The geology of north-western Denbighshire

THE GEOLOGY OF NORTH-WESTERN DENBIGHSHIRE. By Professor P. G. H. BOSWELL, A.RC-S., D.Se., F.R.S., EGoS.

[Written for the Easter Field Meeting, I935·J I. INTRODUCTION. only general account of north-western Denbighshire dates back to the time of Ramsay and was included in his Memoir on the Geology of North Wales, published in 1866. Devoting rather less than two pages to the Denbighshire district in his description of the Denbighshire Grits and Wenlock Shale, Ramsay opined that "to strangers it forms the least known district of North Wales." And before the Easter Field Meeting in 1935, even if we substituted the word geologists for strangers, we should be justified in endorsing his opinion. A second edition of Ramsay's Memoir appeared in 1881 but less than a page was added to the account of the district, the additional notes containing the suggestion that Ludlow as well as Wenlock rocks might be present. Before Ramsay's time, the general rock-succession in North Wales had been studied by Sedgwick, who published his account in 1844-5 and, in reference to our area, introduced the term Denbighshire Flagstones. During the past ten years parts of the area have been described in sundry papers, to which reference will be made in the present description. The re-mapping of the area on the 6-inch scale (Ramsay and Aveline's original survey having been, of course, on the r-inch scale) is still in progress, and is likely to occupy many years before completion. But the stratigraphical succession and areal distribution of the rock-types are now sufficiently well known for a general account to be given. Further field-work and fossil-collecting will doubtless amplify our knowledge in respect of details, but the generalizations will probably not be invalidated. The interpretation of the tectonics, however, is likely to provoke active discussion for a long time. In cartographic outline Denbighshire is irregular. Its north-western part comprises a block-like region from which areas extend like processes north-eastwards and south-westwards on its south-eastern side. This upland country, of which the central part (about 100 square miles in extent) is occupied by the Denbighshire Moors, is the subject of the present paper. The region includes three well-defined major topographic units: (a) the Denbighshire Moors and their marginal belts of arable country, (b) the Vale of Clwyd, and (c) the Clwydian Range. Only incidental reference will be made to the lastnamed unit (which is an outlier of the Silurian mass of the

THE

153

GEOLOGY OF NORTH-WESTERN DENBIGHSHIRE.

Denbighshire Moors), for, although it was frequently seen during the Field Meeting, it was not actually visited. (a) The first-mentioned unit is trapezoidal in plan and dome-like in elevation. It consists of Silurian rocks, belonging mainly to the Ludlow division, flanked by a steep escarpment of Carboniferous Basement-beds and Limestone (Fig. 19). It is bounded on the north by the Irish Sea, on the west by the / R/SH

SEA WH.IES

e,

o , Q.

o

e

Ordovician

FIG. J9.-SKETCH-MAP OF NORTH-WESTERN DENBIGHSHIRE, showing chief

localities and lithological variations of the Silurian rocks. dotted; Carboniferous, horizontal ruling.

Trias

Vale of Conway, on the south by the outcrop of the Ordovician (the junction of which runs roughly along the east-west main road through Pentre Foelas and Cerrig-y-druidion), and on the east by the Vale of Clwyd. Until the last decade, the interior of the area was almost as difficult of access as when Ramsay commented on the fact>, but new roads have recently I He remarked that the tableland was cut by a labyrinth of valleys so numerous and steep that" scarcely any part of Wales was more difficult of access," and that one might walk faster in Den bighsbire than one could drive.

154

P. G. H. BOSWELL,

been made, others widened, and motor-omnibus routes now connect the remote villages. For the most part the area is a tableland deeply dissected by streams; and these two factors have resulted in beautiful valleys, gorges and waterfalls, once accessible (and fortunately some still accessible) only to the pedestrian. The numerous hills are of relict character. Like the dome-shaped central portion of the moorland, they reach elevations of 1,300 to 1,800 feet above O.D. It is evident, therefore, that the apparently monotonous character of the country when seen from a distance is belied on closer acquaintance. In the extreme south-west, near Capel Garmon,the quiet restful landscapes of the Silurian give place to the tugged and diversified country of the Ordovician area of North Wales, fortified as it is by resistant volcanic lavas and tuffs. (b) It has truly been said that no scene in Wales is more lovely of its kind than the fertile plains of the Vale of Clwyd, and the four towns, Ruthin, Denbigh, St. Asaph and Rhyl, lying along the River Clwyd, have been aptly compared with pearls strung on a silver thread. The castles of Ruthin, Denbigh and Rhuddlan bear witness to the interesting mediseval history of the Vale, and were in the nature of outposts facing the wild country of the Welsh fastnesses and the Moors. The Vale runs from south-south-east to north-north-west, being little more than a mile in width at its upper end, and four miles in width at its mouth. It is a remarkable flatbottomed trough, consisting of a down-thrown region of Triassic Sandstones and Coal Measures, bounded by faults. Thus it is an excellent example of a rift valley and, as a scenic feature of this type, is almost unrivalled in Britain. On the east it is steeply flanked by the smooth slopes of the Clwydian Range, the margin being a practically continuous fault-feature. Along the western side, however, instead of a continuous fault (or faultsystem of closely packed members), we find a series of faults en echelon with unconformable junctions of Carboniferous on Silurian between them, so that the flanks of the Vale rise gently and are diversified. Being frequently clothed with glacial drift, they are largely under cultivation. II. STRATIGRAPHY. The stratigraphical succession of the Wenlock and Ludlow rocks exposed in Denbighshire is one of the most complete in Britain. In addition, parts of the Valentian and Upper Ordovician are available for study in the Capel Garmon district, forming the south-western portion of our region. The Carboniferous rocks are present on the north and east, and the Triassic rocks are confined to the Vale of Clwyd,

GEOLOGY OF NORTH-WESTERN DENBIGHSHIRE.

155

TABLE OF STRATA. TRIAS

Red sandstones (Bunter?)

CARBONIFEROUS:

3. Grey and purplish sandstones (Coal Measures and Millstone Grit?) 2. Limestones belonging to the sub-zones of DI, D2 and D3. 1. Basement-beds, consisting of conglomerates, red and green shales and red sandstones. SILURIAN:

3. Ludlovian

2.

Wenlockian

(e) Zone of Monograptus leintwardinensis. 400 feet. Grey and blue shales and hard bluish sandstones; ribbon-banded and striped sandy mudstones, often massive. M. leintwardinensis, and its variety incipiens, and M. d. ultimus. (d) Zone of M. tumescens. 200 + feet. Gritty brown and grey flags, siltstones and thin mudstones. Ribbon-banded soft bluish concretionary mudstones and flags. Thinly-bedded flags with "biscuity" layers. M. tumescens and its variety minor, and M. scanicus. (c) Zone of M. scanicus. 300 to IIOO feet. Fine-grained grey sandstones and mudstones; massive brachiopod-bearing mudstones. Thinlybedded flags with biscuity bands. Also, in Ruthin area only, ribbon-banded siltstones and mudstones. M. bohemicus (small form), M. chimeera and its varieties salueyi and semispinosus, M. dubius, M. rcemeri, M. scanicus, M. uncinatus var. micropoma, M. uarians and its variety pumilis. (b) Zone of M. nilssoni. 1200 feet. Upper part: Micaceous sandy flags and gritty sandstones; blue, brown and grey mudstones and siltstones, and flags with biscuity layers. M. dubius, M. chimera val. salweyi, M. uarians, M. bohemicus, M. nilssoni, M. uncinatus var. orbaius and M. d. tumescens. Lower part: Bluish-grey, well-bedded and concretionary mudstones and flags, often ribbon-banded and forming slabs (Nantglyn Flags in part). M. bohemicus (large form), M. colonus and its variety compactus, M. dubius (long form), M. nilssoni, M. uncinatus var. orbatus, M. uarians and M. vulgaris. (a) Zone of M. vulgaris. ? 400 feet. Bluish-grey flags and mudstones (ribbon-banded), often thinly-bedded (Nantglyn Flags in part). Calcareous soft blue and brown massive mudstones. M. vulgaris and M. dubius (long variety). (c) Zone of Cyrtograptus lundgreni, 400 feet. Bluish-grey ribbon-banded and thinly-bedded mudstones and siltstones (Nantglyn Flags in part). Calcareous soft brown mudstones. M. flemingi (frequently, the variety ~). M. dubius (long variety). and Cyrtograptus lundgreni:

P. G. H. BOSWELL,

I.

Valentian

(b) Zone of C. rigidus, to 3000 feet. Yellow and grey banded mudstones and siliceous shales. Felspathic grey and brown sandstones interbedded with grey shales (Denbighshire Grits). C. rigidus, M. dubius, Also, in extreme west only, calcareous soft blue and brown flags and mudstones. C. rigidus, M.ftemingi, M. priodon, (a) Zone of C. linnarssoni. In the west only: soft calcareous flags and mudstones. The zones of C. symmetricus and C. murchisoni appear also to be present. Zone of M. conuolutus. Bluish, rusty-weathering shales and massive blue mudstones, thin sandstones, black papery shales and paste-rock. Climacograptus scalaris, Glyptograptus tamariscus, M. concinnus, M. denticulatus, M. d. inuolutus, M. limatulus, M. lobiferus, Rastrites d. peregrinus, Higher zones may t ~ present, perhaps as high as M. turriculatus, for from one locality, M. marri and M. exiguus have been identified. Purple" Tarannon " shales also occur.

ORDOVICIAN :

Bala

The succession in the Capel Garmon area is as follows : Massive blue and grey mudstones 100 feet. Soft black slates . . 20 " Blue-black slates, with Di-plograptus multidens var. compactus, Glyptograptus eugiyphus, Climacograptus scharenbergi and C. antiquus 20 feet. Fossiliferous sandstones and shales with a shelly fauna 200 Lithic, vitric and crystal tuffs .. 60 " Gritty fossiliferous slates 100 " Barren grey slates 4 0 0 +" Nodular and fluidal rhyolites and associated tuffs 400 " Barren grey slates (= ? the Glanrafon Beds of Snowdonia). (The black and blue slates appear to belong to the lower part of the Caradocian). II

Stratigraphical Details. The stratigraphy of the small area of Upper Ordovician and Lower Silurian (Valentian) rocks is described briefly in Section III., Traverse 5. It need not be further discussed here. Since the various stratigraphical horizons of the Wenlockian and Ludlovian vary lithologically as they are followed throughout the region, the predominant rock-types of each zone will first be described and then the changes observable as they are traced laterally will be noted (see Fig. 19). The classification of the Silurian rocks as set out in the table above is based on their graptolitic faunas; indeed, in regard to the geologist in Denbighshire, we might adapt Carlyle's epigram on Man as a tool-using animal, "without graptolites he is nothing; with

GEOLOGY OF NORTH-WESTERN DENBIGHSHIRE.

157

graptolites he is all." The fact that our knowledge of the interesting rock-relationships and remarkable structures of this seemingly monotonous area of Denbighshire is entirely due to pioneer work on graptolitic faunas is a tribute to the genius of Lapworth and the inspired labours of Elles and Wood. Just as the impression of topographical monotony, conveyed by only a distant view of the region, is corrected by close acquaintance with the beauty of its gorges, waterfalls and lakes, so the lithological sameness of the strata gains interest and significance when their fossil-contents are disclosed. But the general similarity of rock-types, and their repetition at intervals throughout the succession remain forbidding facts, and the lateral variation of beds within the same lithological limits-a series of sandstones, siltstones, shales and mudstonesdoes not make for ease in elucidation. WENLOCKIAN. The delimitation of the zones of the Wenlockian in Denbighshire has not yet been satisfactorily effected. Graptolites are less plentiful than in the Ludlow, and the conditions of deposition do not appear to have been so favourable for the existence of life or the preservation of fossils. The lower part of the succession (which is probably not Lower Wenlockian, however) consists dominantly of sandstones or grits, with the result that any shales associated with them are strongly cleaved: the finding and identification of graptolites is, therefore, rendered difficult. The local base of the Wenlockian is thus marked by the Denbighshire Grits, a massive formation reaching 3,000 feet in thickness and having an outcrop which, in the south and west of the district, is from two to three miles in breadth. The Denbighshire Grits include within their mass (and at times have at their base and summit) bluish shales and mudstones, often with current-bedded fine-grained sandy stripes. The grits themselves often contain wisps and pellets of dark grey shale, showing clearly the effects of contemporaneous erosion. In places, where they become conglomeratic, they include numerous pebbles of vein-quartz up to the size of hazel-nuts. The only graptolite which has been identified from the sandstones themselves is M. dubius, but from the shales immediately overlying the grit near Derwen, Cyrtograptus rigidus has been obtained by Dr. R. C. Blackie. Thus part at least of the grits appears to lie in the zone of C. rigidus. It is not possible to say at present, however, that the grit occupies the same stratigraphical position all over its outcrop from Derwen to Eglwys-fach. Near Llanrwst the grits peter out, giving place to shales and calcareous mudstones of the Shropshire type, but they come in again north of the town and form imposing escarpments overlooking the Conway Valley as far north as Tal-y-cafn. Still

P. G. H. BOSWELL,

farther north at Glan Conway they are replaced once again by blue and brown calcareous mudstones. The uppermost Wenlock of the moorland area generally consists of shales and flags of Nantglyn type, sometimes ribbonbanded and usually thinly bedded. The prevalent colour is bluish-grey, but brown tints are developed on weathering. The stratigraphical position of the bluish sandy shales and mudstones which underlie them (and overlie the Denbighshire Grit Series) has not been established with certainty, for the few graptolites that are found are poorly preserved. The thinly -bedded and ribbon-banded flags contain M. flemingi (commonly var. 3) and M. dubius, often in abundance, and, in rather rare instances, Cyrtograptus lundgreni. In the western part of the Moors the rock-succession is more richly fossiliferous, and the lithological types differ to some extent from those of the moorland proper. Brown calcareous flags and mudstones are found there, the latter often bluehearted, reminiscent of the corresponding rocks in Shropshire. The siliceous and hackly-fracturing blue mudstones which are the local equivalent of the Denbighshire Grits are succeeded by calcareous flags, tough and irregular,which break with difficulty on the bedding. They occasionally contain bands of , gingerbread' rock, a result of decalcification. Higher in the succession they become thinly-bedded, but retain their general character of soft, brown flags, blue-hearted in the thicker bands. They have yielded graptolites which indicate the zones of C. linnarssoni to C. lundgreni, and possibly C. symmetricus and C. murchisoni (see Traverse No. I, p. 168). LOWER LUDLOVIAN. Zone of M. vulgaris. Many years ago Miss E. M. R. Wood referred to Miss Elles's work as proving that the Nantglyn Flags included the two zones, M. nilssoni and M. vulgaris. This is generally true of the moorland area, but we should also note that the lithological characters which are connoted by the term Nantglyn Flags are displayed by the zone of C. lundgreni and possibly even lower beds; also that in the south-eastern part of the area, the same lithological type extends upwards through the zones of M. scanicus, M. tumescens and M. leintwardinensis. The zone of M. vulgaris, where distinguishable as a zonal unit, consists either of thinly-bedded flags or ribbon-banded siltstones. Only in rare instances is it monotypic; more frequently, the specimens of M. vulgaris are accompanied by the long variety of M. dubius. Beds containing this zonefossil in association with M. colonus, M. dubius and other graptolites characteristic of the nilssoni-zone, are sometimes found; they may be regarded as transitional between the two zones, although it is unnecessary to speak of passage-beds,

PROC. GEOL Assoc., VOL. XLVI. (1935).

:\.-

SLAH-HORIZOK,

NANTCLY~ l~'LAGs: Zone of 1\1,

PLATE

nilssoni .

9

Na ntglvn

q\l~rrics.

B.--CHU:lIPLED

AKD CLEAVED SHAT.ES: Zone of MonoPontpastvnoz. l 'r ion, near Denbigh.

SANDSTONES

grapilts scanicus.

(Reproduced hy pcru.ission of the Council of tile Geological Society.) [To face p. ISS.

GEOLOGY OF NORTH-WESTERN DENBIGHSHIRE.

159

since there is no gap to be bridged and no hard and fast line to be straddled in zonal subdivision. These beds merely mark the survival of M. vulgaris in a later assemblage. Zone of M. nilssoni. The beds referable to this zone can be usefully divided into an upper and a lower group, each of which displays distinctive lithological features, with slight differences in fauna. The lower part of the zone consists of ribbon-banded mudstones and siltstones of the type known as the Nantglyn Flags. The characteristic banded sheen, or ribbon-banding, is imparted to the rocks by the alternation of layers in which mud and silt respectively predominate, each about half of an inch to one inch in thickness (Pi. 9, A.). The upper beds split less regularly along the bedding and often only with a sub-conchoidal fracture; the lower part of the series is more thinly-bedded. Concretionary structures, of the same colour as the rock, are often found in the massive mudstones, a gradual transition from the bedded rock into the oval concretion being usual. At a few levels, mostly high up in the flags, it is evident that the rock originally included thin calcareous bands: for, as a result of the leaching-out of the lime, surfaces of ' gingerbread' colour and appearance, sometimes crowded with casts of brachiopods, lamellibranchs and graptolites, are not infrequently seen. When first exposed the Nantglyn Flags are dark-blue or bluish-grey; but on weathering they become yellowish-brown, and change eventually into pale yellowish clay. Fossils are abundant at certain horizons; they include, besides graptolites, Cardiola (Slava) interrupta, Orthoceras primceoum, and Periechocrinus [ActinocrinusJ pulcher, the last-named sometimes showing stem, calyx and arms (' lilies '). When the Nantglyn Flags are traced northwards and eastwards, they are found to become more siliceous and micaceous; but they retain their characteristic ribbon-banding and , whetstone' concretionary structure. In the Bettws-Abergele country, strong grits make their appearance in the flags. The syncline of sandy scanicus-beds, which gives rise to the hills west and south-west of Denbigh, forms the approximate division between the typical muddy and silty upper Nantglyn Flags of the moorland area and the more siliceous equivalents on the north and east. The latter consist of massive blue or grey sandy mudstones in the lower part, passing upwards into brownish and greyish more thinly-bedded mudstones and flags, which are in places also micaceous, and sometimes striped with sandy layers. The upper part of the nilssoni-zone consists of more sandy and thinly-bedded deposits, each layer being usually not more than half an inch in thickness. More massive mudstones alternate with these flags and sandstones. Some of the flaggy

160

P. G. H. BOSWELL,

beds are best described as very fine sandstones or siltstones; they have a characteristic striped appearance, and miniature current-bedding of the lamina: is frequently seen. The usual colour is grey, but some of the beds weather to a brown, softer rock, which suggests that they may originally have been slightly calcareous. Like the Nantglyn Flags, they become more siliceous when traced northwards. Thus, in the Cynant and Mynydd du country, about four miles south-south-west of Abergele, they include many bands of strong grit, and give rise to hills, similar to those of the sandy scanicus-beds. Zone of M. scanicus. The thinly-bedded striped flags just described pass upwards into a series of beds of varying lithological characters. The actual passage can be seen at a few localities, one of which, between Ruthin and Derwen, is referred to in Section III., Traverse 4. The lower part of the zone consists of red-weathering thinly-bedded and striped flags and sandy mudstones, often highly cleaved, and some beds in this part show whitish or yellowish biscuity surfaces. These beds are most prolific in graptolites, of which the usual assemblage has already been mentioned in the table above. The thinlybedded flags pass up into massive blue or grey mudstones, usually displaying strong cleavage, and then into alternating sandstones and shales (PI. 9, B.). Bedding is often difficult to detect in the mudstones, and graptolites are rare or absent. But shelly faunas are by no means uncommon, and brachiopods such as Lepta-no rhomboidalis, Dayia navicula, Rhynchonella nucula and Chonetes striatella, together with crinoids and lamellibranchs, frequently occur. A similar fauna is found in, the sandstones, especially in the bands of ' gingerbread ' rock which appear to have been originally fine-grained calcareous grits, since decalcified. Current-bedding and evidence of contemporaneous erosion are frequently seen. The sandstones contain occasionally grains of felspar and detrital white mica and chlorite, together with small fragments of quartzite and rhyolite or fine tuff. A likely source of this material is the Ordovician volcanic rocks. It is probable that the gritty sandstones are impersistent and occur at several different levels. Individual beds are frequently four to six feet in thickness, but occasionally reach 20 feet. On being mapped, they are usually found to thin rapidly and disappear, their place being taken by sandy or argillaceous grey shales, similar to those with which they are interbedded. The shales are generally well-cleaved and often puckered, as are the thinner grit bands. Both types of rock are sometimes squeezed out into phacoidal bands; also, the phenomenon of internal crumpling- is not uncommon. The 1 L.

J.

Wills and .B. Smith.

Quart. [ourn. Grot, Soc., lxxviii. (1922), p, 206.

GEOLOGY OF :-IORTH-WESTER~ DE~lllGHSHIRE.

16r

grits frequently show the effects of pressure in their undulating and shattered character, and occasionally, when sufficiently fine in grain, they take on a rough cleavage or close jointing; Being the most arenaceous division of the Ludlovian succession, the scani'Cus-beds form most of the hilly country in the northern and eastern part of our area. Succeeding the gritty beds is another series of biscuity flags and well-bedded striped mudstones. These appear to pass up into the thinly-bedded flags of the zone of M, tumescens, but the passage is nowhere visible. Having described what might be termed the typical' facies of the scanicus-beds of the region, which here reach a thickness of not less than 1,100 feet, we must note an interesting lithological change which appears in the Ruthin area towards the southeast. The sandstones and massive cleaved mudstones are absent in this area and the thickness of the zone, as exposed, does not exceed about 300 feet.' Further, while the biscuity and thinly-bedded flags still form an essential part of the succession, they are associated with relatively uncleaved mudstones displaying ribbon-banding of the Nantglyn type. The whole series is graptolite-bearing, the beds with shelly faunas being subordinate or absent. Thus, these beds were either laid down in deeper waters than those farther north, and are consequently thinner and more argillaceous, or the sandy and massive members have been cut out by faulting. There is also the possibility that the overlying beds of the zone of M. iumescens have overlapped the upper part of the scanicuszone. On the whole, the evidence is rather in favour of lateral change in lithology. Zone of J1. tumescens. No continuous section through this zone has been discovered. The area occupied by it is less than that of any other Silurian zone in the district, and the outcrops all have faulted contacts with those of the adjoining zones. But it is possible to piece together to some extent the isolated sections, although the thickness of the zone cannot be estimated. It will be remembered that the zone of AI. tumescens elsewhere in Britain is usually monotypic, that is, only the zone fossil (with its variety, minor) occurs. Such is the case also in Denbighshire, but there happen to be two areas where the zone-fossils of the underlying and overlying zones respectively have also been found. It is only reasonable to suppose that in these areas we are fortunate in finding the lowermost beds and the uppermost beds respectively, of the zone of M. tumescens, The first of these areas is about equidistant from Denbigh and Ruthin. It consists of an inlier, fault-bounded on all sides, on the slopes of Foel Uchaf, nearly a third of a mile PROC. GEOL.

Assoc..

VOL.

XLVI.,

PART 2,

1935.

II

16z

P. G. H. BOSWELL,

south of Porth. The beds here differ from those of the neighbouring zones of M. nilssoni and M. scanicus, for they consist of well-bedded argillaceous and sandy flags, usually in regular beds about half an inch in thickness; they are sometimes striped and contain biscuity surfaces. A noteworthy feature is their complete freedom from cleavage, although the neighbouring scanicus-beds are highly cleaved. Only 10 feet of beds are here visible, but fortunately they are richly fossiliferous, M. iumescens being abundant, with rare examples of M. scanicus. We may therefore regard the deposits as belonging to the bottom of the tumescens-zctie. The localities where the upper beds of the zone are found occur near Ruthin and Nantclwyd, that is, in the area where the zone of M. Ieinttaardinensis occupies. much ground. Here again, however, the outcrops of the tumescens-zone are faultbounded, and no actual passage into the overlying zone can be traced. At a number of localities in the northern part of Pool Park (see p. 174), also near Bodyngharad, still farther north (in exposures 220 yards south, and 400 yards south-east, of this farm), and near Nantclwyd Station (in the Station yard, also 200 yards north and zoo yards north-east of it) M. tumescens is found in association with occasional examples of M. leintwardinensis. The rocks are blue and brown calcareous flags and mudstones, often concretionary. There remains, of course, the question as to whether these beds should not be assigned to the higher zone. Either the first appearance of M. leintwardinensis (which is the common practice), or the level at which it begins to predominate over M. tumescens (which is convenient in this district) can be used to mark thc base. In either case, the upper beds of the tumcscens-zotv: appear to be present here. A few localities are known where only },1. tumescens and its variety minor have been found. The beds containing them are bluish or greyish mudstones and striped flags, sometimes calcareous and concretionary, with 'gingerbread' bands. A typical fossiliferous exposure is the roadside quarry, close to Bron-haul, on the south-eastern slope of Foel Gasyth, about 3t miles S. 35° W. of Denbigh (Fig. 20). In the Ruthin area a similar type of lithology is met with, but the mudstonesbecome ribbon-banded and of the Nantglyn type, in common with all the Ludlovian zones. A typical exposure of such beds is seen at the Lodge gates of Pool Park, half a mile N. woW. from the House. These beds are usually, but not invariably, free from cleavage. As no graptolites of either the scanicus-zone or the leintwardinensis-zone have been found, we may regard such mudstones and flags as referable to the main mass of the tumescens-zone, Assembling the evidence of all the exposures, we can

GEOLOGY OF NORTH-WESTERN DENBIGHSHIRE.

r63

estimate a thickness for the beds exposed of only zoo or possibly 300 feet, but there is no reason for thinking that we are able to see the whole thickness of the zone. Zone of M. leintwardinensis. The lower part of this zone consists of thinly-bedded brown and blue flags alternating with more sandy, striped sediments. Higher in the zone the latter become thicker, and fine-grained bluish sandstones make their appearance. 'Gingerbread' bands of decalcified sandstone, current-bedded and reaching half an inch in thickness, are not uncommon. Graptolites occur throughout the zone, being abundant in the biscuity flags associated with the sandstones, or in the sandstones themselves. In the latter case, they are often accompanied by numerous brachiopods and species of Orthoceras and Cardiola. The shelly fossils and the graptolites frequently occur together on the same bedding-planes. In addition to the lithological types mentioned, massive and slab-like ribbon-banded mudstones and siltstones, like those of the Nantglyn Flags, occur less frequently. The shales associated with the sandstones are usually well-cleaved and sometimes exhibit the phenomenon of internal crumpling.. Petrologically, the sandstones are often highly micaceous, and are indistinguishable from those of the other Ludlovian zones. CARBONIFEROUS. The Carboniferous succession in the district has been worked out by Dr. E. Neaverson, and the following notes are extracted from his paper. I Except for the Basementbeds, which appear to be of 52-Dr age, all the deposits belong to the Dibunophyllum-zone, of which three subzones can be distinguished, namely, those of Dibunophyllum bourtonense (Dr), Lonsdaleia floriformis (Da) and Lonsdaleia duplicata (D3). The distribution of the outcrops of these subzones around the Vale of Clwyd and along the sea-coast is shown on the map (Fig. zr). Of considerable interest is the change of facies displayed by the beds in the district. For details of this phenomenon reference should be made to Dr. Neaverson's paper, but some of his main conclusions are included in the following brief account of the stratigraphy. Dr. Subzone of Dibunophyllum bourtonense. Most of the limestone outcrops on the western side of the Vale belong to the lowest division of the Dibunophyllum-zone. A conspicuous feature of these limestones is a strong development of finegrained porcellaneous rocks (calcite-mudstones or 'chinastones ') which have ' standard' limestones occasionally interbedded with them. The latter become more frequent as the I

Faunal Horizons in the Carboniferous Limestone of the Vale of Clwyd.

Geol, Soc., zv, (1929), pp. IIi-I33.

Proc, Liverpool

P. G. H. BOSWELL,

succession is traced upwards. The most characteristic fossil is the large brachiopod Daviesiella llangollensis (Dav.), which often occurs to the exclusion of other shells, but is sometimes associated with Productus hemisphcericus J. Sow., Overtonia fimbriata (J. de C. Sow.) and other productids. Corals are scarce. Well-preserved specimens of land-plants, of which Archaosigillaria vanuxemi (Gopp.) is the most abundant form, occur in the lower beds of the zone at some places, a famous locality being the Graig quarry at Denbigh. It is inferred that deposition must have taken place near to a shore. Higher in the succession the limestones become white and purer, and new forms of fossils take the place of those found in the lower beds. Corals become more abundant, some typical species being Lithostrotion martini E. & H., L. irregulare Phill. and L. [unceum (Flem.). Also Dibunophyllum bourtonense Garwood and Goodyear, and Paleosmilia inurchisoni E. & R.> which are typical of DI in the South-Western Province, occur Such frequently at Coed-talwrn and Craig-adwy-wynt. brachiopods as Athyris expansa (Phill.), Productus d. hemispha:ricus J. Sow., Overtonia fimbriata (J. de C. Sow.) are often abundant, while gastropods are locally conspicuous. The faunal change which accompanies the increase in purity of the limestones appears to have depended on physical conditions, the white beds having been deposited in a more open sea than the lower strata. Dz. Subzone of Lonsdaleia floriformis. The index fossil has not been found in the Vale below the "Upper Grey" limestones of Plas Einion (south-south-east of Ruthin), which form an outcrop extending for about a mile southwards. These are grey limestones with numerous chert nodules. Sirriilar limestones crop out in the faulted area nearer the head of the Vale, south of Ty'n-Llanfair. The chief faunal change comprises the incoming of the compound corals Lonsdaleia floriformi« (Martin), Palaosmilia regia (Phill.) and Lithostrotion maccoyanum (E. & R.), with large productids, which, however, are not the typical P. giganteus (Mart.) of higher horizons. D3. Subzone of Lonsdaleia duplicata. The highest limestones. in the south of the Vale contain a rich and varied fauna which is especially marked by the abundance of corals. The prolific fossil-localities of Faenol, Ty'n-Llanfair and Melin-y-coed are worthy of special mention. Many of the fossils are distinctive of high horizons in the Carboniferous Limestone of the north of England, which are now referred to the subzone D3. The lowest members of this subdivision are exposed on Craig-feehan, a hill near the south-eastern extremity of the Vale. Some dark grey limestones at this locality, crowded with the

GEOLOGY OF NORTH-WESTERN DENBIGHSHIRE.

r65

foraminifer Saccammina carteriBrady, r recall the famous" Spotted Post" of the north of England. The beds also contain Productus giganteus (Mart.), and Lithostrotion irregulars (Phil1.). A conspicuous band of shale, one foot in thickness, divides the Saccammina limestone into two beds, and contains the following fossils :-Rhipidomella michelini (Lev.), Composita d. ambigua {J. de C. Sow.). Productus spp. and GrifJithides sp. Higher in the succession, coral-beds are present in reef-like masses, the coral association being similar to that which occurs in the faulted blocks of Faenol and Ty'n-Llanfair. The dendroid Lonsdaleia duplicata (Martin) is especially characteristic, and a small form of L. floriformis referred to the variety floriformis is also present. Species of Lithostrotion are very abundant, and occur in large-sized masses; they include L. irregulare (Phill.), L. junceum (Flem.) and L. portlocki E. & H. Another distinctive coral is Orionastrcea Phillipsi (M'Coy), which has long been recorded from the well-known outlier of Carboniferous Limestone near Corwen, but which is found also at Faenol, Ty'nLlanfair and Melin-y-coed. A remanie-bed at the top of the Faenol quarry has yielded numerous simple corals, mostly in a rolled and water-worn condition, together with broken pieces of compound forms. The most distinctive of the simple forms belong to the species-group of Dibunopbyllus« muirheadi Thoms. & Nich. In the cherty limestones at the northern end of Craig-fechan A ulophyllum fungites mut. cumbriense has been found, and large zaphrentids have been observed in contiguous beds. The brachiopods, which form a distinctive association, include the typical Productus giganteus (Martin), examples of P. latissimus J. Sow. in abundance, Composita ambigua (J. de C. Sow.) and spiriferids. Thus it is evident that these higher limestones with their intercalated sandstones, show the closest relations, in lithological features and faunal characters, with the Yoredale facies of the north of England. They appear to be represented in the Prestatyn area by knoll limestones which contain Goniatites crenistria and some of the corals mentioned above. COAL MEASURES (?). A triangular outcrop of micaceous sandstones and shales referred to the Coal Measures (but possibly including the Millstone Grit), lies in the Vale of Clwyd between Denbigh and St. Asaph. A smaller outcrop occurs about a mile and a half N. 80° W. from Rhewl, between Denbigh and Ruthin. Exposures in these outcrops are not numerous, for there is a considerable covering of Drift. The beds appear to lie conformably on the Carboniferous Limestone and to be unconformably covered by the Triassic deposits. The sandstones and shales are yellowish and grey, except where they have r Now referred to Saccamminopsis.

166

P. G. H.

BOSWELL,

taken a purplish or reddish stain from the overlying Trias. No fossils have been found in them. The zoning of the limestones has increased the uncertainty concerning the age of these beds, for they may include representatives of the uppermost Carboniferous Limestone as well as the higher beds to which they are usually assigned. TRIAS. Cropping out on the borders of the Vale of Clwyd and doubtless occupying most of the floor of that depression, although largely covered by Glacial Drift, are soft sandstones of vivid red tint which have been referred, on account of their lithological characters and mineral composition, to the Lower Bunter division of the Trias. These sandstones are frequently current-bedded, a spectacular section of this type occurring in roadside cuttings on the eastern outskirts of Ruthin, 700 yards east-south-east of the market-place. The quartz-grains of the sandstones are often well-rounded and felspar is not uncommon. T. Mellard Reade described in detail" the occurrences of these sandstones, and Mr. 1. S. Double investigated their mineralogy.> Mr. Double found that the petrological similarity between the sandstones of the Vale of Clwyd and those of other areas round the Irish Sea Basin, namely, the Liverpool district, the Carlisle district, Arran and Antrim, was well-marked and suggested that the outcrops belonged to one Triassic basin of deposition. PLEISTOCENE. The Pleistocene and Post-Pleistocene history of north-western Denbighshire has not yet been investigated in sufficient detail to enable a connected story to be told. And the same is true of the development of its puzzling river-systems. Comparatively little has been published on the former subject and nothing on the latter. In the course of work on the stratigraphy of the older rocks, notes have been made regarding glacial or later deposits; with a few exceptions the glacial phenomena can hardly be regarded as of striking character. The Glacial Drift is widespread, and over most of the area is composed of local rocks. It then consists of grey sandy or rubbly boulder' clay' (true clay being rare), or of sands and gravels containing the common Wenlock and Ludlow rocks of the district. It is encountered at all levels up to the highest elevations of the moorland, but becomes thicker and more persistent on the lower slopes, where for considerable distances the solid rocks are invisible. Banked up against the Carboniferous escarpment which bounds most of the area, it occupies the depression in front of it, where the low country has been the I The Trias of the Valley of Clwyd,

Proc, Liverpool Geol, Soc., vi. (1891) pp. 278-289.

2 The Petrography of the Triassic Rocks of the Vale of Clwyd, Ibid., xlv. (r926), pp, 249-262.

GEOLOGY OF NORTH-WESTERX DEXBIGHSHIRE.

167

dumping ground of piles of the larger boulders,chiefly sandstones from the scanicus-zone and Denbighshire Grit Series. Near the Carboniferous escarpment the Drift sometimes changes in colour to purple or purplish-red, as a result of incorporation within it of red Basement-bed material, blocks of red sandstone being abundant. Following his investigations of the area lying south of the escarpment at Llandulas and Abergele, Mr. Montag has shown that the gorge of the river Dulas from Rhyd-y-foel to Llandulas was blocked by Irish Sea ice, which left Northern Drift in the gap. The Dulas ice-flow was thus diverted from its northerly direction and, being prevented from turning westwards by Fernant Hill, took an easterly course through the low ground towards Pen-y-coed at the southern end of the Abergele gap, where it deposited boulder-clay with a mass of large erratics.' Apart from the rubbly drift itself, large erratics are scattered over the area, sometimes singly and at other times in groups, at levels varying from 1,000 feet to 350 feet, but mostly at 500 to 400 feet above O.D. These accumulations seem to mark pauses in the retreat of the ice-sheet, and in this connection it should be noted that good morainic mounds occur at 1,000 feet, 700, 480, 420 and 300 feet, notably at three localities, (a) Pwllglas. near Ruthin (300 feet)," (b) north-west of Denbigh (420 feet) and (c) Llangerniew (480 feet). Other phenomena of the retreat stages, such as flat-bottomed channels, now dry or nearly so, and fluvio-glacial terraces and fans, are occasionally seen. Strise and polished surfaces are not visible except on the Carboniferous Limestone, possibly because the lithological character of the Silurian rocks did not lend itself to the preservation of such phenomena. In the eastern part of the area the erratics are almost entirely of local origin, but in the western part, boulders of fluidal rhyolite, nodular rhyolite and acid tuff, sometimes up to 10 feet in diameter, are met with. These are clearly , foreign' and are derived from the Ordovician volcanic rocks. The ice which covered the area must therefore have come (at least in part) from the Snowdonian region or from the country south of the Cerrig-y-druidion road. The latter seems the more probable, but whatever region was the source, it is certain that the ice was fairly clean when it invaded the Denbighshire Moors. Erratics from the Arenig Mountains, consisting of andesites, felsites and tuffs, are found in the Vale of Clwyd and in the extreme south-eastern part of the moors. r E. Montag. Erratics from the Boulder Clay of Pen-y-coed, Abergele, Proc, Liverpool Geol, Soc., xv, ('929), pp, 144-5. 2 Kendall, P. F., and J. Lomas. On a Terminal Moraine at Pwll-glas near Ruthin, G/acialists' Magazine, iii. (1896), pp. 171-1~5.

168

P . G. H . BOS WELL,

The North Welsh (or local) Dr ift extends down almost t o sea-level into th e Vale of Clwyd and to the Irish Sea coast. But, as is well known , a red or purplish-red bould er-day of northern origin is found on th e coast and for some miles southwards up the Va le from the mouth of the River Clwyd. This boulder-d ay is t erm ed th e Nort hern Drift because of the evidence of its abundant erratics derived from Scotla nd and the Lak e District. Th e red colour is due to the gro und-up mat erial of th e Bunter Sandstone , picked up in all proba bility from the floor of the Irish Sea. This red drift contains marine shells in a fragmentary conditio n. The rela tionship of th e red Northern Drift t o th e grey Local Dr ift has no t been visible in the area for some yea rs, but sectio ns formerly exposed at Colwyn Bay, St. Asaph and other localit ies showed that th e Nor thern Dr ift always overlay the Welsh Drift. We can feel assured th at the northern ice never ad vanced more than a mile or tw o inland on to the high ground bordering the coast or the Vale of Clwyd, but whether it was stopped by th e accumulation of retreatin g local ice, or whether it s vis a tergo was exhausted, we arc not able to say . Man y of the larger valleys of th e area are pre-glacial. Th ey are wide and open and their slopes and floors ar e covered wit h Dr ift , especially in t heir lower courses . But in man y inst ances the rivers have either left th eir old courses and are excavating beautiful rock-gorges, or have cut through th e drift into solid rock. It is, however , difficult to point to an y consta ncy of level for th e upper limit of reju venation, i.e., the head of a gorge (often marked by waterfalls). As examples of the vari et y of levels not ed, for exam ple, we m ay mention 900 , 1 ,130 , 1 ,1 8 0 , 1 ,2 5 0 and 1, 3 00 feet . The simplest explanation of thi s variation seems to be that the present posit ion of the head of the gorges is determin ed by the area of th e ga thering-ground and th e am ount of wat er availabl e for river-erosion . III. SOME REPRESENTATIVE TRAVERSES. 1. Glan Conway to Ffrith-y-Mynydd, south of Colwyn Bay. The section begin s at Glan Conway , south-west of Colwyn Bay, with a series of isoclinally-Iolded blue and brown mudst ones, often vertical, observable in good exposures in the railw ay cutting and in the side of the m ain road from Llandudno Junction t o Llanrwst. These rocks seem t o belong to the group termed by Miss Elles the Benarth Grits and Flags, but the beds do not appear for the most part to be so low in the succession as those found at Benarth, on th e oppos ite shore of the Conway, where the lowest zone of th e Wenlockian , that of Cyrtograpius mur chisoni, is well ex posed. T he lithology is rather different from th at at Benarth, alt hough at one locality, near the Pensarn

GEOLOGY OF :-lORTH-WESTERN DENBIGH SHIRE.

I69

cross-roads, beds of the Benar th type have yielded C. murchisoni, At Glan Conway , grapt olites are found in moderate abundance in certain bands of the mudstones. The assemblage includes Cy rtograptus symmeiricus, M . prio don, M. dubius and M. vomerinus (?). At ot her . localiti es, situat ed a short distance eas t of the road-sections (e.g., adjoining the Church and oneeighth of a mile north-north-east of it), cleaved brown calcareous mudstones and flags have yielded C. symmetricus (?), M. uomerinus, M. fl emingi and M . priodon (?). Continuing th e t raverse eastwards and ascend ing th e hill past Br yn-Rhys, we find exposures of similar flags which at localities on the strike such as Bryn Ei steddfod, yield M, priodon, M, vomerinus, and C. linnarssoni. In th e isoclina l folds also, beds with C. rigidus, .71,1. priodon and M. fl emingi ar e found (as at Gydar). Thus th e zones of C. symmetricus , C. linnarssoni and C. rigidus are present. The isoclinal folding then ceases, and is either replaced by gentle dips or a fault intervenes and brings in beds with a low dip on the east. The east erly dip results in a series of westward-facing escarpments rising one above the other eastwards of Bryn-Rhys. H ere the succession of zones can be traced without difficult y in the banks of the lane s which lead past t he farm s Pan t-yr-haiarn, Rhyd-Itan-bach and Goetre. Between the first t wo farms named, t he flags contain abundant specimens of 111. fle mingi var. 8 and var. comp actu s, C. lundgreni (?), and M . du bius, suggesti ng the pre sence of the zone of C. lundgreni. The easterly dip then br ings in wellbedd ed flags with M, colonus and .H . dubius, visible in exposures one-e ighth of a mile east-north-east of Rh yd-Ifan-bach , The se clearl y belong to the lower part of the zone of M, nilssoni, but the int ervening zone of .M . vulgaris has not been iden tified with certainty. As th e sect ion is followed eastwards to Goetre , the beds become more gritty and t hinly-bedded, until in r oadside exposur es at that farm, we reach the t ypical sandy facies of the upper part of the zone of M. nilssoni . The zone-fossil it self is plentiful and is accompa nied by M. uarians, M. u ncinatus va r. orbatus , and M . chimera and its va riet y salweyi . Ascending an other escarpment we encounter massive blue and green flags and mudst ones, highly cleaved, and similar in lithology to those of the zone of M. scanicus, but they have yielded no graptolites. T hese flags and mudstones are overlain by strong bluish grits which make the hill of Ffrith-y-Mynydd. The grit s and their interbedded grey cleaved shales resemble closely those of the middle part of the scanicus-zone farther eas t. Continuing along the road, we reach a small quarry situa te d ISO yards south-east of Llwydcoed School, where thinlybedd ed flags with biscuity surfaces have yielded poorly preserved spe cimens of M . dubius and M. scanicus.

17 0

2.

P. G. H. BOSWELL,

From Pentre Foelas to Denbigh, along the main south-west to north-east road. For the first three miles of its length from Pentre Foelas the road ascends north-eastwards through the Denbighshire Grit Series. Strongly-cleaved grey or bluish shales alternate with gritty sandstones, the latter less frequent in the southern (or lower) part of the succession, but becoming dominant as we emerge from the cultivated country on to the open moorland area. Graptolites are rarely found and are poorly preserved, so that the exact stratigraphical position of these beds is uncertain. At and near Bwlch-gwyn, two and a half miles northeast of Pentre Foelas, roadside quarries furnish excellent exposures of the Denbighshire Grit and associated shales. Graptolites arc to be found, but can be identified only as Monograptus. Crinoid ossicles and brachiopods are not uncommon. Other quarries situated at intervals on the roadside as far north-eastwards as Pont-y-Clogwyn, afford good opportunities for the inspection of the horny, yellow and grey-banded or blue hackly mudstones, which are the characteristic rock-types that succeed the Denbighshire Grit (or are sometimes interbedded with it) in this part of our district. They have yielded only poorly-preserved specimens of Monograptus, but apparently their zonal position approximates to that of Cyrtograptus rigidus or C. linnarssoni. The gentle north-easterly dip continues and the beds pass upwards into well-stratified flags of the Nantglyn type, but usually rather more thinly-bedded. They are often ribbonbanded, and their more muddy nature is indicated by their weathering product-a Naples-yellow clay. No satisfactory exposure is seen on the road, but a useful quarry has been opened on the strike, about a mile and a quarter N. 60° \V. from Pont-y-Clogwyn, and a third of a mile west-north-west of the mouth of Llyn Aled. Here the flags yield abundant specimens of M. jlemingi var. 3. Although Cyrtograptus has not yet been found, there is little doubt, from the lithology and the character of the specimens of AI. jlemingi, that the flags lie in the zone of C. lundgreni. (A similar and confirmatory section, in which C. lundgreni has been found, occurs near Clawdd-newydd, north-west of Derwen, referred to on p. 174.) Returning to the main road, we find a quarry a short distance from the road and 700 yards S. 60° W. of the Sportsman's Arms, in which flags with M. vulgaris make their appearance. Thus we have here passed from the top of the Wenlockian into the base of the Ludlovian. The quarries near the Sportsman's Arms reveal beds a little higher in the succession, for they are excavated in Nantglyn Flags which contain abundant graptolites, such as M. colonus and M. dubius (long variety), characteristic of the

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E. N.E . Gorsed d Brim

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Llanrh ai adr .

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Flags (tllm~st~n3

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FI G . ~ O .-S E CTI OK FROM THE MIDDL E OF THE . D ENDI G Il :\ IoO RS T O T il E V A L E OF C LWYD .

(Reproduced by permission

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the Council

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the Geological Society.)

"17 2

P . G. H . BOSWELL,

nilssoni-zone; th e occasional occurrence of 1\1 . vulgaris, however. indicates t hat the beds lie in the lower part of the zone. At this point we ha ve reached the highest part of the Denbighshir e Moors, where the strata no longer maintain their north-eastward dip, but are gently rollin g. Within a short distance (a little m ore than a mile and a half east -north-east of the Sportsm an 's Arms) are the fam ous Nan tglyn qu arries, the type-locality for the Nantglyn Flags (PI. 9, A.). Here the slabs have been worked for many centur ies, and indeed, for a long period by th e fam ily of th e pr esent qu arry-m aster . Mr. ]. Howell Evans. They are quarried and cut for use as tombstones, st eps, sills and tank s, and for building purposes generally , ad vantage being taken of th e excellen t joint ing and the bedding, which enables them to be split into sla bs about two inches in thickness . These ofte n show " graveyards " of M. bohemicus (large form) or of M, COI01t1IS and M. dubiu s, and less commonly, specimens o f }'1. nilssoni and beautiful examples of Periechocrinus [ActinocrinusJ pulcher, of dr ooping daffodil-like form. Returning to the high road and continuing the traverse north-eastwards, we cross a number of fault-blocks comp osed of grits and cleaved flags belonging to the zone of M. scanicus (see Fig. 20). Other sections in the scanicus -beds being mor e prolific as regards grapt olites, reference t o stratigraphical deta ils is deferred (see Traverse 3 below). At Groes, three miles from Denbigh, on th e north-east ern side of the outcrops of the zone of J[. scanicus, there is a good roadside section of flags which bea r a considerable resemblance to the Nantglyn Fl ags, bu t a re perh aps slightly more siliceous. Their graptolitic faun a , which is abunda nt, includes M . colonus, 1\1 . dub ius, 1\1 . orbatus and M . uarians , and indicates that they ar e still in the lower p art of th e zone of 1\1. nilssoni. , The lithology illustrat es the fact , already mentioned, t hat the nilssoni-beds become more siliceous as they pass north-eastwards from the centre of the Mcorland area. Other roadside sect ions nearer Denbigh (e.g., east of Groes-bac h and east of Gwayn ynog-b ach) reveal the upper beds of th e nilssoni-be ds, which here consist of sandy striped mudstones and flags, often thinly-bedded, and containing chiefly M. uarians and M. dubiu s. 3.

Abergele to Llansannan via Llanfair Talhaiarn. This traverse is select ed as the most suitable for a det ailed examination of the lithology of the zone of M. scanicus: it follows approximately the main road running from north to so uth. After passing through the gap in the Carboniferous Lim est one, with Tower Hill on the east, we reach the unconformable junction of the Carboniferous Basement-beds and Siluri an mudstones, about one mile sout h of Abergele. The red sandstones of the Basement- beds are seen in the bed of t he

GEOLOGY OF NORTH- W ES TE RN DENBlGHSHIRE .

173

River Gele, west of Plas Uchaf, dipping northwards off bluish mudstones believed (from their lithological characters) to belong to the zone of M , scanicus . Actually the nearest fossiliferous localities lie farther south-east , but even there they occur but a short distance from the south-east erly trending outcrop of the Basement-beds. At these localities the following gr aptolites, charact eristi c of the zone of 111. scanicus , have been found :-M. scanicus , M. chimaira, M. rtemeri, M , dubius and M . varians and its variet y pumilis. Better opportunitie s for observing the rock-succession a nd lithological t yp es are afforded on the slopes and summit of the hill Mynydd Bodrochwyn, two miles farther south. In the quarries sit uated 200 yards ea st and 800 yardsN . 75° E. of the intersection of the cross-roads at Wern-fach, the typical assemblage of graptolites of the scanicuszone can be collected from the biscuity bands which occur in gritty bluish mudstones and well-bedded flags. These beds, which have a general southern dip, are seen to pass upwards into sandstones, the individual beds of which are gritty and reach a thickness of 10 feet. They crop out frequently as folds on the summit of the hill, the anticlines and synclines being shifted by numerous faults of north-north-westerly trend. A similar but probably more compl ete section of the beds of this zone m ay be ex amined still farther south, at the sides of the new road from Llanfair Talh aiarn to Llansannan. Near Plas-Bela, half a mile east-south-east of the former village, massive cleaved blu ish mudstones with brachiopods are well exposed , and farthe r south, close to Pentre-du, the biscuity graptolite flags have been quarried. The rolling of the beds brings in the grits which usually underli e th e massive mudstones near Bron-yr-haul, the biscuity flags appearing again from under them st ill farther south. The roadside sections in these flags half a mil e north of Llan sannan prov ide rich collecting-grounds for the graptolites of the zon e. 4.

From Ruthin to Braieh, north of Derwen. A trav erse in this area affords the best opportunities for studying the upper portion of the Ludlovian succession. Northwest of Ruthin, in th e charm ing Lad y Bagot's Drive (the valley of the Clywedog, a tributary of the River Clwyd ), and west of th e town, from Bont Uchel t o Gyffylliog, are good sect ions in fossiliferous mudstones and flags of the nilssoni-zone, the dip being generally to the west or south-west. These beds extend for many miles south-westwards, but on the sout h-eas tern side, down-faulted against them , are areas consisting of outcrops of th e zon es of "AI. scanicus, J/ . tumescens and JI.leintwardinensis. The road to the villa ge of Clocaenog passes,about two mile s from Ruthin, the northern p art of Pool Park, where there are numerou s faulted outc rops of flags, siliceous mudst ones and

:£74

P . G. H. BOSWELL,

gritty beds from which M. iumescens and its variety minor, with occasional specimens of M .leintwardinensis, can be collect ed. A typical exposure of flags, not differing appreciably in appearance from the more siliceous types of Nantglyn Flags, is seen in the roadside quarry at th e Lodge gates, half a mile N. ro " W. from the house in Pool Park. . Other fossiliferous localities in the siliceous beds, which appear to belong to the upper part of the zone, occur near -by in the Park at distances of 400 ya rds N . 80° E . and 220 yards S. 70° E . from the Lodge gates. Proceeding south-westwards, we cross a gently-rolling serie s of mudstones and tough siliceous flags folded on east-west axes and belonging to the zone of M . leintwardinensis, which occupi es mu ch of the area of the Pool Park Estate. One of the most prolific localities for graptolites is the roadside quarry, about 500 yards N. 60° E. of Clocaenog, where M. lemtwardinensis and its variety incipiens may be collected, but there are many other fossiliferous exposures in the neighbourhood. A fault of north-north-easterly trend, running through Clocaenog, brings up the zone of M: scanicus on the west. In this district the latter zone consists of thinly-bedded biscuity flags and ribbonbanded mudstones and flags of Nantglyn type. A quarry in the latter, a quarter of a mile north of the church, yields a rich fauna, including M. scanicus, M. varians, M. chimaira and its varieties salweyi and semispinosus, and M. dubiu s. Th e biscuity flags are best exposed in the lane and stream west of the village; indeed, the secti on in the lane, where it passes north of the farm named Fron, affords one of the best opp ortunities for tracing the passage upwards from the scanicus-zou e to the nilssoni-zone, the zone-fossils and their usual associated s pecies being plentiful. The next point of interest is rather more than a mile farther southwards, where the road from Clocaenog joins that from Clawdd-new ydd, In this neighbourhood, close to Pen-y-rnynydd and Foel-fach , are numerous quarries in typi cal Nantglyn Flags containing abunda nt graptolites characteristic of the lower part of the zone of M, nilssoni ; farther westwards ill . vulgaris has been found. Continuing westwards along th e road, against the dip, we reach the upper beds of the Wenl ockian in roadside expo sures, north of Braich, where Cy rtograptus lundgreni and large spe cimens of M. jlemingi var. ~ occur in flags of the same lithological type. Still farther westwards, near Pont Petryal, the Denbighshire Grit escarpment is crossed by the road, the Grits being br ought into contact with the zone of C. lundgreni by the east-west tear-fault through Braich.

5.

The Conway Falls to Nebo via Capel Garmon. Beginning in the valley at Conway Falls Cafe (at 384 feet a bove O.D.) and reaching to over 1,000 feet above O.D . in the country of the Denbighshire Grits at Nebo, this traverse affords

GEOLOGY OF NORTH-WESTERN DENBIGHSHIRE.

175

a good opportunity for the examination of the Upper Ordovician and Lower Silurian (Valentian) rocks of the district. The wild and rugged pastoral scenery of the Ordovician country presents a striking contrast to the smooth and undulating cultivated area of Silurian rocks. The steep hillsides which rise north-eastwards from the road are composed largely of resistant volcanic rocks. A convenient roadside quarry close to the Falls of the Conway permits an examination of the same fluidal rhyolite as that over which the Falls themselves discharge. This mass of rhyolite forms the picturesque hill of Dinas Mawr, 800 feet in height, which overlooks the valley and affords fine views of Snowdonia. The eastern and north-eastern slopes of the hill are covered by a skin of nodular rhyolite, here very well displayed, which forms the upper part of the flow. A thin band of acid tuff, partly glassy and partly lithic, succeeds the rhyolite. The traverse is then most usefully diverted eastwards, towards the farms named Pen-rhyddion, where we cross outcrops of barren grey highly-cleaved slates. At the farms are exposed volcanic tuffs of vitric and vitric-crystal type, belonging to an outcrop which runs north-eastwards, the dip being to the south-west. Succeeding the tuffs is a series of interbedded gritty slates and sandstones, here frequently containing an abundant shelly fauna referable to the Bala. This gritty series is succeeded by wellcleaved blue-black and then lustrous black slates (although perhaps not directly, for there may be intervening strike-faults), exposures of which are found 150 yards S. 35° E., and about the same distance due east, of Pen-rhyddion-bellaf. The black shales bear a close lithological resemblance to the black Dicranograptus slates of Dolwyddelan. From the blue-black slates, graptolites referable to the following species were collected by Dr. W. F. Whittard and a field-class of students of the Imperial College: Diplograptus multidens var. compactus, Climacograptus scharenbergi, C. antiquus, Glyptograptus euglyphus and Orthograptus intermedius. These graptolites indicate that the slates belong to the lower part of the Caradocian. Continuing the traverse still farther eastwards we encounter the highest Ordovician beds of this area, a series of more massive bluish mudstones and grey slates, containing a shelly fauna which has not yet been investigated. The nodular and fluidal rhyolites and tuffs appear, from the evidence of the overlying fossiliferous sandstones and slates, to be approximately on the horizon of the Lower Rhyolitic Series of Snowdon, the nearest section described in detail with which we can attempt correlation being that in the Dolwyddelan syncline. I The Pen-rhyddian tuffs would then be correlatable with the Upper Rhyolitic Tuffs. But when the intervening I Williams, H., andO.M. B. Bulman. The Geology of the DolwyddeIan Syncline (North Wales). fJ-rt. ]uu.""G«JI. Soc., lxxxvii. (1931), .25-of511.

}'. G. H. BOSWELL,

ground is mapped, these tentative correlations may require modification. If the traverse is resumed by way of the lane from the Pen-rhyddion farms to the Capel Garmon road, we cross the first member of the Capel Garmon fault-system, of northwesterly trend. Another of the faults lies north-east of the road. On the north-eastern side of these faults are the Valentian rocks. The traverse is most advantageously continued, however, from a point farther north-westwards on the road, namely, at Capel Garmon itself. Taking either the lane which runs north-eastwards by Bryn-Han or the path in the same direction past the school to Ffynnon Garmon, we pass over a succession of mudstones and grits with a strike varying but little from east and west. The mudstones are blue and massive and difficult to break on their bedding planes: they have yielded graptolites, but the forms are rather too longranged to be of use zonally. If no faults intervene (Which is unlikely, for the area has every appearance of having been badly shattered) the mudstones would attain a thickness of 2,000 feet. In their upper part they contain siliceous bands; grits also make their appearance (e.g., at Ffynnon Garmon). Still higher in the succession the gritty bands are interbedded with pale-grey thin mudstone (" paste-rock ") and black-striped shales. In places the shales and mudstones are richly fossiliferous (e.g., at Pen-y-ffridd and Bryn-rhyg), the following assemblage of graptolites being present :-Climacograptus scalaris, Glypto-

graptus tamariscus, Monograptus concinnus, M. denticulatus, M. d. involutus, M. limatulus, M. lobiferus and Rastrites d. peregrinus.' By taking the lane which runs eastwards south of Bryn-Rhyg to Bryn-gwnog, we pass other roadside exposures in blue mudstones which have yielded a similar assemblage. There is little doubt but that we have here the upper part of the Lower Valentian and possibly the lowermost part of the Upper Valentian. The lane, continued eastwards, reaches Nebo. Here another of the great shatter-belts trending north-west to south-east is plainly in evidence. On the north-eastern side of this Nebo fault-system the Denbighshire Grits rise in picturesque bold scarps, with a general dip to the north-north-east. The grits can be examined in numerous exposures. They are felspathic and often coarse and pebbly. Pellets of shale frequently occur. Shelly fossils and graptolites occur in the sandstones, but the graptolites are poorly preserved. IV. TECTONICS. The areal distribution of the zonal outcrops of the Silurian 1

For the identification of these graptolites I am indebted to Dr. W. F. Whittard.

GEOLOGY OF NORTH-WESTERN DENBIGHSHIRE.

177

rocks gives a preliminary idea of the general structure of the district. The oldest rocks (the Ordovician and the Valentian) are exposed on the south-west, and the newest (the top of the Lower Ludlovian) appear on the east and south-east. The next oldest rocks, the Wenlockian, form a fringe along the whole of the western and southern flanks of the region. When we have resolved the complications due to faulting and minor folding, we find that the significant structure is a broad syncline pitching towards the east-north-east from the south-western margin of the area at Capel Garmon. If the structure were regular and symmetrical, the zone of M. leintwardinensis (at the top of the Lower Ludlovian) would be present in the Denbigh district. As a result of varying pitch and faulting, however, the next older zones, those of M. tumescens and M. scanicus, are there exposed, the latter as a broad spread and the former as small down-faulted outcrops. Although not exposed at Denbigh, the zone of M. leintwardinensis occupies a considerable area in the country farther south-south-eastwards, where it is brought in .by the pitch of a subsidiary syncline running parallel to, and south of, the main syncline, and north of the Derwen anticline (see Fig. 21). In front of the Carboniferous escarpment, which forms the eastern and north-eastern limit of the area, a variation of pitch of the main syncline (resulting possibly from irregularity of the pre-Silurian floor), or less probably the effect of contemporaneous cross-folding, brings to the surface the zone of M. nilssoni (see Fig. 20). This upward roll may be connected with the former anticlinal character of the now down-faulted rocks of the Vale of Clwyd (see Fig. 21). The effects of block-faulting and possibly of torsional movement have torn out the outcrops of the zone of M. scanicus and distributed them beyond the centre of the syncline, so that they sweep south-eastwards to Ruthin on the one hand, and north-westwards to Abergele on the other. As already noted, the scanicus-beds are the most arenaceous of the Lower Ludlow rocks, and give rise to grit-ribbed hills, frequently exceeding 1,000 feet in height. From this ridge of rounded hills, staggered by faults, the country slopes downwards both to the north-east and south-west (or more precisely, to the north, north-east and east, and to the south, south-west and west). Thus the central part of the moorland area is composed of the Lower Ludlovian zones of M. nilssoni and M. vulgaris, constituting the Nantglyn Flags, and rising from under the hills of scanicus-beds. It is this middle region which, with the Wenlockian, forms the Denbighshire Moors proper; it is largely covered with heather and peat-bogs, and given over to grouse. Rock-exposures are relatively few; thus the tracing of structural lines into it from the surrounding country is difficult. Rising PROC. GEOL. Assoc., VOL. XLVI., PART 2, 1935.

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GEOLOGY OF NORTH-WESTERN DENBIGHSHIRE.

I79

in dome-like form, it reaches a culminating height of I,700 feet near the Sportsman's Arms and Shooting-box, half-way between Denbigh and Pentre Foelas on the main road. The dome-like area is encircled (that is, for the part of the circle now remaining between the Irish Sea and the Vale of Clwyd) in front of the strong escarpments of the main Denbighshire Grit, by a slight depression occupied by the softer shales and mudstones of the Upper Wenlock. The Denbighshire Grit forms a southern and western rampart to the moorland area, rising to hills of I,70Q-I,800 feet overlooking the Conway Valley and Cerrig-ydruidion road, but breached near Llanrwst, where the main road communications across the Moors has been facilitated by the gap. The descending slopes from the Denbighshire Grits to the low ground on the west and south are in part occupied by Valentian rocks, reduced in thickness or even eliminated at times by strike-faulting. The Upper Ordovician slates, mudstones and volcanic rocks form a natural boundary, usually faulted, to the Silurian rocks on the south and west. FOLDING. Although the axis of the main syncline now runs from west-south-west to east-north-east, the folding of individual beds, when it can be traced, is dominantly on east-west lines (except in the west of the area). Examples of this minor folding are seen at Bettws Abergele, Mynydd Bodrochwyn, Mynydd Tryfan, Moel Tywysog, Foel Feehan, etc. Also, the cleavage (to be referred to in detail later) has a strike varying a few degrees north or south of an east-west direction. Both folding and cleavage are clearly manifestations of post-Silurian and pre-Carboniferous, i.e., Caledonian, movement. The development of a general east-north-east trend for the main syncline from a series of beds buckled on east-west lines has been effected by movement on a number of master-faults of north and south trend or north-west and south-east trend, which have shifted successive fault-blocks more and more to the north as we go farther and farther east. FAULTING. A glance at the published maps gives some idea of the degree and complexity of the faulting. There, are, of course, far more faults than have been shown; indeed, they are probably far more numerous than the geological surveyor would be justified in indicating as a result of mapping rocks so similar in lithology throughout the succession. The chief faults are seen to fall into a number of well-defined groups. Beginning in the south-east, in the country south-west of Ruthin, the dominant trend is north-easterly; the" grain" and, consequently, the valleys and roads have their trend determined by the fracturelines. The close-spacing and relative parallelism of these faults

180

P. G. H. BOSWELL,

might almost strain credulity, but an inspection of the ground is sufficient to remove any doubt. The subsidiary faulting has mainly an east-north-east trend, which is also the direction of the. folding here. Thus the country is broken up into a series of rhomboid areas. The movement on the major faults seems to have been, in part if not dominantly, of tear-character, with a translation towards the north-east on the east of the fault. These faults abut on the south against the east-west Braich fault, which forms part of the Llanelidan fault-system, closing the southern end of the Vale of Clwyd. The Braich fault has been proved to be also a tear-fault. On the north, the north-easterly fault-system and the northsouth system (which is a feature of the eastern part of the moors from Ruthin to north of Denbigh) break into one another at angles of about 30°. In this district, not only is the northsouth trend shown by the master-faults which dislocate the Silurian rocks, but also by those which cut the Carboniferous and Triassic rocks and form the western margin of the Vale of Clwyd. A subsidiary fault-system of east-west, or east-southeast to west-north-west trend, divides the area into crustal blocks. As a rule the east-west faults do not cut through the master-faults; the two series may have been simultaneously developed, or the latter may be the later set. Passing from the Denbigh area north-westwards towards Colwyn Bay, we note that the northern part of the region is traversed by master-faults which swing slightly from the northsouth direction or even take a north-north-west trend. Once more, subsidiary east-west faults breakup the country into well-defined blocks which form topographical features, and again faults of the master-trend cut the Carboniferous Limestone and Basement-beds which form the coastal belt. Some of these are known, on field-evidence, to be tear-faults just as those of similar direction, cutting the Ordovician rocks of Conway, have been proved by Miss Elles' to have a lateral component of shift. Once again, the dominant fault-direction changes. The western belt of the region, bordering the Conway Valley, is characterized by the north-westerly trend of the master-faults. As a detailed map of this area has not yet been published, we may mention as examples the Mochdre, Pentre-felin, Eglwysfach, Nebo and Capel Garmon faults. The grain of the country and the run of the roads here are determined in part by this trend and in part by the north-south strike of the rocks. The subsidiary faults have yet to be worked out in detail, but many are of north-south trend, thus dividing the country into lozenger The Relation of the Ordovician and Silurian rocks: of Conway (North Wales). ]0t4In. Geol. Soc., Ixv. (r909), p, 173.

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shaped areas in a manner similar to those in the south-eastern area between Ruthin and Derwen. CLEAVAGE. All the Silurian rocks of the area have had a cleavage imposed on them, but its intensity varies considerably in different lithological types. The Nantglyn Flags (using the term in a lithological sense to cover part of the Upper Wenlock and Lower Ludlow) of the dome-like part of the moors are cleaved least of all; indeed, in quarry exposures where the rocks have been deeply excavated, the cleavage is often difficult to detect. Weathering, however, soon reveals it, and then it is more effective than bedding and jointing in determining the direction of rock-fracture. The cleavage is most intense in the shales interbedded or associated with the sandstones and grits, as, for example, in the Denbighshire Grit Series or the scanicusbeds. Repeated attempts have evidently been made in many districts to work these cleaved shales for roofing slates, but the results seem to have proved unsatisfactory (except in the old days, when the cleaved flags were used for roofing farm buildings). Apparently the rocks are too sandy or have been insufficiently compressed and mineralogically rearranged to yield thin slates like those of the Ordovician or Cambrian, but the cleavage is sufficient · to defeat any attempts to find graptolites on the bedding-planes. The strike of the cleavage-planes displays a remarkable constancy of direction over the whole of the area. Generally speaking, it is east-west in trend, but fluctuates frequently up to ID o or sometimes 20° north or south of this direction. In proximity to certain master-faults, such as the Conway Fault, which are probably of Hercynian or even later origin , the cleavage may be swung round to any direction, even north and south. Such cases are rare. While the strike of the cleavage-planes remains relatively constant, their dip varies regularly, being southwards in the northern part of the area and northwards in the southern part. There is thus a central belt-the middle of the cleavage syncline-where the cleavage is vertical or the dip oscillates rapidly north or south at high angles (usually 80° or more). Away from the middle of this cleavage syncline-or" cleavagefan," as it is commonly called-the amount of dip of the cleavage is progressively reduced with increasing distance. The significance or explanation of this cleavage-fan is not at present clear, but mention should be made of the fact that a similar fan, or possibly two fans, can be distinguished in the Ordovician slates of Snowdonia, farther to the west. An additional point of interest is brought out by a study of the position of the middle of the cleavage-fan. The belt of vertical or oscillating cleavage, which is usually about a

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mile in breadth from north to south, varies in latitude in different parts of the Denbighshire Moors. These differences in location are clearly related to the master-faulting of northsouth or north-north-west to south-south-east trend. The centre of the fan has been moved farther and farther northwards as we travel eastwards from the Conway Valley. Now the translation of a vertical belt of rock by a fault is clear evidence that movement on the fault is, in part at least, lateral. Thus the master-faults which shift the cleavage-fan have the character of tear-faults. The rocks farthest west have, speaking figuratively, been " glued on" to the Ordovician, while those towards the east have found relief by moving northwards into the area now occupied by the Irish Sea. This movement appears to have been of Hercynian date and connected with the torsional effects seen in north-eastern Wales. IMBRICATION. A relatively small area of rocks of unusual character, about four square miles in extent, is found in the western part of the region to the north-west of the centre of the moors. It lies between two of the master-faults of roughlynorth-south trend, the LIangerniew fault-system on the east and the Wenlli fault-system on the west. Here a series of mudstones and flags have been piled up irregularly in imbrications, yielding a characteristic "schuppen" topography (PI. ro, A.), not found elsewhere in North Wales. The reader is referred for details to a previously-published description of the peculiar features of the area (see list of references). To state the case briefly, the uppermost beds of this part of the moors, probably belonging to the zone of M. nilssoni (but possibly including also mudstones and flags, although not grits, belonging to the scanicus-zone) have been broken up and piled upon one another. as wedges to a thickness of about 500 feet. Some of the streams of the district have excavated their valleys through the imbricated mass and have revealed below it the normal graptolite-bearing flags of the nilssoni-zone. In a few sections the contact of disturbed and undisturbed beds can be seen, one of the most easily accessible being the roadside quarry near Ty'n-y-ffordd, a mile and a half S. 10° W. from LIangerniew (PI. 10, B.). Here well-preserved and readily identifiable graptolites (M. nilssoni, M. colonus, M. bohemicus and M. dubius) can be collected within an inch or two of the plane of dislocation, although the beds containing them exhibit drag-phenomena in their joints and oblique shear-planes. The disturbed beds above display a variety of structures of which upward-riding listric surfaces and" balling" into masses, one foot to several feet in diameter, are typical. Often, however, these beds are sheared into a medley of lenticles and fragments. Graptolites have been destroyed, but fragments of brachiopods and lamellibranchs are occasionally to be detected.

PROC. GnOL. Assoc., VOL. XLV I. (1935).

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[To face p. 184'

GEOLOGY OF NORTH-WESTERN DENBIGHSHIRE.

185

The area occupied by these confused rocks is indicated on the sketch-map (Fig. 22), and sections across the area drawn to scale (Fig. 23) illustrate the relationships of the beds. The cause of the development of this curious structural unit is not clear. That its origin is tectonic is beyond doubt, for neither land-slipping nor glacial disturbance can be invoked to explain the occurrence of a cake of composite rock-material 500 feet in thickness forming the highest ground for miles around. It does not lie in the middle of the syncline and is not composed of beds uppermost in the stratigraphical succession; therefore, the suggestion that the middle of the syncline has given way under compression from the limbs on the north-west and southeast, although attractive, is not entirely acceptable. But the situation of the area of disturbed rocks in that part of the moors where the district of east-west strike (from Denbigh to Llanfair Talhaiarn, Llansannan and Llangerniew) abuts against the belt of rocks of north-south strike (bordering the Conway Valley from Colwyn Bay to Llanrwst) appears to be significant. We know, from the work of Mr. C. B. Wedd, Dr. Bernard Smith and Professor L. J. Wills, that north-eastern Wales was involved in powerful torsional movements, mainly post-Carboniferous in age. Such spiral movements appear to have effected the region of the Denbighshire Moors, as shown by the tectonics of the Ruthin area, the movements on the Llanelidan fault-system, and the shifting of the centre of the cleavage-fan of the moors already referred to. The most reasonable explanation of the imbricated area seems, in the present state of our knowledge, to be that the rocks found relief from the clockwise spiral movements by riding northwards towards the region now occupied by the Irish Sea, and westwards in an upward direction over themselves, on a number of listric surfaces.

REFERENCES. BOSWELL, P. G. H. 1926. A Contribution to the Geology of the Eastern Part of the Denbighshire Moors. Quart. ] ourn. Geol, Soc., lxxxii., pp. 556-85. - - - . 1928. The Salopian Rocks and Tectonics of the District South-West of Ruthin (Denbighshire). Ibid., lxxxiii., pp. 6897 10 . - - - . 1928. The Cleavage-Fan in the Silurian Rocks of the Denbighshire Moors and Clwydian Range. Proc. Liverpool Geol, Soc., xv., pp. 69-77. - - - . 1930. The Pre-Carboniferous History of the Vale of Clwyd, Ibid., xv., pp. 230-240. - - - . 1932. On the Occurrence and Significance of an Area of Imbrication in the Ludlow Rocks of the Denbighshire Moors. Ibid., xvi., pp. 18-32. - - - and I. S. DOUBLE. 1934. The Silurian Rocks of the Northern Part of the Denbigh Moors between Abergele and Llanfair Talhaiarn. Ibid., xvi., pp. 156-172. DOUBLE, I. S. 1926. The Petrography of the Triassic Rocks of the Vale of Clwyd. Ibid. xiv., pp. 249-262.

186

GEOLOGY OF NORTH-WESTERN DENBIGHSHIRE.

KENDALL, P. F., and J. LOMAS. 1896. On a Terminal Moraine at Pwllglas near Ruthin. Glacialists' Magazine, iii., pp. 171-185. MONTAG, E. 1929. Erratics from the Boulder Clay of Pen-y-coed, Abergele. Ibid., XV., pp. 144-5. NEAVERSON, E. 1929. Faunal Horizons in the Carboniferous Limestone of the Vale of Clwyd. Ibid., XV., pp. III-I33. READE, T. MELLARD. 1891. The Trias of the Vale of Clwyd. Ibid., vi., pp. 278-289. STRAHAN, A. and A. O. WALKER. 1879. On the Occurrence of Pebbles with Upper Ludlow Fossils in the Lower Carboniferous Conglomerates of North Wales. Quart. Journ. Geol, Soc., xxxv., pp. 268-274. MEM. GEOL. SURVEY. 1866 (rst Edn.), 1881 (znd Edn.)., vol. iii. The Geology of North Wales. By A. C. Ramsay. - - - - 1927. The Geology of the Country around Wrexhatn (Sheet 121) (for Mynydd Cricor). By C. B. Wedd, B. Smith and L. J. Wills.