THE MYNYDO CRICOR INLIER.By BERNARD SMITH, M.A., Sc.D., F.R.S., F.G.S.

[Written for the Easter Field Meeting, 1935J.

THE Vale of Clwyd Syncline and the Clwydian Anticline attheir southern ends abut at right angles upon the northern

flank of the Mynydd Cricor Anticline, from which they are partedby the Llanelidan Fault. This anticline-actually a muchfaulted semi-dome-with its general axial trend from west toeast, constitutes a separate segment of the Horse-shoe Anticline,"which, enclosing most of the Flintshire Coalfield within its greataxial curve, sweeps asymmetrically round the eastern terminationof the Harlech folding.

The Cricor Anticline is separated from its eastern neighbour,the Cyrn-y-brain Anticline, which is composed of similar rocksand forms another segment of the Horse-shoe Anticline, by theGreat Bryn Eglwys Fault. Where the latter crosses this strung­out folding obliquely, the Mynydd Cricor and Cryn-y-brainsegments of the fold are both distorted-the Cricor saddlebending south-eastward towards the fault and declining (Fig. 24),probably as the combined result of pitch and diminishing foldamplitude.

To the north of its axis the Cricor Anticline is faulted againstSilurian and Carboniferous rocks, whilst to the south-west, southand east it is followed in normal sequence by Wenlock strata,in which, on the south-western margin, the well-known Denbigh­shire Grits attain their eastern termination.

The area was subjected to glaciation by ice which moved inan easterly direction from the Arenig Mountains.

In the Cricor Anticline the following rock formations arepresent :-Silurian.

Wenlock Series :-Pen-y-glog Stage: slates and shales with Pen-y-glog Grit in

upper part in western area.Upper Valentian or Tarannon Series :-

Dark-blue striped shales and slates: (Zone of M. crenulatus).Pale greenish-grey and purple mudstones: (Zones of M. gries­

tonensis, M. crispus, M. turriculatus).Lower Valentian or L. Llandovery Series:-

Cleaved gritty greyish-green mudstone and shales with thinsandy seams (up to zone of M. cyphus).

Ordovician.Ashgillian Series:-

Cricor or Corwen Grit (at or near top).Cyrn-y-brain beds: cleaved blue shales with thin sandstones or

sandy bands.

ORDOVICIAN.The Cyrn-y-brain Beds are the equivalents of the Ddolhir

I Geology of Flint, Hawarden, and Caergwle, (Mem. Geol, SU1'V.), 192~. p, 10.

OF THE MYNYDD CRICOR

INLIER.

Succession.d, Carboniferous Lime­

stone Series;b7, Ludlow Beds;

be, Wenlock Series (Pen-y­glog Grit shown bycircles) ;

b 5, UpperValentian (Cren­ulatus Beds, white) ;

b4-5, Lower Valentian;b 3, Cyrn-y-brain beds with

Cricor Grit.

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THE MYNYDD CRICOR INtIER.

Beds of Glyn Ceiriog, and form the core of the Cricor Dome, thenorthern half of which has been dropped or squeezed out of sightby the Llanelidan Fault. They consist of at least 1,000 ft. ofshaly and micaceous greywacke slates of bluish tint, the beddingbeing usually marked by sandy laminas along which fossils liesomewhat unevenly. In places, more especially towards the top,the sandy laminse are more common and form thin layers of grit.The beds are well exposed on the moorland of Mynydd Cricor,which rises above the swathing boulder-clay, for example eastof Cerrig-oerion where fossils are common; but the best sectionis perhaps that in a dingle about 400 yds. south-south-west ofCricor Farm.

As a rule fossils are fairly easily obtainable; it is not difficultto find a number of forms characteristic of the horizon, such as"Leptaena rhomboidalis (Wilck.)," Clijtonia spiriferoides (M'Coy),Orthis calligramma Dalm., O. (Platystrophia) bijorata (Schloth.),M eristina crassa (J. de C.Sow.), Sowerbyella sladensis O. T. Jones,or rare trilobites, including Cybele.

On Mynydd Cricor, the Cricor or Corwen Grit is essentiallylenticular and may consist of one or more lenticles at any par­ticular spot or may actually fail to make a show at all. Wheremassive, it is white to greenish and quartzitic, the green coloura­tion being due chiefly to chlorite. The grit may, however, beinterleaved with a great number of narrow bands of shale, about1 inch in thickness, when it weathers with a ribbed surface. Itcan be examined near Cricor Farm, but the best exposures lieto the south and east of the summit of the mountain.

To the east of the Tan-y-graig Fault (a north to south fracturethat passes west of the summit) the gritty material, commencingas nodules of calcareous grit, rapidly increases eastward until,about 300 yards due south of the summit, we find over 20 ft.of grits with shale partings. Here an interesting phenomenonmay be observed; the grit bands, which individually attain athickness of 4 inches, have been broken up into elliptical phacoidsby the stresses that have induced cleavage in the interveningshales.

Exposures some 600 yards east-south-east of the summit givethree main grits with interbedded shales, the lowest grit, con­taining massive beds up to 6 ft. in thickness, measures 100 ft.,and is succeeded by 35 ft. of fossiliferous banded grits and grittyshales with species of Sowerbyella [Plectambonites] like those ofthe Slade Beds of Haverfordwest.

On paleontological evidence Messrs. Groom and Lakeassigned the Glyn Grit to the Ordovician, and our experience ofthe Grit at Corwen and Mynydd Cricor, where it is associatedwith shales or mudstones of Ddolhir rather than Valentian type.confirms this view.

B. SMITH,

SILURIAN.

The actual passage upwards from the Ordovician into theSilurian is not seen here, but, on analogy with what occurs atCorwen, is thought to be one of conformity.

In this neighbourhood the Valentian (or Llandovery) strataconsist (I) of a lower group of Lower Valentian (Birkhillian)slates of a bluish-grey colour, often with sandy partings anddarker spots, streaks and blotches similar to those of the' MottledBeds' of the Machynlleth district (Jones and Pugh), and, (2) ofa higher group which have been termed Upper Valentian orTarannon, and consists of smooth pale greenish-blue or greycleaved mudstones, with inclusions of striped dark blue and greygraptolitic mudstones. At their top these become more pro­nouncedly blue, and pass up into rocks either splintery oralternately displaying resemblances to the Tarannon mudstonesand to the banded slates of the Lower Wenlock Shales. Theyalso contain thin representatives of the' spotted' or 'mottled'beds. These passage beds by their peculiar lithology form auseful index horizon in working out the structure of the districtand carry the fauna of the highest Tarannon graptolite zone, thatof Monograptus crenulatus.

In the Cricor Anticline the Lower Valentian is about 600 ft.thick, and the Upper Valentian nearly I,200 ft., as against atotal for the two of 250 ft. in the Glyn Ceiriog country. Thebeds having the Tarannon lithology appear to be conformableto the Lower Valentian, but on palseontological grounds and asurvey of a larger region it seems probable that there may be anon-sequence between the two. South of Cricor summit, forexample, the highest beds with the Lower Valentian lithologycontain an assemblage of forms indicative of the zone of M ono­graptus cyphus and appear to be overlain by the Upper Valentianwith conformity (the zones of M. gregarius, M. convolutus withCephalograptus cometa, and M. sedgwicki being apparentlywanting); yet on Cyrn-y-brain the latter overlies the M.convolutus zone and at another point beds with a fauna suggestingthe M. gregarius zone.The beds with the Lower Valentian lithology contain brachiopods

of Lower Valentian types, and a few possibly of Upper Valentianalso.

It seems probable that all the Tarannon zones are presenthere. That of M. turriculatus is inferred; it was found near theLlanelidan Fault a few miles farther east. The other three havebeen proved at several points: that of M. crispus east of theTan-y-graig Fault and north-north-east of Rhos-lydan, that ofM. griestonensis near Nantuchaf ; east of the Tan-y-graig Fault,and east-north-east of Rhos-lydan and that of M. crenulatus nearNant-uchaf, Rhos-lydan, in Nant-y-Garth and at other places.

THE MYNYDD CRICOR INLIER. 191

At these and other localities the following characteristicfossils, in addition to the zone fossils themselves, have beenobtained :-M. discus Tornq., M. marri Perner, M. nodiferTornq., M. nudus Lapw., M. pandus Lapw., M. priodon (Bronn),M. runcinatus Lapw., M. spiralis (Geinitz), M. vomerinus (Nich.),var. basilicus Lapw. and Retiolites (Gladiograptus) geinitzianus(Barr.), var. angustidens Elles & Wood.

Little need be said here about the Wenlock Beds or Pen-y­glog Slates or of the Silurian Beds which together frame theolder rocks of the anticline. The lowest part of the WenlockSeries, the zone of Cyrtograptus murchisoni, everywhere succeedsthe Crenulatus Beds with conformity and may be seen at severalpoints-perhaps best in Nant-y-Garth where they are bluish,banded, sandy shales and thin micaceous flags usually weatheringto an ashy tint. To the west and south-west of the CricorAnticline the higher Wenlock zones are associated with theDenbighshire or Pen-y-glog Grit-an important element in theDenbighshire moorlands west of the River Clwyd. This thinsout and disappears, however, east of a line passing north fromMoel-Fferna in the Berwyns to Mynydd Cricor. It occurs inforce between two faults near Pen-y-cae, less than a mile south­west of Cricor Farm, but makes its last appearance as lenticlesand detached masses in the shales about a mile and a half southof Cricor.

North of the Llanelidan Fault which is crossed with seemingindifference by Nant-y-Garth, the banded flagstones of thenilssoni zone of the Ludlow Series of the Silurian may be studiedat several points, more especially near the Lodge close to thefifth milestone from Ruthin.

TECTONICS.

Throughout the Cricor and surrounding areas the cleavage isfairly intense-though never strong enough to have made slatesof commercial value-its normal strike being roughly east andwest and its inclination northward ;' but the subsequent faultinghas brought about changes in strike and dip of the cleavagewhich have helped to reveal the positions of the faults themselves.

The main Llanelidan Fault, with the general effect of a normalnorthward downthrow in this part of its course, combinesevidence of great compression and horizontal movement,s forit not only suppresses the intervening syncline between thesouth end of the Clwydian Anticline [Clywdian Range] and theMynydd Cricor Anticline, but in great part also the northern limb

I This being due in the writer's opinion to a deep-seated underdrive from the south.

2 For a discussion of the tectonic principles involved in the case of this and other importantfaults in this part of N. W.Wales, see .. Geology of Wrexham," (Mem. Geol, S"",.), Pt. 1.• 1927,PP. 23-<27·

THE MYNYDD CRICOR INLIER.

of that anticline. Strong evidence of the great compression andmovement along this fault may be seen especially well about200 yards south of the Lodge, in a ravine opening north intoNant-y-Garth. The rocks are strongly smashed and pugged,and the individual smash belts dip north or north-east at highangles.

All discernible movement of the great fault, both horizontalor vertical, was-like that of the smaller tranverse faults-laterthan the cleavage; it was also later than the deposition of theCarboniferous Limestone Series and probably also of the CoalMeasures, and later than the elevation of the Cricor Anticline.

In conclusion a few words may be said about Nant-y-Garth.This striking valley, although showing signs of fairly recentover-deepening in its lower reaches, is really a pre-Glacial gorge,since boulder-clay is found on its floor, as far down at least asthe point where it crosses the Crenulatus Beds. Its indifferenceto rock texture and to tectonic structure suggests superimposeddrainage; i.e., that following upon an uplift a previously mean­dering stream was able to sink its thalweg into the underlyingrocks rapidly and without reference to their relative resistance toerosion. It is tentatively suggested that this may have occurredwhen the supposed Pliocene uplift took place which gave rise tothe Menaian and associated marine platforms of the western partof North Wales and of Anglesey-when the Dee, the Tanat, theVyrnwy and the Clwyd became incised streams. Before thatperiod of uplift it is possible that the Vale of Clwyd and theCheshire and Shropshire Plains were much more encumberedwith New Red Sandstone than they are at present, and its rapidremoval may have assisted the rivers to sink their courses fairlyquickly into the older rocks.