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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 







17 



III. Th 



II. The Coast Sections 



19 



IV. Su 





I. Introduction 



PAGE 



III. The Deipt of the Interior ... 38 



IV. Summary and General Conclusions . 45 



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 Trimmer'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 



