Quarterly Journal of the Geological Society of London/Volume 33/On the Silurian Grits of Corwen, North Wales

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12. On the Silurian Grits of Corwen, North Wales. By Prof. T. McKenny Hughes, M.A., F.G.S. (Read December 20, 1876.)

I beg to offer a few notes, made chiefly in the summer of 1876, while endeavouring to correlate the base of the Silurian rocks of North Wales with the corresponding beds in the Lake-district and South Wales.

I found that some important changes in the mapping and consequent classification of the rocks in the neighbourhood of Corwen were necessary. These corrections being made, I obtained a key by which I was able to detect, over the whole of the district I then examined, a very variable but yet a more satisfactory and easily recognized base for the Silurian rocks.

It will be seen by reference to the Geological-Survey map of the Corwen district, that the Denbigh Grits are represented as thrown down by a fault (running W.S.W. and E.N.E.), while no Grits are are shown on the map between this and Pennyglog Quarry.

I found, however, that the Grits at Corwen were not the Denbigh Grits, as seen e.g. on top of the Flags at Penyglog Quarry E.S.E. of Corwen, but that the Corwen Section is as follows (fig. 1):—

Fig. 1. Diagram Section from Corwen to Penyglog.


The section runs from W.N.W. to E.S.E. from Corwen to Nantcaweddu, then nearly W. to E. to Nant Llechog, thence E.N.E. to Penyglog,

f. f'. Denbigh-Flag series. g. Grit in Denbigh-Flag series.
h. Band of nodules near base of f'.
k. Pale slates (including part of what was previously called Bala).
l. Corwen Grit. m. Bala beds.

The nearly vertical Grits of New Corwen seem to be brought in by faults, two of which are pretty clear. South and east of these faults there are two large quarries near the church—one in Bala beds, which have yielded Stygina latifrons, Illænus Davisii, Glauconome disticha, Atrypa marginalis, various species of Orthis, and corals.

The other quarry is a little further west. In it the Grits, to which I propose to give the name of the "Corwen Grits," are worked, chiefly for road-metal. These are obviously thrown down by a small fault from the grits seen in the cliff immediately to the south. In this cliff I picked them up, and traced them S.W. for some distance in the bed of the stream that runs down Nantcaweddu. Here they begin to show a very variable character. In one place just south of the moorland boundary-wall, they contain quartz pebbles from the size of a pea to that of a hen's egg. In another they run into fine sandstones. I here obtained one fossil from the coarser part, which, though not well preserved, can with great probability be referred to Favosites alveolaris. At about one mile south of Corwen a slight fold throws the Grit outcrop about 1/4 mile to the east, beyond which I followed it west of the shooting-box known as Liberty Hall, and lost it south of Moel Ferna. Along this line it is frequently very thin and is generally of a finer texture than in Nantcaweddu, being very often a grey-and-white sandstone with wavy lines of bedding and subordinate patches and lines of slate.

To return to Corwen. The Grit runs along the cliff south of the rectory to Nant Llechog, where it may be seen rolling towards the north, the stream following the face of the beds along some of the folds for a considerable distance. It is then lost under the drift and talus north of Penyglog. Nearly south of Bonwmuchaf the Grit, which is rather coarse, weathering yellow or white, contains what look like fragments of cleaved slate; but this is not clear, as elsewhere it certainly contains small pans and lenticular patches of mud; and these, when pinched up in a bed of different lithological characters, such as sandstone or grit, might often appear like included fragments, and have cleavage produced in them alone. In such cases there is generally a kind of uniformity in the direction of the cleavage-planes; but when the cleavage-planes of the included pieces of slate lie in all directions, the probability is that they are fragments of a previously cleaved rock.

Along this line of section the Grits are frequently seen overlying Bala Shale, in which, immediately below the Grit, I found, in one place, Orthis Actoniæ. In the Grit itself here I have found only some undeterminable fragments of Orthis and Petraia.

In Nant Llechog the Grit is represented by a white saccharoid sandstone, sometimes ripple-marked and false-bedded, sometimes with black lines and bands of slate in it, at others quite homogeneous. In several places along this line of outcrop I noticed in the Bala beds a kind of double cleavage, to be referred to two successive movements causing lateral pressure, not in the same direction, and giving to the rock, when viewed across the broken edges of the cleavage-planes, an elongated lozenge-shaped structure. In the series overlying the Grit there seemed to be but one cleavage. I could not make out the original directions of these several movements, as it was clear that there had been many subsequent disturbances in that area which had affected both the singly and doubly cleaved rocks—such as, for instance, the great movements which let in the Hafodycalch Mountain-limestone, which, with its associated shale, is not cleaved. This test of double and single cleavage cannot be expected to hold everywhere, as it must frequently happen that the direction of the second pressure may coincide with that of the first; and though perhaps the result may be more intense cleavage in the previously cleaved rocks, no crossing of cleavage-planes could be produced; and, moreover, in cases where the second cleavage is very intense, or the rock very susceptible of cleavage, this would entirely obliterate all previously existing divisional planes, as notably in the volcanic slates of the Lake-district, where we find joints and faults quite healed, and the rock splitting along the cleavage-planes only.

The beds below the Grits are, as a rule, bluish rab, i. e. a shivery mudstone breaking along bedding, joints, and imperfect or double cleavage into irregular, often somewhat prismatic fragments. Frequently, however, these beds have, especially where much weathered, a pale grey colour, much resembling that of the beds above the Grits. This may perhaps be because they are derived directly from the felspathic ash of some volcanic district, while the Silurian beds above the Grits have been formed later on from the waste of such beds.

These pale beds above the Grits pass up into the "Pale Slates" of the Survey, which in turn pass up into the striped flaggy Slates of Penyglog, on the top of which come Grits to be referred to the true Denbigh Flag- and Grit-series, and which I traced for about two miles to the N. side of Moel Ferna. In the flaggy Slates I found Graptolithus priodon, Cyrtograpsus Murchisoni, and Retiolites, sp., with Orthoceras primævum and O. subundulatum. I again found some Graptolites in a small watercourse N.E. of Moel Ferna, on what seemed to be the same horizon. The dip is here about 10°, N.E. to N.N.E., and the cleavage 30° N. I would here acknowledge much useful information which I got from Mr. Phillips, of the Penyglog Quarries.

I then tried to apply this key to other districts.

First, then, there is a patch of Bala coming in south of the fault near Bryn Gorlan, at the south end of the Yale of Clwyd, as shown in the section (fig. 2).

Fig. 2. Section at Bryn Gorlan, Vale of Clwyd.


a. New Red. b. Basement bed of New Red, sometimes conglomeratic.

c. Carboniferous Sandstone, generally stained red. d. Mountain Limestone.
e. Basement bed of Mountain Limestone (shale, sandstone, and conglomerate).
f. Denbigh Flags.
k. Pale slates (including part of what was previously called Bala).
l. Corwen Grit. m. Bala beds.

Here I found at the north end the Denbigh Flags. These may be examined along the road to Llanfairdyffrynclwyd, where Graptolites (chiefly G. priodon) occur. South of the Bryn-Gorlan fault we find the Bala beds in places highly fossiliferous, as e. g. near Cerrigoerion, where I found Leptæna sericea, L. transversalis, L. scissa?, Strophomena depressa, Palæarca, &c.

On the top of the hill, sticking out through the flat peaty surface soil, runs a ridge of whitish Grit about 50 feet thick, which becomes finer as we follow it west, until it is represented by a set of nodular striped sandy beds.

The beds below the Grit have the same kind of double cleavage, noticed in some cases near Corwen in beds below the Corwen Grit.

The beds above the Grit are pale imperfectly cleaved slates, passing up into the beds mapped as "Pale Slates" by the Survey, which are, in turn, overlain by the Denbigh Flags.

On the west the same beds might be expected at a short distance below the Pale Slates; but I have only hastily run over some of the ground between Llanfihangelglynmyfyr and Cerrigydrudion since I made out the Corwen Section; and though I came upon fragments which I believe should be referred to this rock, I did not find it in place. Mr. Etheridge informs me that he has Pentamerus oblongus recorded from Cerrigydrudion.

On the east, after crossing the Llandegla and Bryneglwys faults, we come to rather a difficult bit of ground about Cyrnybrain. I will, however, give the results of a very imperfect examination, as it may save time if any one should go over the ground more carefully before I can offer any thing better.

Following up the stream by Penycae to the south-east, about one third of a mile south-east of Penycae we find some crushed Sandstone in the bed of the stream, but no clear section. If we proceed straight on, climbing the hill on the north-east of the gully, we soon come upon a long line of fragments of white Grit, nowhere seen in place, but, as far as it can be traced, keeping a uniform distance from the Pale Slates, as if they were portions of a rock broken up along its outcrop. It is true that there are also scattered along the hillside many boulders of felspathic rocks from the high mountains to the west; but they do not occur in a line like the fragments of Grit. In this Grit there are a good many fossils, generally in the form of casts, among which I was able to make out Petraia subduplicata, P. crenulata, and Meristella crassa; and Pentamerus oblongus is recorded by Mr. Salter from Cyrnybrain.

The character of this rock is like that of Corwen; the position of the line of fragments relative to the Pale Slates is the same.

To the E. and E.S.E., however, I was unable to follow it into the ravine near Plasuchaf; but up that dingle a thick series of fossiliferous sandy mudstones occur, dipping at a high angle (50° to 70°) to the south.

In these beds the following fossils occur—Meristella crassa, Orthis sagittifera, Petraia subduplicata, and P. crenulata.

I would suggest a comparison of these beds with those in the tramway-section at Llansantffraidglynceiriog; while the beds on the hill about half a mile to the N.W. of Plasuchaf seem to be exactly like those in the valley south of Llansantfrraid, on the road to Llanarmon.

Along the strike of the Llansantffraid mudstones toward the S.E. we find a curious bed, indicated by a blue line in the Survey Map. This consists of alternations of bands of Limestone and fine Sandstone with wavy lines, very like that into which the Corwen Grit passed in several places. In this I saw several traces of fossils. The only ones I was able to make out were Favosites alveolaris and a large form of Orthis calligramma.

Both the Limestone and the tramway mudstone pass under the Pale Slates; and these are overlain by the Denbigh Flags, which are worked in the large quarries near Llansantffraid. A bed of Sandstone seen on the hillside north of the village may represent the Grit of Penyglog, near Corwen.

The points that seem to me clear are:—that the Corwen Grits are distinct from the Penyglog Grits; that there is more evidence of a discordancy at their base than at the base of the Pale Slates or of the Penyglog Grits; that there are generally some beds of conglomerate, sandstone, or limestone with sandstone on the horizon of the Corwen Grits; that the general facies of the few fossils obtained from these beds in the district examined is that of the May-Hill rocks; that the mudstones of the ravine north of Plasuchaf are the same as those of the tramway-cutting south-west of Llansantffraid.

Other questions remain to be worked out. Are these mudstones (which have not yet been found immediately underlying the Corwen Grits) merely a local development of those Grits? or are they a lower part of the same group locally developed to a greater thickness? or are they a higher part of the underlying Bala series here and there overlapped by the Silurian Rocks?

I have thought it better to bring forward what I have done, and invite cooperation along the same line of investigation, rather than to wait till I could offer more definite results.

 

Discussion.

Prof. Ramsay said that about Builth and all round the Longmynd area, and, indeed, over a great part of South Wales, we find Cambrian and Lower Silurian rocks overlain unconformably by the Pentamerus-limestone or Upper Llandovery rocks, upon which, north of the Builth country, and also in part of South Wales, come the Tarannon shales and the Denbigh grits with Wenlock fossils, overlain again by the ordinary Wenlock shales. He was particularly pleased, therefore, to hear that Pentamerus oblongus occurred in the beds where Prof. Hughes said it was present, as this was strongly in favour of his own opinion that the Upper Silurian strata were transgressive in the Corwen district.

Mr. Hicks considered the Lower (or Corwen) Grits to be the equivalents of the grits at the base of the Lower Llandovery rocks in South Wales. The Upper (or Penyglog) Grits are nearer to the horizon of the May-Hill Sandstones. There was no visible unconformity in the Corwen area between the Lower Grits and the underlying Bala beds in any of the sections he had examined; and he was inclined to believe that this area remained under water during nearly, if not quite, the whole period that the more central parts of Wales were above sea-level. No one had attempted to deny that there was marked unconformity in the Longmynd and some other districts between the May-Hill Sandstones and the underlying rocks; but the beds which could in any way be classed with the Llandovery rocks attained there only to a few hundred feet in thickness at the most, whilst thousands of feet represent that period in South Wales, and apparently also in parts of Denbighshire.

Mr. Hopkinson gave a list of Graptolites he had found in the flaggy slates, which included forms characteristic of beds at the summit of the Coniston Mudstones or the base of the Coniston Flags, and stated that a similar series occurred in equivalent beds in Scotland.

Prof. Hughes, in reply, pointed out that the Graptolites found in the slates of Penyglog were not those of the Graptolitic Mudstones or Stockdale Shales, but agreed exactly with those of the Coniston Flags. So also near Austwick, on the borders of the Lake-district, the Graptolitic Mudstones had not yet been discovered, though they were well developed not far to the north in the Sedbergh district. He had not himself succeeded in finding Pentamerus oblongus in the Corwen beds; but Mr. Salter recorded it from Cyrnybrain. He thought the Corwen beds were on the horizon of the calcareous conglomerate of Austwick, and that the Penyglog grit was the equivalent of the Austwick grit, while the flaggy slates of Penyglog represented the flags between the Austwick conglomerate and Austwick grit.

The President insisted strongly on the necessity of studying both the Palæontology and the Field-geology of any district, before attempting to come to any definite conclusion as to its geologica structure and the relative age of the deposits forming it.