HISTORICAL GEOLOGY OF THE SKERRIES

Thomas Pennant makes what may be the first geological observation regarding the Skerries, as an aside to his description of the serpentinite formation out-cropping on the adjacent mainland at Mynachdy [1].

Not far from hence [Cemlyn] I saw the noted quarry of marble, common to this place, some parts of Italy, and to Corsica, and known in the shops by the name of Verde di Corsica. Its colours are green, black, white and dull purple, irregularly disposed. In different blocks; one or other of the colours are [sic] frequently wanting; but among the green parts are often found narrow veins of a most elegant and silky white asbestos. It is a compound species of marble, part is calcareous, and may be acted on by aqua fortis.[1a] The green parts partake of the nature of jasper. It is apt to be intersected by small cracks, or by asbestine veins, therefore incapable of taking a high polish. This quarry lies on the lands of Monachdy in the parish of Llanfair-ynghornwy; and it is found again in the isle of Skerries, off this parish.

Rather unfortunately perhaps, Pennant was certainly mistaken in his belief that serpentine exists on the islands, as Rev. Freeman clearly realised in his passage quoted in the section on the early private lighthouses. In fact, Pennant never succeeded in visiting the Skerries, so one must assume that he misinterpreted a description of some other formation on the islands, the rocks of which are generally of a dark greenish colour.

The islands received rather perfunctory mention in Sir Andrew Ramsay’s Geology of North Wales[2] and on the 1st Ed. 1-inch-to-the-mile geological map, where the single sentence ‘hard conglomerates and grits’ appears. Edward Greenly describes this earlier work and his own Geological Survey of the Skerries which formed part of his larger survey of Anglesey as a whole, carried out between 1895 and 1919 [3], in his autobiography [4]. He writes

Eight miles from Holyhead is the little archipelago known as The Skerries, with its celebrated lighthouse. All that was known of its geology was four words and a symbol on the old 1” map but one of those words, conglomerates, was enough to shew that it must be of the first importance. There are 17 islets, bare and rugged, so it was clear that the survey of them would be a work of several days. In the summer of 1911, through the influence of Teall [5] as Director, I was accorded leave by the Elder Brethren of the Trinity House to stay in the lighthouse as long as I needed. The Trinity House is one of our many historic anomalies. Originally a religious corporation, it was permitted to survive the Dissolution of the Monasteries and to become the germ of the great institution which now watches over the safety of the shipping round our coasts.
      The Elder Brethren sent instructions to their Superintendent at Holyhead to take me out and bring me back in their own vessel the Triton. The Trinity Stores at Holyhead is a most important centre, for with its ever-cruising Triton it supplies the lighthouses and buoys over most of the Irish Sea. Food and water for the keepers, coal for their rooms and cooking oil for the lamps, and petrol for the foghorn engines: these are but the principal things of which no lighthouse must run short.
       Although the Elder Brethren’s permit did not include my Colleague [Mrs Annie Greenly], yet I longed for her to go to the Skerries, for it was just the sort of place that would be after her own heart. So I took her with me when I went to see the Superintendent, guessing that when he had met her there would be a much better chance. And my guess was quite correct. He agreed to ‘put his blind eye to the telescope’ and break the rules so far as to give her the voyage there and back. Here is a chivalrous episode which shows how she had won his heart, and the hearts of the crew as well. When she returned to Holyhead the tide was too low for the Triton so she had to land by boat. There was a gap of a yard or two between the boat’s head and the beach, across which a plank was laid. At a signal from the Superintendent, the boat’s crew leapt into the water and, smiling brightly, raised an oar on either side, to make a parapet for her hands. She came in, too, for perfect weather; and the delight these voyages were to her is to this hour a delight to me.
      My colleague had given me ample stores in case of being detained by weather. As there is no lack of space, the house and work-rooms are not cramped, indeed my bedroom was quite large. The Skerries have a staff of four, but they come ashore by turns to work in the Stores at Holyhead so a spare bedroom is always to be had. Of one rule I was told at once. At night all windows must be shuttered, for if any but the light were to appear, passing ships might be misled.
      So on July 25th 1911 I was landed by the Triton’s boat and marooned upon the islets for the inside of a week. Nowadays there is wireless but in my time communication with the land was by flashlight only. To my surprise, moreover, no boat was kept there. ‘Had they a boat’ said the Superintendent ‘they would be sure to go a-fishing and sooner or later come to grief. This gave at once an impression of peculiar helplessness. The impression was quite different from what we have on board a ship; for a ship has, ipso facto, the faculty of locomotion, and can take one to the land. Here, one was at the mercy of the men at Holyhead. Or should the flashlight fail, what would happen to one then? I never felt like it before or since.
      Everyone knows of lighthouses, all of us depend on them for all sorts of things: but how often do we ask what lighthouse life is like? I had the interest of seeing it from the inside. One feature, somewhat unexpected, is that it is very busy. Work of one sort or another seemed to be going on all the time. Night, too, as well as day, for the lantern is never left at night, and the night-man has to watch the weather, lest need arise to blow the horn. Wicks, of course, must be trimmed, and every pane of the great lantern has to be cleaned every day. The clockwork for the revolving shutter (which to minimize friction floats in a pool of mercury) must always be in perfect order, as must be the horn-engines. Strict accounts are kept of stores, everything must be bright and clean, and all the while the men are their own cooks and housemaids. I said to the Superintendent ‘Men shut up with one another are somewhat apt to quarrel. Does that ever give you trouble?’ ‘Never. I take good care that they have no time to quarrel’. Being favoured with good weather I was out nearly all the time, but took my meals with the keepers and was entertained by racy pictures of life in different lighthouses. The Skerries light gives two flashes with a few seconds interval, and then a much longer interval before the succeeding pair; and the blasts of the foghorn are at similar periods. One day, at a meal time, there was a little mist for an hour, so the horn was set a-going. Between the blasts in the two or three seconds dead silence reigned, all knew what was coming, it wasn’t worthwhile to speak. This horn is worked by a petrol engine which drives compressed air into the throat and the keepers are proud of it, for it is said to be the loudest on the whole of the British coast. On my first morning there, a ‘rise’ was taken out of me. The keepers took care not to tell me that they were going to test the engine, I went to my room to get my maps, and the horns were three or four yards from the window. Suddenly there came a blast which I can compare to nothing but the very Trump of Doom.
      All too soon my work was done, though the conglomerates gave the key to the chronology of the Mona Complex. So the Triton, returning from the Dee, where she had been to ‘gas the buoys’ [6] , took me off and landed me at Holyhead. But geologically and in every other way, I count that week upon the Skerries as one of the greatest privileges of a much-privileged career.

Greenly later recalled that when preparing Geology of Anglesey for the press [7], he and his wife were anxious that some examples of their large-scale coloured maps should be included. Both the N.W. corner of Anglesey and Llanddwyn Island, were included because of their complexity. He continues

Then, why did we include the Skerries ? Well, we said to one another:-‘The Skerries are of surpassing importance, they are sure to be visited by geologists who, from lack of time there, will need a very lucid map’. All of which was true enough. But I fear that our real motive was a certain feeling of romance, in regard to this remote and rugged archipelago. Were we wrong? I do not think we were. For ought not a geologist to be something of a poet ?

The islands give their name to a distinct formation within Greenly’s classification of the supposed Pre-Cambrian rocks of Anglesey. Together with their mainland equivalents, they form the so-called Skerries Group. On the islands, this formation consists of grits and conglomerates. Greenly writes

All ... the most interesting and important characters of the rocks [of the Skerries Group] are far more strikingly developed on The Skerries than at any other place ... They are hard greywackes [8] , generally rather coarse and often pebbly; green with subordinate grey mottling ... The most remarkable thing about them is their unusual massiveness. On most of the sections no bedding whatever can be seen. They consist of quartz felspar and composite fragments with abundant secondary epidote, chlorite and sericite and some iron-ores, most of which appear to be ilmenite. The felspars are chiefly albite with some albite-oligoclase and micropegmatite. But the clastic grains are not closely crowded as in ordinary grits; they are visibly isolated by the matrix. This ... is of a ... pistacio-green colour ... and is identical in appearance with the green ... of the Church Bay tuffs ... [under high magnification] it is resolved into minute granules and prisms of epidote ... sericite ... and abundant chlorite ... it is evident that much of the rock is of pyroclastic [eruptive] origin. The Skerries Grits ... are far from being normal sedimentary rocks; the pyroclastic element in them preponderates, and ... to this is due their extraordinary massiveness. [9]

The Skerries are unique among the supposed Pre-Cambrian rocks of Anglesey in that only on these Islands are true conglomerates found

There, and there alone, are true conglomerates in the Mona Complex. Bands occur at intervals all over the little archipelago, the pebbles being usually 3 to 4 inches [7.9 to 10.3 cm] across. But on Ynys Arw the conglomerate may be called a boulder bed, being full of great oval blocks, most of which are 6 inches [15.4 cm], many of them a foot, and some nearly 2 feet [61 cm] in diameter. They are well rounded and being light in tint, stand out in strong contrast to the dark green matrix, so that the beds are very striking in appearance. But so tough and resistant is the matrix that the boulders do not always weather out, some even weathering into cavities. The grits contain many short bands, an inch or two [2.54 - 5.1 cm] in thickness, of a fine epidositic mudstone, with minute clastic quartz, which is of the same nature as the matrix. These bands may be finely bedded but in the boulder beds they are broken up into long strips, and these into fragments, many of which are well rounded, so that contemporaneous erosion was at work. All but a few of the larger pebbles in the Skerries Conglomerates are of acid igneous rocks ... textural range from quartz-rhyolite to granolite and are very fresh and well preserved ... compact ... of a clear blueish-purple tinge and consist of a cryptocrystalline matrix that may once have been vitreous, in which are phenocrysts of quartz ... and felspar. At the other extremity of the suite are granites [see b/w Fig.28] with ... micropegmatite but no porphyritic crystals. [Some] contain the same phenocrysts as the felsites but the matrix ... is relatively large of grain and sometimes ... micropegmatitic [see b/w photograph 5 below]. It is evident that all are products of one and the same acid sodium-magma. None are foliated. The rocks from which these interesting pebbles are derived are never seen in situ [10] .

Similar pebbles occur on the East Mouse though they are neither so common nor so large. In the Skerries conglomerates Greenly observed other pebbles, derived from quartz, jasper and schistose grit. He believed that the Skerries Group was older than both the Holyhead quartzite and the New Harbour Group and therefore suggested the rocks of the Gwna group as the source of these pebbles, as he believed that formation to be older still and it contains all the required mineral types. Until recently it was believed following the work of R.Shackleton that the Skerries Group was stratigraphically below the Gwna Group and above the others but since jasper occurs in association with the New Harbour group spilites there seemed no mystery as to the source of these other pebbles. However, opinion has recently changed and is currently uncertain. Greenly’s description is reproduced below:

local bedding has been observed in some 5 places, where short bands of mudstone or the disposition of pebbles render it perceptible, especially on the little northern peninsula and on Ynys Arw, dipping at high angles to the north. By the deep cleft in Ynys Arw these mudstones are undoubtedly true beds, and in place. But elsewhere (and they are seen in many places), especially in the conglomerates, they are broken up, rolled, and cut off in such a way as to show that contemporaneous erosion is the cause, not cataclastic deformation. The best locality is on the south cliffs of Ynys Arw ... the most powerful deformation is at the lighthouse garden, where boulders of granite have been rolled out into long lenticular strips [see Greenly’s line drawing opposite] ... On the western cliffs, north of the footbridge, a 1/3” [85 mm] quartz vein that cuts the foliation at right angles is itself shifted many times along it, thus revealing a second movement along the same planes ... Two thrust planes traverse Ynys Arw and others may be seen on the lighthouse crag [see Greenly’s line drawing opposite] but the most powerful thrust plane is that along which a little sound has been eroded, isolating the Toucan, the peninsula and Berchan. The grits are mylonised [11] upon it ... The most conspicuous series is a system of jointing with south-easterly and north-westerly dips, that simulates bedding. It is not a pure jointing, for at the North end of the large ‘footbridge’ island and on the northern peninsula some feeble crushing has been observed, and it is older than the Palaeozoic sills which are injected along it. Finally, there is a system of late faults trending approximately north and south, which have initiated several of the little sounds. Isolated pebbles may be found anywhere, and beds of conglomerate occur all over the islands. The nature of the pebbles is much the same everywhere but it may be mentioned that the 2” [5.08cm] pebble of Gwna quartzite [12] was found on the south slope of the lighthouse crag and the best one of ... jasper under the wall on the eastern side of the garden. Ten beds of conglomerate have been laid down upon the 25” [1:2500 scale] maps; a broad one on the southern cliffs of Ynys Arw and on Flood point islet, five thinner ones at the Lighthouse and garden, one on the south point of the footbridge island, one on Cave Point, one on the little northern peninsula and one on the Toucan. The broad zone on Ynys Arw is the great boulder-bed alluded to, and is the most remarkable zone in the islands. On the west cliffs of the deep cleft, internal bedding can be seen in it, but the great granitoid boulders are upon the brows of the southern cliff. [13]

With regard to the numerous Palaeozoic basic igneous sills intruded into the rocks of the Skerries, Greenly writes as follows [14]

The dolerites of the Skerries are ... sills, for they have been injected along pre-existing divisional planes, which, however, are not bedding, or even foliation but old jointing of the massive schistose grits [see previous passage]. The sill below the lighthouse is about 20 feet thick [6.1m] in all, but is rather complex and includes large masses of the grits [see coloured photographs ]. Inclusions on the small scale have not been seen so often in the sills as in the dykes.

and later on

The six sills on the Skerries are closely allied to the [mainland] dyke-dolerites. The one below the lighthouse is exceptional, being an amygdaloidal basalt with well preserved augite [15] and labradorite [16] in the matrix. It reappears on the other side of the footpath. The junctions have been sheared in places. That on Ynys Arw is only six inches [15.2 cm] thick, and the one on the adjacent islet was inaccessible when surveyed. The Haven sill, about a foot [30.5 cm] thick, is a fine basalt with acicular augite. The small patch furthest away to the north-east near Race Point is an outlier, sending down veins along the foliation of the grits. It is porphyritic and has a good chilled selvage full of little lath-shaped felspar phenocrysts that have a parallel arrangement as in the selvages of the dykes, but the matrix is crystalline, a mesh-work of later felspars, which are labradorite, partly albitised [17] . This and the large one at the Foot Bridge between the two chief islands, which are both hard, fresh, dark green rocks ... have undergone a peculiar kind of anamorphism, the whole of their pyroxene having been converted into green hornblende without any deformation of the felspars. They are therefore ophitic epidiorites. The Footbridge rock is seen to be a true sill, its upper surface, which has a good chilled selvage with lath-feldspars, winding about at a low angle. A feeble zone of shearing, about a yard [92cm] wide, passes through it, parallel to the foliation of the grits, probably of the same age as that which cuts the quartz vein [see previous passage] [18].

In summarising his views of the evolution of the coastline of Anglesey, Greenly writes as follows

the Mona Complex and [associated] Palaeozoic rocks must have been ... enclosed within a ... wider periphery [of softer Mesozoic sediments] ... the present confines of Anglesey coincide roughly with the limits of this old hard nucleus ... There is evidence that the nucleus ... is the core, several times planed off, of an early Tertiary anticline ... the erosive forces under whose power the softer periphery gradually wasted away, must have received a sharp check when they had cut back to the margin of the hard nucleus ... the margin, probably sketched out for us by the fringe of outer isles which curves round from Puffin, by Dulas and the three Mouse islets to the Skerries, would have been the pre-glacial outline of a land which we may, for the first time, call Anglesey, Thence, but much more slowly, ensued a retreat to a second pre-glacial outline whose position coincided very nearly with that of the present coast, in so far as that coast is composed of solid rock ... The glacial episode, however, once more considerably extended the land by enclosing the old coast in a periphery ... of boulder clay. The present line has been developed by a second erosion of soft material [back to the hard rock limit] ... considerable tracts of boulder clay remaining to be cut away from the mouths of pre-glacial valleys. The forest bed submergence has accelerated the destruction of the boulder clay periphery and has also enabled the sea to attack the hard cliffs once more along considerable stretches from which it was ... excluded. Of the three open-sea coasts, about one-sixth is determined by the present stage of retreat of the glacial drifts. The remaining five-sixths are essentially a restoration of the last limit of pre-glacial erosion ... varying from a trifling minimum to a maximum of two miles at the Skerries [19] .

Of the presence of glacial and post-glacial features on the Skerries Greenly writes:-

There is hardly any drift upon the Skerries, but beneath some angular rubble in two hollows that cross Ynys Arw is a little true boulder clay [see colour plates below]], resting in one case upon a striated surface. Erratics are rare and none of definite significance have been identified [20] .

and also:-

The Skerries are strongly ice-worn, almost every islet having marked onset and lee sides, while striae have been found at four places [marked on his 25” / 1:2500 map; see coloured Fig.14]. The direction is the SSW normal on the northern coast and differs by some 35° from that of Carmel Head and the NW corner of Anglesey [21].

We have summarised Greenly’s observations on the geology of the Skerries, especially as it relates to the supposed late Precambrian geology of North West Wales, because in fact there has been no more recent published study of its special features during the intervening ninety years. The exotic conglomerates seen there have in recent years acquired a new interest, for they may possibly be evidence of the recently discovered widespread glaciation which affected the earth during late Proterozoic time. This is discussed in the section on recent developments.

1 Pennant Tour in North Wales
1a Nitric acid HNO3
2 A.C. Ramsay Memoirs of the Geological Survey-The Geology of North Wales Longmans, London 1866
3 E. Greenly Memoir of the Geology of Anglesey
4 E. Greenly A Hand through Time
5 Dr. J. J. H.Teall Director of the Geological Survey. Author of British Petrography: with special reference to the igneous rocks 1888
6 This note relates to Trinity House’s use, from 1860 onwards, of buoys lit by the burning of pressurised acetylene gas. This gas was manufactured by Trinity House from oil, at their Blackwall headquarters on the Thames, and taken to sea in pressurised tanks by Tender vessels like the Triton. At sea, the gas was pumped into cylinders in the buoy bodies at a pressure of six atmospheres. These gas lanterns are said to have been real works of craftsmanship, cast in bronze, and their light could be seen in most conditions at a distance of several miles. They began to be replaced from 1916 on, and had all gone by the outbreak of World War II. See M. Tarrant Trinity House pp.21-22.
7 E. Greenly A Hand through Time p.337
8 Grits with angular un-weathered grains
9 E. Greenly Geology of Anglesey vol.1 p.59
10 Greenly ibid vol.1 p.60
11 Shattered by crushing along a thrust plane
12 Greenly says ‘Gwna quarzite’ but this is not clear any longer with changing views about Anglesey geology..
13 ibid p.320
14 ibid p.507
15 The commonest clino-pyroxene
16 A plagioclase Na-Ca felspar with roughly equal amounts of each element.
17 All calcium replaced by sodium in the felspar crystalline matrix
18 ibid p.532
19 ibid p.894-5
20 ibid p.741
21 ibid p.740

Greenly's 'boulder clay' deposit on Ynys Arw

Detail of the 'boulder clay' on Ynys Arw Photograph by Nigel Campbell-Bannerman