Oct. 6, 1882.] 



KNOWLEDGE 



305 



GEOLOGY OF LLANDUDNO AND lUlYL. 



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



THE arrangement of the rock masses whicli form tlie 

 north-east of Wales is tolerably clear and easy to 

 make out, contrasting strongly with the broken and in- 

 tricate nature of the strata which lie further west. In this 

 article I shall describe the geological structure of the 

 country lying cast and north of a line drawn from the 

 mouth of the Conway to Cerrig-y-Druidion, and thence to 

 Chester. Either Llandudno or Khyl form convenient 

 centres from which to explore this area, which has Denbigh 

 almost in its centre, and Mold near its eastern boundary. 



The Government Geological Map, sheet 79 (in four parts, 

 price 3s. each), includes the greater part of the district, but 

 Llandudno lies in Quarter Sheet 78 N.E. The purchaser 

 of the new edition of Prof Ramsay's excellent Survey 

 Memoir on the Geology of North "Wales (Stanford, 21s.) 

 ■will, however, find included in that book an extremely 

 clear coloured geological map of the whole of Wales, 

 although, of course, it is on a comparatively small scale 

 (10 miles to an inch). 



Surface Features of the District. — Starting from 

 almost any point in the Tale of Conway, say Llanwrst, 

 and walking eastwards, we ascend a long slope, and soon 

 find ourselves on the table-land of Denbigh, a barren and 

 scantilypeojiled district, traversed by few travellers, and 

 seamed by many little streams (of which the Elwy is the 

 principal), running eastwards to pour their waters into the 

 Clwyd. 



The strata numbered from 1 to 8 constitute the solid 

 yologij of the district : they liave been up-heaved, or 

 tilted-up, on the west, so that they liave a general easterly 

 slant or dip ; the glacial and alluvia! deposits rest irregu- 

 larly upon the upturned edges of these older rocks. The 

 diagram (fig. 1) represents the relation of the strata to one 

 another : — 



Fig. 1. 



Unfortunately, the real position of the rocks is not 

 nearly so simple as that shown in the diagram: the 

 several rock-masses have been so upheaved, broken, and 

 bent by the various movements of the earth's crust which 

 have taken place since their formation, that their actual 

 niode of occurrence is considerably more complex. 



Lower Silurian Rocks. — Walking southwards from 

 Llandudno, we come to the low ridge of the Diganwy Hills, 

 formed of a cream-coloured felsite (an ancient lava) of 

 Lower Silurian age ; the felsite is overlaid by grey grits 

 and blackish slates seen in several quarries. Tracing these 

 beds southwards by Conway Mountain, they are found to 

 increase greatly in thickness ; they are, in "fact, the well- 

 known Caraduc and Dala Beds which form the entire mass 

 of Snowdon. 



Crossing a breadth of ten miles of this table-land, we 

 come to the eastern limit, overlooking the fertile Yale of 

 Clwyd, which lies parallel to it. Continuing our easterly 

 walk, the Vale is quickly crossed, but on its eastern side 

 we get a long climb up the highest of the hills (Moel 

 Fammau, 1,823 ft.), whose rounded summits form another 

 north and south ridge; beyond this is a narrow table-land, 

 or rather an elevated shallow valley, bounded on the east 

 liy a lower ridge, whose slopes run down to the plain of 

 Cheshire. 



Geoloc.ical Form.\tioxs — The country over which our 

 imaginary walk has extended is composed of eight main 

 beds, or layers of hard rock, all resting one upon the other, 

 with an easterly inclination, so that the lowest (and oldest) 

 stratum forms the Vale of Conway, while the highest (and 

 latest-formed) constitut"s the plain of Cheshire. 



The laV)ours of Sedgwick, Murchison, and the officers of 

 the Geological Survey have proved these beds to follow 

 one another in the following order : — 



Fitnnation. Suh-diiisions. 



Thickness i 

 11. feet. I 



Alluvium OtooO 



Glacial Drift to 100 ' 



8. Bunter Sanilstoiie 5()0 



Cl. Coal-measures l.-SOO 



fs FoR3i,VTlox j e. Millstone Grit 7.W 



(.5. CarlionifeiDUS Limcstoiio l.lHH) 



(• 4. Wenlock Shule 2.r>(IO 



UrPEB SiLuniAX ] 3. Uenbi-hshire Grits I,5ti0 



( 2. TaranuoQ Shale 500 



1. Caradoc or Bala Beds ... 1,000.^ 



Rkcent ... 



Pl.KISTOCENE 



Trias 



Carbo.nii 



Lottf 



L^pper Silurian Rocks. — To examine the strata next 

 in succession — the Tarannon Shales — we must cross over to 

 the town of Conway ; from Conway due south to Llanbedr, 

 a distance of about five miles, we shall find a band of fine 

 smooth grey or blue slates, 500 feet thick, lying between 

 the Bala Beds and the Denbighshire Grits ; "but no such 

 smooth slates are to be found between Diganwy and the 

 Llandudno Junction Station ; in this area the gritty strata 

 lie directly upon the Bala Beds. This local alienee of the 

 Tarannon Shale is due to an overhi/) ; the Bala Beds formed 

 a sea-bottom on which the Tarannon Shales were laid 

 down ; a rapid subsidence of this sea floor led to an irre- 

 gular recession of its coast-line, so that the succeeding 

 sandy deposit, the Denbighshire Grits, overlapped, or ex- 

 tended beyond tlie margin of the earlier mud, which, 

 consolidated and compressed, now forms the Tarannon 

 Shales (fig. 2). 



This diagram also shows the great vncon/ormabilit;/, or 

 want of parallelism, between the Lower and the Upper 

 Silurian rocks ; a fact which points to a great interval of 

 tunc having elapsed between the formation of the two sets 

 of strata. 



To examine the Denbi;/L<ihire Grits we must walk along 

 the eastern slopes of the Vale of Conway ; except as a local 

 development of sandy grits with intorstratiUed shale, tliese 

 beds arc not interesting, for their fossils are few and hard 

 to find. The same is the case with the overlying Weiilocl: 

 Shales, which, with the (Jrits, help to form the wide and 

 barren plateau that lies between the Conway and the 



