Oct. 20, 1882.] 



KNOWLEDGE • 



3:3: 



GEOLOGY OF LLANDUDNO AND EHYL. 

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



{Continued from paje 306.) 



Tde Carboniferocs Fobmatiox. — Beds of the ^[o}'-nfain 

 or Corhoniffrous Limestone, preserved by their position at 

 the base of a synclinal curve, form Great Orme's Head : 

 the change of dip from northerly to horizontal, and then to 

 southerly, is well seen T\-hile taking a drive almig the mag- 

 nificent road of five miles in length -nhich encircles the 

 " Head." Little Onue's Head is a continuation of the 

 same rock, while Llandudno lies in the gap between the 

 two Heads, produced by the forces of denudation acting 

 on strata thrown into an anticlinal curve, and thereby 

 specially laid open to the atmospheric and marine forces. 



Striking out to sea from Little Orme's Head and the 

 corresponding headland on the south side of Ehos Bay, the 

 Mountain Limestone cur\es round southwards beneath the 

 water, and strikes the coast again at Colwyn Bay, forming 

 the picturesque heights west of Abergele. From this point 

 the limestone strata extend southwards in a long narrow 

 line past Denbigh to the extremity of the Tale of Clwyd. 

 But while the strike of the limestone beds is north and south, 

 their dip is to the east In this direction they disappeai- 

 under the Xew Red Sandstone which forms the level sur- 

 face of the Vale of Clwyd, to be brought up again by a 

 fault which ranges along the eastern side of the valley and 

 exposes the Limestone in several patches, as near Bodfari 

 and Llan Yedr. At the north-east end of the Vale, the 

 Mountain Limestone forms the hill called Moel Hiraddug, 

 whence a line of hills runs eastwards to Prestatyn, and 

 thence southwards, forming the hills that lie west of Holy- 

 well and Mold. 



Everywhere the Mountain Limestone presents a 

 similar appearance, as a thick bed of compact grey 

 stone, the central portion being lighter in colour, or 

 nearly white. Fossils are plentiful, especially in the 

 higher beds, chiefly such marine shells as Prodiiclvs, 

 X/ii/nchoneUa, and Spiri/era, branching corals called 

 Syringopora, and the little cup-coral {CijutJwphyUum) ; 

 while the- rings which were the joints of sea-lilies 

 (Crinoids), lie thickly on almost every slab. In the 

 quarries above Prestatyn I obtained many formidable- 

 looking fish-teeth belonging to the genus Bhi:odus. In the 

 Minera district, further south, Mr. Morton has recognized 

 four zones — Lower Grey, Lower White, Upper White, and 

 Upper Grey — each characterised by certain fossils, in the 

 Mountain Limestone, and any geologist who makes a stay 

 at Llandudno or Ehyl should bear this fact in mind, and 

 endeavour to trace these zones in his neighbourhood. 



^lany bdes, or fissures fiUed with metallic ores, are 

 known. The most important of these is worked at 

 Talargoch, about three miles south-east of Rhyl ; it yields 

 about i',700 tons of zinc ore (value £12,000) and 900 tons 

 of lead ore richly impregnated with silver (value .£9,200), 

 annually. The limestone itself is largely quarried for 

 burning, for road-metal, and for building purposes. 



Certain lieds of red sandstone and conglomerate underlie 

 the Mountain Limestone on the west side of the Vale of 

 Clwyd ; they occur in Ffernant Dingle, about one mile 

 souti of Llysfaen, also near Henllan. Tliese bods' have 

 been mapped as O/d Red Sandstone by the Survey, but I 

 believe they are identical with similar strata known in 

 the Lake District and in the Isle of Man, and that they 

 form the base of, and belong to, the Carboniferous Lime- 

 stone. 



The Milhtone Grit is well seen resting upon the Lime- 

 stone at Gronant, Gwespyr, Holywell, and at many points 



west and south of !Mold. There is a small patch of it in 

 the valley south of Little Orme's Head, and another round 

 St. Asaph. The lower beds are black and flinty-looking : 

 they are a kind of chert, and are quarried at Gronant to be 

 used in the Staflbrdshire Potteries ; above these cherty 

 bed come sandstones, not very coarse in texture, worked 

 at Gwespyr and other places for building-stones, mill- 

 stones, scythe-sharpeners, iVc. Fossils, with the excep- 

 tion of plant-markings, are scarce. Further south, near 

 Oswestry, the marine shells of the Mountain Limestone 

 have been found to range high up into the Millstone 

 Grit, and it would be a most interesting task for any 

 geologist residing in the neighV)Ourhood of Holywell or 

 Prestatyn to endeavour to determine whether the same 

 thing occurs further north. 



The Coal-hteagures form a little coal-field along the Dee, 

 at Flint and Bagillt ; dipping under the river, the beds 

 rise again on the Cheshire side, where there is a colliery at 

 Xeston. The possibility of the occurrence of coal-seams 

 underneath the Vale of Clwyd has been considered, and in 

 1860 a borehole was made near the Foryd, close to Rhyl, 

 which passed through the following beds : — 



ALLVTUt Deposits, 54 (t. ; Gravel and Clay. 



Glacial Beds. -t4ft. ; Bed Clay and Sand. 



Xew Red Saxdstoxe, 500 ft. ; Red Sandstones and Shales. 



CAKBOxiFERors ? 150 ft. ; Red ilarls and Shales. 



Although it is a pity that the boring was not continued 

 to a greater depth than 7i8 feet, yet the probability is 

 against the occuiTence of useful coal-seams in this district. 



The Trias or Xew Red Sandstone.— The soft red 

 sandstone which forms the level floor of the Vale of 

 Clwyd represents the Lower Bunter Sandstone of the 

 Midlands. Here, as elsewhere, it is quite unfossiliferous. 



Glacial Deposits.— Nearly all the low ground of north- 

 east Wales has a coating of what is called Boulder Claij. 

 which is a reddish imstratified clay, containing an abund- 

 ance of scratched stones of aU sizes. This deposit creeps 

 up the hill-sides to a considerable height, while higher 

 still beds of gravel and sand are met with, also referable 

 to glacial times. Many travelled blocks of granite arc 

 found along the coast, some of which can be identified with 

 Lake-district rocks, having been carried southwards during 

 the glacial period by glaciers or icebergs. The smoothed 

 and rounded (»io!(to?inee) outlines of the hills. (best seen 

 from a distance), clearly tell of the passage of a hea\'7 mass 

 of ice over the district. Considered in this light, a study 

 of the pebbles on the beach will lie found interesting : at 

 Rhyl, since the town stands on a dead alluvial flat, this 

 will be the only use to which a geologist can put his 

 hammer ; an examination of the low sand-hills which here 

 fringe the coast will, however, show him the geological 

 phenomenon known as " false-bedding " in perfection. 



Pre-histokic :Man. — Owing to the absence of flint, the 

 hills are not so productive of weapons fashioned out of that 

 (to savage tribes) favourite material as the Downs of 

 Yorkshire or Sussex. There are, however, numerous well- 

 preserved remains of tumuli, camps, and hut-dwellirgs. 

 Caves are not unfretjuent ; the best known is situated at 

 Cefn, about two miles south-west of St Asaph, half-way up 

 a vertical limestone clifl" ;W0 ft. high, overlooking a grand 

 ravine cut by the river Elwy. Here flint and stone im- 

 plements, with many teeth and bones of certain extinct 

 animals, have been found embixlded in the stalagmitic floor. 



From these brief notes it will be seen tliat in this region 

 there is much for the beginner to learn, and many pro- 

 blems which an experienced geoloirist might render aid in 

 solving. My difficulty has been to compress a notice of 

 even the principal facts into the necessarily restricted limits, 

 of this article. 



