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Reply by the author
213 postulated are of course diagrammatic, but bear some relationship to contours, explain the positions and drainage directions of the channels, and account for the form and the placing of the major glacial deposits. There is certainly no reason to suppose that stagnant ice persisted in the Mold Valley. No pitted deposits remain on the flat floor, across which the sand and gravel deltas thrown out from the Bellan gorge and the Flint-Alyn trench-valley extend for some distance downstream. Similar deposits to those of the Mold Lake occur in the former glacial lake areas farther down the Lower Alyn, at lower levels, supporting the postulation of a progressive series of pro-glacial lakes. In my opinion the evidence of sub-glacial drainage is insufficient to explain most of the retreat phenomena observed. REFERENCES DERBYSHIRE, E. 1962. Late-Glacial Drainage in Part of North-East Wales: an Alternative Hypothesis. Proc. Geol. Ass., Lond., 73, 327-34. EMBLETON, C. 1956. Late-Glacial Drainage in Part of North-East Wales. Proc, Geol. Ass., Lond., 67, 393-404. - - - . 1964. Sub-Glacial Drainage and Supposed Ice-Dammed Lakes in NorthEast Wales. Proc. Geol. Ass., Lond., 75,31-8. PEAKE, D. S. 1961. Glacial Changes in the Alyn River System and their Significance in the Glaciology of the North Welsh Border. Quart. J. geol. Soc. Lond., 117, 335-66. SISSONS, J. B. 1960-1. Some Aspects of Glacial Drainage Channels in Britain. Scot. geogr. Mag., 76, 131-46, and 77, 15-36. D. S. PEAKE
104 Old Farleigh Road Selsdon, Surrey REPLY BY THE AUTHOR
Mrs. Peake raises first a number of points concerning the Sam Adda meltwater channel and, on her interpretation, the associated lake delta deposits in the Bellan area. As she observes, the highest delta surface at Bellan had an outer edge at about 500 feet before gravel working commenced, but the surface rises to a little over 530 feet around Bryn-Bellan. Between here and the Sam Adda channel four miles to the south, there are no signs of lake deposits but merely a very narrow and doubtful terrace feature at 500-525 feet near the Mold-Gwernymynydd road, and at about 520-535 feet a little to the south. There is no proof that the lake in which the Bellan delta formed was other than a local one, or had a long extension for four miles to connect directly with the Sam Adda channel. I prefer to envisage the ice margin between the Bellan lake and Sam Adda as one closely following the hill-side at a height above 500 feet, and I find no evidence that marginal drainage followed it continuously. I see no reason why deltas laid down in local pro-glacial lakes should not possess a well-defined form. The Sam Adda channel is clearly cut in a broad open col of probably pre-glacial origin between Leeswood Hill and the Treuddyn spur. Mrs. Peake agrees that ice must have been present in this col before the channel was cut. She suggests that the channel was initiated by meltwater flowing at about 600 feet marginally beside ice occupying the col, and that the channel was subsequently deepened as a spillway, but this does not explain the anomalous long profile of the channel. The hypothesis that the channel developed as a sub-glacial col-gully has the advantage of accommodating this point, and admits of comparisons with several other similar channels in north-east Wales. There is no reason to dispute the
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suggestion that the excavation of the channel may have commenced in an earlier glaciation, the channel being deepened in later glaciations; this surely applies to many other channels. Mrs. Peake doubts whether stagnant ice persisted in the Mold Valley. Yet she does not mention the Padeswood gravels a little downstream from Mold, which are full of kettle holes and very characteristic of dead-ice topography. It is by no means impossible that other dead-ice deposits now lie buried in the Mold Valley by the most recent alluvium. Certainly they are abundant and impressive in the Wheeler Valley. Moreover, general considerations of ice thicknesses show that the ice in the Mold area must have been stagnant during most of the stages enumerated by Mrs. Peake; even at the 'Leeswood Stage' there can, according to Mrs. Peake's ice margin positions, have been little more than 100-150 feet of ice in the Mold Valley. Cessation of movement and stagnation were inevitable, and conditions probably very favourable to sub-glacial and en-glacial drainage. I still maintain that revision of Mrs. Peake's ice margin positions is needed. For instance, the Sam Adda ice margin is depicted as falling from over 650 feet on the south of Halkyn mountain to 550 feet around Bellan, a gradient of 1 in 60; the Pont-Blyddyn ice margin falls from 600 feet on Halkyn mountain to only 400 feet east of Mold, a gradient of 1 in 90. Almost certainly these marginal gradients are too steep for the Irish Sea Ice-sheet which is here involved. And if the PontBlyddyn ice margin stood at only 400 feet east of Mold, how did the ice at this time also cover the nearby Buckley and New Brighton uplands, which reach over 550 feet? The form of the ice cover at Mrs. Peake's Pont-Blyddyn stage is in fact very difficult to picture. Mrs. Peake also raises the matter of multi-glaciation. There can be no doubt that several glaciations of varying magnitude have affected the area. Some evidence relating to earlier glacial episodes is given by Mrs. Peake, though I do not share her view that the Padeswood gravels belong to a moraine deposited by a late readvance of the Irish Sea Ice. However, the papers of Derbyshire and myself were concerned first and foremost with the landforms and deposits associated with the last deglaciation, which hardly merits the designation of a 'minor glacial episode' since it involves the existence of Irish Sea Ice in the area up to approximately 700 feet in height. There is much scope for further research in the glaciation of north-east Wales. We know, for instance, little about the form and extent of the cover of Welsh Ice in the last main glaciation. Concerning the last deglaciation, it seems to me that there is evidence of both sub-aerial and sub-glacial meltwater drainage; there were undoubtedly some small pro-glacial lakes, but I am not convinced that it was the outflow of these which was responsible for the creation of large meltwater channels such as Sam Adda. In the later stages of deglaciation, the ice cover seems to have been in a stagnant condition so that meltwater was usually draining through or beneath the ice in preference to adopting sub-aerial routes. CLIFFORD EMBLETON
Department ofGeography Bedford Co/lege London N. w.i
104 Old Farleigh Road Selsdon, Surrey REPLY BY THE AUTHOR
Mrs. Peake raises first a number of points concerning the Sam Adda meltwater channel and, on her interpretation, the associated lake delta deposits in the Bellan area. As she observes, the highest delta surface at Bellan had an outer edge at about 500 feet before gravel working commenced, but the surface rises to a little over 530 feet around Bryn-Bellan. Between here and the Sam Adda channel four miles to the south, there are no signs of lake deposits but merely a very narrow and doubtful terrace feature at 500-525 feet near the Mold-Gwernymynydd road, and at about 520-535 feet a little to the south. There is no proof that the lake in which the Bellan delta formed was other than a local one, or had a long extension for four miles to connect directly with the Sam Adda channel. I prefer to envisage the ice margin between the Bellan lake and Sam Adda as one closely following the hill-side at a height above 500 feet, and I find no evidence that marginal drainage followed it continuously. I see no reason why deltas laid down in local pro-glacial lakes should not possess a well-defined form. The Sam Adda channel is clearly cut in a broad open col of probably pre-glacial origin between Leeswood Hill and the Treuddyn spur. Mrs. Peake agrees that ice must have been present in this col before the channel was cut. She suggests that the channel was initiated by meltwater flowing at about 600 feet marginally beside ice occupying the col, and that the channel was subsequently deepened as a spillway, but this does not explain the anomalous long profile of the channel. The hypothesis that the channel developed as a sub-glacial col-gully has the advantage of accommodating this point, and admits of comparisons with several other similar channels in north-east Wales. There is no reason to dispute the
214
suggestion that the excavation of the channel may have commenced in an earlier glaciation, the channel being deepened in later glaciations; this surely applies to many other channels. Mrs. Peake doubts whether stagnant ice persisted in the Mold Valley. Yet she does not mention the Padeswood gravels a little downstream from Mold, which are full of kettle holes and very characteristic of dead-ice topography. It is by no means impossible that other dead-ice deposits now lie buried in the Mold Valley by the most recent alluvium. Certainly they are abundant and impressive in the Wheeler Valley. Moreover, general considerations of ice thicknesses show that the ice in the Mold area must have been stagnant during most of the stages enumerated by Mrs. Peake; even at the 'Leeswood Stage' there can, according to Mrs. Peake's ice margin positions, have been little more than 100-150 feet of ice in the Mold Valley. Cessation of movement and stagnation were inevitable, and conditions probably very favourable to sub-glacial and en-glacial drainage. I still maintain that revision of Mrs. Peake's ice margin positions is needed. For instance, the Sam Adda ice margin is depicted as falling from over 650 feet on the south of Halkyn mountain to 550 feet around Bellan, a gradient of 1 in 60; the Pont-Blyddyn ice margin falls from 600 feet on Halkyn mountain to only 400 feet east of Mold, a gradient of 1 in 90. Almost certainly these marginal gradients are too steep for the Irish Sea Ice-sheet which is here involved. And if the PontBlyddyn ice margin stood at only 400 feet east of Mold, how did the ice at this time also cover the nearby Buckley and New Brighton uplands, which reach over 550 feet? The form of the ice cover at Mrs. Peake's Pont-Blyddyn stage is in fact very difficult to picture. Mrs. Peake also raises the matter of multi-glaciation. There can be no doubt that several glaciations of varying magnitude have affected the area. Some evidence relating to earlier glacial episodes is given by Mrs. Peake, though I do not share her view that the Padeswood gravels belong to a moraine deposited by a late readvance of the Irish Sea Ice. However, the papers of Derbyshire and myself were concerned first and foremost with the landforms and deposits associated with the last deglaciation, which hardly merits the designation of a 'minor glacial episode' since it involves the existence of Irish Sea Ice in the area up to approximately 700 feet in height. There is much scope for further research in the glaciation of north-east Wales. We know, for instance, little about the form and extent of the cover of Welsh Ice in the last main glaciation. Concerning the last deglaciation, it seems to me that there is evidence of both sub-aerial and sub-glacial meltwater drainage; there were undoubtedly some small pro-glacial lakes, but I am not convinced that it was the outflow of these which was responsible for the creation of large meltwater channels such as Sam Adda. In the later stages of deglaciation, the ice cover seems to have been in a stagnant condition so that meltwater was usually draining through or beneath the ice in preference to adopting sub-aerial routes. CLIFFORD EMBLETON
Department ofGeography Bedford Co/lege London N. w.i