerous Formation.— On the west side of the promontory of Langness, east of Castletown, a conglomerate of varying thickness (5 to 50 ft.), containing many quartzite pebbles, rests very unconformably on the Silurian slates (see Fig. 3). Traced northwards, towards the College, the conglomerate gradually passes upwards into a limestone—the well-known carboniferous or mountain limestone. At Peel, beds of red sandstone are seen similarly to rest on the slates, forming a strip about two miles in length, and perhaps 300 ft. in thickness. These red beds at Peel dip north-west towards the sea, and are overlaid by limestone, which, however, is beneath the water. Both the conglomerate of Langness and the red sandstones of Peel were formerly assigned to the period of the Old Red Sandstone, a formation which Hugh Miller did so much to render famous ; they have yielded no fossils, and it is certain that they really form the base of, and belong to, the Carboniferous Limestone.

The latter rock itself occupies an area of ten or twelve square miles in the extreme south of the island round Castletown. It forms a "happy hunting-ground" for the fossil-seeker, for Cumming obtained no fewer than 222 species of shells, &c., from Pool vash The strata lie in a trough, or synclinal, dipping towards a common axi (see map). The lowest beds are brownish limestones, with partings of shale, exposed in two quarries at Castletown, and altogether about 200 ft. in thickness. These are overlaid by the light-coloured limestones of Poolvash, while the hard dark-coloured limestone (called black marble), worked in a quarry south of Poolvash, forms the highest known stratum. It is the upper beds which are most fossiliferous, and they contain a remarkable mixture of ferns and calamites with marine shells, indicating the proximity of land at the time of their formation. Thin, corrugated shells of the genus Posidonia characterise these upper limestones.

Although the carboniferous rocks only occur at two points in the Isle of Man (Castletown and Peel), yet, if an elevation of the district were to take place, we should doubtless find the red sandstone and the limestone completely encircling the Silurian strata, but at present the later-formed rocks of Carboniferous age lie under water, except at the two points just mentioned.

Igneous Rocks.—The coastline between Castletown and Poolvash affords most interesting evidence of volcanic action during early Carboniferous times. The Stack of Scarlet is a boss of basalt which fills an ancient pipe or vent ; a dyke of similar rock runs from it in a north-west direction, while beds of volcanic ash are interstratified with the neighbouring limestones.

Glacial Period.—Beds of clay, sand, and gravel rest irregularly upon the edges of the Silurian and Carboniferous rocks which form the solid geology of the Isle of Man On the south of Ramsey, a bluish sandy clay, containing boulders of Scotch granite, is seen resting upon the slates ; it is overlaid by a bed of sand 10 ft. thick, above which is a second bed of boulder clay, 6 ft. thick, containing scratched stones, &c. A bluish clay full of stones covers most of the limestone in the south of the island; when it is removed, the surface of the rock beneath is frequently seen to be grooved. All this boulder clay was brought by a large glacier which, during the glacial period, advanced from the south of Scotland, ploughing its way down the Irish Sea, and grinding over the Manx hills—a fate to which their rounded and mammillated outlines still bear witness.

This period of intense cold was succeeded by a steady though slow, submergence. Down went the island ! but a peak remained to tell of its existence, or to afford a refuge for a vestige of that alpine flora which elsewhere continued to linger on, and which still exists on the Welsh [] Cumbrian hills. It is to the lifting action of coast ice continued during this depression, that we must look for an explanation of the fact that numerous blocks of the Foxdale granite have been raised more than 600 feet, so that they now lie on the very top of South Barrule—a mountain only a mile and a half south of the Foxdale boss.

Post-Glacial Beds. —The submergence alluded to above was succeeded by an elevation so great, that the entire floor of the Irish Sea appeared above the waters, and the British Isles formed a peninsula jutting out from the western coast of Europe. The flat surface of the Curragh formed a part of this old plain, whose surface was studded with small lakes and peat bogs ; the Great Irish Elk roamed freely over the district, and individuals got bogged and lost their lives ; their bones are now often found in the marl pits of the Curragh. Numerous marine shells of species still living in our seas also occur in the sands and gravels of this northern flat portion of the island

The magazine Knowledge was founded in 1881, folding in 1917, and sold at 2d per issue. The intent behind it was to make science accessible to the general public - founded and edited by Richard Proctor, an astronomer who wrote many of the articles himself before his death in 1888.

William Jerome Harrison (1845-1908) wrote much on chemistry, physics, photography and geology - in 1880 appointed Chief Science Master under the Birmingham School Board, previously for many years had been curator of Leicester Town Museum. [see Wikipedia article]

The article is aimed at the amateur geologist - it predates by 20 years the magisterial work of Lamplugh.

The map is also based on the pre-O/S map of 1867 - probably derived from that in Cumming's book.

		see A224
Any comments, errors or omissions gratefully received The Editor HTML Transcription © F.Coakley , 2026