II.—The Glacial Deposits of Western Carnarvonshire. by T. J. Jehu, M.D. (Edin.), M.A
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II.—The Glacial Deposits of Western Carnarvonshire. By T. J. Jehu, M.D. (Edin.), M.A. (Camb.), F.G.S., Lecturer in Geology at the University of St Andrews. (With Four Plates and One Map.) (MS. received January 12, 1909. Read March 15, 1909. Issued separately May 3, 1909.) CONTENTS. PAGE PAGE I. INTRODUCTION 17 III. THE DRIFT OF THE INTERIOR ... 38 II. THE COAST SECTIONS .... 19 IV. SUMMARY AND GENERAL CONCLUSIONS . 45 I. INTRODUCTION. Since the time when AGASSIZ and BUCKLAND made known the former presence of glaciers in the mountain valleys of North Wales, much has been written concerning the glaciation of Snowdonia, but comparatively little attention has been given to that of Western Carnarvonshire. This part of the country stretches south-westwards as the broad promontory of Lleyn between Carnarvon Bay and Cardigan Bay. Western Carnar- vonshire for the most part lay outside the paths followed by the native glaciers. None of the larger valleys of Snowdonia trend in this direction, and so the marks of recent glacia- tion are not so fresh and striking in this region as they are further east. Hitherto no one has attempted to give any detailed account of the Drift deposits over the whole of Lleyn, but various references to the glaciation of the region are found scattered in the literature dealing with the geology of Wales. Among these one of the most interesting and most accurate is that in JOSHUA TLUMMER'S Practical Geology and Mineralogy (1841), where he writes of the " Diluvial Deposits of North Wales " (p. 398). Concerning Lleyn he says : " The promontory which divides the bays of Cardigan and Carnarvon is covered with diluvial deposits of variable thickness, but frequently exceeding 100 feet. They consist of sand, clay, and gravel, changing rapidly from one to the other, through which are dispersed broken marine shells, and, amidst much local detritus, fragments derived from the north and west. Granite is rare, but they afford fragments of Antrim chalk, often very slightly abraded. The south-western termination of this promontory affords several instances of the accumulation of the transported matter with shells, covering masses of angular detritus of the rock immediately subjacent, which may be supposed to have accumulated slowly under the atmosphere, after the manner of a talus, or to have been collected from the weathered surface of a bare rock by the first rush of the diluvial currents." This extract proves that TRIMMER had detected the " rock rubble "or " head" which underlies the Drift deposits at the western end of the promontory. TRANS. ROY. SOC. EDIN., VOL. XLVII. PART I. (NO. 2). 3 18 DKT. J. JEHU ON Sir ANDREW RAMSAY, in his Survey Memoir, "The Geology of North Wales" (2nd ed., 1881), makes only occasional references to the glaciation of Lleyn. Describing the district, he says : " Like Anglesey it partly consists of metamorphic rocks, which are supposed to be of Cambrian age, and of Lower Silurian strata, through which numerous bosses of rock protrude of a syenitic and dioritic character. Some of these are of great extent, and in Bwlch-mawr and Moel Penllech, Yr Eifl or The Eivals, Cam Bodvean, Mynydd Nevin, Cam Madryn, Mynydd Mynytho, Mynydd-y-Rhiw, and other minor heights, they generally rise high above the plain of glacial drift that shrouds so much of the country, and thus form the loftiest hills of the district. The stratified rocks, indeed, generally lie in ground so low, and, except on coast cliffs, they are mostly so obscured by sands and marine boulder clays, that it is difficult to make out the details of this stratification" (p. 206). Again he states (p. 221) that "from the heights of Yr Eifl the promontory of Lleyn, but for the chain of Puys, looks like a mere plain." He mentions "an angular breccia of post-Tertiary date resting on the slaty strata" at Porth Ceiriad and underlying the Drift deposits (p. 208). Beds of angular breccia are also noted as occurring in Aberdaron Bay (p. 211). With regard to Bardsey Island he observes that " like the mainland the country has been moulded by ice during the Glacial period, but the mammillated roches moutonnees have since been roughened by weather" (p. 212). He describes the ice-sheet which overspread Anglesey as continuing "its onward course to some unknown distance, for on the ice-ground rocks above the sea- cliff of Trwyn-y-tal, a mile north of the seaward flank of Yr Eifl, well-marked striations are seen pointing towards the Straits 43° north of east" (p. 273). In RAMSAY'S Physical Geology and Geography of Great Britain (6th ed., 1894, p. 246), mention is made of the occurrence of shell-t>earing deposits in the district of Lleyn. It is also stated (p. 248) that "at various levels on the low ground between Carnarvon and Criccieth there are extensive deposits of sand and gravel, well stratified, and much resembling those of Moel Tryfan, but apparently without shells." These are overlain by boulder beds. He held the view that there had been two epochs of glaciation in Wales separated by an epoch of submergence during which the land was drowned to a depth of 1400 feet or more. Mr CARVILL LEWIS in The Glacial Geology of Great Britain and Ireland, 1894 (p. 355), states that " deep till, with many large boulders, covers the land, and makes the moorland extending from Afonwen to Carnarvon." He also noted the peat at Ynysand Pant-glas and the " hills of till" at Brynkir ; the " very large beds of coarsely stratified drift, full of large boulders and knob-like in shape," occurring near Penygroes. " All this," he concludes, " probably belongs to the great ice-sheet, not to the Welsh glaciers." He gives a Glacial Map of England and Wales in which the margin of the northern ice is depicted as reaching North-Westem Lleyn but leaving the southern part of the pro- montory free. In common with many other geologists, Mr CARVILL LEWIS denied that the so-called "Intermediate Sands and Gravels" indicate a submergence of 1400 feet. He regarded the high-level shell-bearing sands, such as those of Moel Tryfan, simply as THE GLACIAL DEPOSITS OF WESTERN CARNARVONSHIRE. 19 rearranged material derived from the bottom moraine of an ice-sheet which reached this elevation. Mr T. MELLARD JREADE, in a paper on " The Drift Beds of the Moel Try fan Area of the North Wales Coast," * describes the coast sections from Dinas Dinlle to Nevin and also some sections inland. He favours the view that the sands and gravels are marine, and that they indicate a submergence of this area in Glacial times to at least 1400 feet. The base of the Lleyn promontory lies just outside the mountains of Snowdonia, and has a breadth of about 5 miles. South-westwards the breadth varies from 3 to 5 miles, becoming reduced to about a mile at the extreme end. None of the great valleys of Snowdonia trend towards Lleyn. The stratified rocks of this district are chiefly of Lower Silurian (Ordovician) age, and through these protrude a number of igneous masses which form an interrupted chain of hills along the whole extent of the promontory. Hard grits and shales of Cambrian age constitute the high ground between Porth Nigel and St Tudwal's Road, and metamorphosed rocks which are now regarded as pre- Cambrian extend from Porth-dinlleyn to the end of the promontory and form the Isle of Bardsey. These metamorphic rocks rise to heights of 500 feet at the extreme end of Lleyn. The lower parts of the district are covered by a mantle of Drift, and if this were entirely removed considerable parts of Lleyn would probably be submerged. II. THE COAST SECTIONS. The following account of the superficial deposits begins with the sections at Dinas Dinlle and the coast is followed south-westward to the end of Lleyn and thence eastwards on the south side of the promontory to Criccieth. From Dinas Dinlle south-westwards as far as Gwydir Bay the coast-line is for the greater part bounded by cliffs of varying heights which are described on the Geological Survey Map (1 inch to the mile) as consisting of stratified clay, sand, and gravel. The bed-rock is not seen anywhere along this stretch of coast. To the north of Dinas the coast is flat, consisting of alluvial land which extends towards the entrance of the Menai Straits. The character of this flat area is shown by a boring recently made for water a few yards behind the hotel at Dinas. Eighteen feet of grey sand were passed through, in which the shells of Cardium edule were abundant, as well as the shells of Scrobicularia piperata, with the two valves often still in apposi- tion. This sand is evidently a recent marine deposit. Below the sand a very tough bkiish-grey boulder clay was reached, and this extends to an unknown depth. Dinas Dinlle.—This is a dome-shaped mound surmounted by an ancient encamp- ment. One side of the mound is being cut away by the sea, so that good sections, which have been described by Mr T. MELLARD R,EADE,f are exposed. The cliff varies in height up to a maximum of about 100 feet. * Proc. Liverpool Geol. Soc., vol. vii. (1893), p. 36. t Ibid., p. 42. 20 DR T. J. JEHU ON The following succession can be traced in the deposits :— 4. Soil and Blown Sand. 3. Upper Stony Clay. 2. Sands and Gravels. 1. Lower greyish-blue or purple Boulder Clay.