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The Editor
349
REFERENCES C. A. 1913. The Geology of Bardsey. Q. Jl geol. Soc. Lond., 69 514-53. SAUNDERS, G. E. 1963. 'The Glacial Deposits and Associated Features of the Lleyn Peninsula of South- West Caernarvonshire.' Unpublished. MSc. thesis, University of Bristol, 1963. MATLEY,
G. E. SAUNDERS
Department of Geology and Geography Sir John Cass College London The Editor Dear Sir,
I welcome the comments made by Mr. Saunders concerning my paper 'The Glacial Successionin West Caernarvonshire', and am pleased to reply to each of three main points of divergence that he has listed. (1) That the terminal moraine of the upper of the two Northern deposits failed
to make a deep penetration through the Pant-glas col as far as Brynkir. The placing of an important glacial stage at Brynkir is based on the dramatic change of the drift topography that takes place south of that point. The terrain is hummocky to the north and smooth to the south. The presence of a large spread of outwash sands and gravels that extends south from the drift ridges and moulds at Brynkir suggests an important glacial stage at that place. Objections to such a glacial limit on the grounds that the ice-sheet failed to extend above 600 ft. O.D. in North Wales do not seem valid when compared with the results of detailed mapping of this drift limit in Ireland. Round the northern edge of the Wicklow Mountains the altitudinal range of the ice limit varies between 500 and 1300 ft. O.D. (Farrington, 1948), and between 500 and 750 ft. O.D. in south-east county Limerick (Geological Survey of Ireland records, 1959-60). For this reason I would suggest that the line chosen by Mr. Saunders could represent a later minor stage when the Brynkir lobe had been cut from its source and stagnated in situ. (2) That the upper Welsh till of the south coast is contemporaneous with the upper till (Gravelly Northern till) of north Caernarvonshire. The researches of Mr. Saunders show that his blue-grey clay-rich northern till, which appears from his description to be the same as my purple 'stoneless' clay-till, extends farther east than Pwllheli. Clearly, then, this till reached farther east than I stated. As the latter is associated with overlying outwash (the Middle Sands and Gravels of Jehu) that pre-dates a till of local Welsh origin, it is clear that they cannot be exactly contemporaneous. There is little evidence to suggest the time-interval between these two tills. The absence of this upper Welsh till or outwash on top of the Northern clay-rich till at Porth Neigwl, and also the very regular striae pattern on St. Tudwals peninsula reported by Nicholas, suggest to the author that the two tills were associated with the same ice-sheet at an earlier phase of glaciation. The difficultyof equating the upper Welsh till with the youngest Welsh till in north Caernarvonshire is the fact that the till surface is so severely cryoturbated or frost-disturbed at Criccieth. As such severe disturbance of the till-fabric was never observed in exposures of the youngest drift of the northern area it was concluded that the latter must be younger.
350 (3) Apparently the striae pattern in Lleyn is much too inadequate to draw any general conclusions about iceflow. The evidence produced by Mr. Saunders shows that the ice flowed from north to south during much of the main glaciation by northern ice. This suggests that the south-west flow suggested by me for this glaciation is not correct. REFERENCE A. 1948. The Glacial Drifts of the Leinster Mountains J. Glacial, I, 22{}-5 .
FARRINGTON,
F. M. SYNGE
Department of Geography Aberdeen University
REFERENCES C. A. 1913. The Geology of Bardsey. Q. Jl geol. Soc. Lond., 69 514-53. SAUNDERS, G. E. 1963. 'The Glacial Deposits and Associated Features of the Lleyn Peninsula of South- West Caernarvonshire.' Unpublished. MSc. thesis, University of Bristol, 1963. MATLEY,
G. E. SAUNDERS
Department of Geology and Geography Sir John Cass College London The Editor Dear Sir,
I welcome the comments made by Mr. Saunders concerning my paper 'The Glacial Successionin West Caernarvonshire', and am pleased to reply to each of three main points of divergence that he has listed. (1) That the terminal moraine of the upper of the two Northern deposits failed
to make a deep penetration through the Pant-glas col as far as Brynkir. The placing of an important glacial stage at Brynkir is based on the dramatic change of the drift topography that takes place south of that point. The terrain is hummocky to the north and smooth to the south. The presence of a large spread of outwash sands and gravels that extends south from the drift ridges and moulds at Brynkir suggests an important glacial stage at that place. Objections to such a glacial limit on the grounds that the ice-sheet failed to extend above 600 ft. O.D. in North Wales do not seem valid when compared with the results of detailed mapping of this drift limit in Ireland. Round the northern edge of the Wicklow Mountains the altitudinal range of the ice limit varies between 500 and 1300 ft. O.D. (Farrington, 1948), and between 500 and 750 ft. O.D. in south-east county Limerick (Geological Survey of Ireland records, 1959-60). For this reason I would suggest that the line chosen by Mr. Saunders could represent a later minor stage when the Brynkir lobe had been cut from its source and stagnated in situ. (2) That the upper Welsh till of the south coast is contemporaneous with the upper till (Gravelly Northern till) of north Caernarvonshire. The researches of Mr. Saunders show that his blue-grey clay-rich northern till, which appears from his description to be the same as my purple 'stoneless' clay-till, extends farther east than Pwllheli. Clearly, then, this till reached farther east than I stated. As the latter is associated with overlying outwash (the Middle Sands and Gravels of Jehu) that pre-dates a till of local Welsh origin, it is clear that they cannot be exactly contemporaneous. There is little evidence to suggest the time-interval between these two tills. The absence of this upper Welsh till or outwash on top of the Northern clay-rich till at Porth Neigwl, and also the very regular striae pattern on St. Tudwals peninsula reported by Nicholas, suggest to the author that the two tills were associated with the same ice-sheet at an earlier phase of glaciation. The difficultyof equating the upper Welsh till with the youngest Welsh till in north Caernarvonshire is the fact that the till surface is so severely cryoturbated or frost-disturbed at Criccieth. As such severe disturbance of the till-fabric was never observed in exposures of the youngest drift of the northern area it was concluded that the latter must be younger.
350 (3) Apparently the striae pattern in Lleyn is much too inadequate to draw any general conclusions about iceflow. The evidence produced by Mr. Saunders shows that the ice flowed from north to south during much of the main glaciation by northern ice. This suggests that the south-west flow suggested by me for this glaciation is not correct. REFERENCE A. 1948. The Glacial Drifts of the Leinster Mountains J. Glacial, I, 22{}-5 .
FARRINGTON,
F. M. SYNGE
Department of Geography Aberdeen University