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370 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. WEDD, C. B. & W. B. R. KING. 1924. Geology of the Country around Flint, Hawarden, and Caergwrle. Mem, geol. Surv. U.K. PEAKE,
D. S. PEAKE
104 Old Farleigh Road Selsdon Surrey REPLY BY THE AUTHOR
In reply to Dr. Peake's discussion of my paper on the deglaciation of the Alyn and Wheeler Valleys, it is necessary to stress that it attempted to review the ground evidence left by the last ice to occupy this small area. Evidence for the diversion of drainage in an earlier ice-retreat, summarised by Embleton (1957), is not reviewed, although I remain critical of the assumption that it was necessarily marginal. It is not enough merely to state that subglacial meltwaters could scarcely have been the major factor in the formation of some diversion gorges. Moreover, the unqualified statement that the two limestone gorges west of Rhydymwyn and the Bellan gorge are 'too deep to have been excavated in a single retreat' begs a multitude of questions, among them the problem of the rate and periodicity of the retreat. Severe modification of pre-Glacial (or inter-Glacial) cols and stream valleys by subglacial torrents has been accepted in Europe for many years and examples from Britain have been recognised recently (e.g. Sissons, 1960, 143; 1961,21, fig. 8). Large examples commonly have very low gradients. While repeating that there is no evidence of late-Glacial channelling at Star Crossing, the kame deposits clearly indicate abundant meltwater in association with immobile ice. Ice gradients, at least until the ice surface was reduced to 600 feet O.D., appear to have been from west to east, a situation consistent with englacial water movement to the east in the upper Wheeler Valley (cf. Derbyshire, 1963,333). On the other hand, no evidence of substance has yet been offered for the view that the drainage was marginal, the waters lacustrine, and the bedding deltaic. A demonstration of the reality or otherwise of the supposed terracing at 600 feet O.D. is still lacking. Two small misreadings of my paper should be mentioned. My reference to Dr. Peake's paper (Derbyshire, 1963,328) concerns the morphology of the gravels, not their content. Secondly, subglacial meltwaters cut the surface of the gravels in the Gwysaney Hall col, which is neither mapped nor referred to as a glacial drainage channel (Derbyshire, 1963,333). Dr. Peake's contribution on the character of the unsorted drift fills a gap in my paper. I agree that while the f1uvioglacial deposits in the area in question are overwhelmingly of local origin, far-travelled material can be found. But the presence of these northern erratics in no way invalidates my general hypothesis. This views the later phases of the deglaciation as essentially a period of downmelting of the ice cover which came increasingly under morphological control, especially in the narrow valleys, until stagnation (local certainty, regional possibly) set in, accompanied by drainage which was dominantly englacial and subglacial. This applies to all ice in the Alyn-Wheeler area, regardless of erratic content. The paucity of channelling, the lack of correlation between channel heights and proposed lake levels, the lack of channelling at the outlets of some proposed
371 lakes, the undoubtedly pitted character and wide altitudinal extent of the fluvioglacial deposits and the complete absence of lake-floor deposits is consistent with this view. If pro-glacial lakes existed during the deglaciation of this small tract, they were localised and very ephemeral, for they have left no undoubted trace.
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. 1957. Some Stages in the Drainage Evolution of Part of North-East Wales. Trans. Inst, Brit. Geogr., 23, 19-35. SISSONS, J. B. 1960, 1961. Some Aspects of Glacial Drainage Channels in Britain. Scot. geogr. Mag., 76, 131, 146; 77, 15-36. E. DERBYSHIRE
Department of Geography Newcastle University College Tighe's Hill, 2N New South Wales
D. S. PEAKE
104 Old Farleigh Road Selsdon Surrey REPLY BY THE AUTHOR
In reply to Dr. Peake's discussion of my paper on the deglaciation of the Alyn and Wheeler Valleys, it is necessary to stress that it attempted to review the ground evidence left by the last ice to occupy this small area. Evidence for the diversion of drainage in an earlier ice-retreat, summarised by Embleton (1957), is not reviewed, although I remain critical of the assumption that it was necessarily marginal. It is not enough merely to state that subglacial meltwaters could scarcely have been the major factor in the formation of some diversion gorges. Moreover, the unqualified statement that the two limestone gorges west of Rhydymwyn and the Bellan gorge are 'too deep to have been excavated in a single retreat' begs a multitude of questions, among them the problem of the rate and periodicity of the retreat. Severe modification of pre-Glacial (or inter-Glacial) cols and stream valleys by subglacial torrents has been accepted in Europe for many years and examples from Britain have been recognised recently (e.g. Sissons, 1960, 143; 1961,21, fig. 8). Large examples commonly have very low gradients. While repeating that there is no evidence of late-Glacial channelling at Star Crossing, the kame deposits clearly indicate abundant meltwater in association with immobile ice. Ice gradients, at least until the ice surface was reduced to 600 feet O.D., appear to have been from west to east, a situation consistent with englacial water movement to the east in the upper Wheeler Valley (cf. Derbyshire, 1963,333). On the other hand, no evidence of substance has yet been offered for the view that the drainage was marginal, the waters lacustrine, and the bedding deltaic. A demonstration of the reality or otherwise of the supposed terracing at 600 feet O.D. is still lacking. Two small misreadings of my paper should be mentioned. My reference to Dr. Peake's paper (Derbyshire, 1963,328) concerns the morphology of the gravels, not their content. Secondly, subglacial meltwaters cut the surface of the gravels in the Gwysaney Hall col, which is neither mapped nor referred to as a glacial drainage channel (Derbyshire, 1963,333). Dr. Peake's contribution on the character of the unsorted drift fills a gap in my paper. I agree that while the f1uvioglacial deposits in the area in question are overwhelmingly of local origin, far-travelled material can be found. But the presence of these northern erratics in no way invalidates my general hypothesis. This views the later phases of the deglaciation as essentially a period of downmelting of the ice cover which came increasingly under morphological control, especially in the narrow valleys, until stagnation (local certainty, regional possibly) set in, accompanied by drainage which was dominantly englacial and subglacial. This applies to all ice in the Alyn-Wheeler area, regardless of erratic content. The paucity of channelling, the lack of correlation between channel heights and proposed lake levels, the lack of channelling at the outlets of some proposed
371 lakes, the undoubtedly pitted character and wide altitudinal extent of the fluvioglacial deposits and the complete absence of lake-floor deposits is consistent with this view. If pro-glacial lakes existed during the deglaciation of this small tract, they were localised and very ephemeral, for they have left no undoubted trace.
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. 1957. Some Stages in the Drainage Evolution of Part of North-East Wales. Trans. Inst, Brit. Geogr., 23, 19-35. SISSONS, J. B. 1960, 1961. Some Aspects of Glacial Drainage Channels in Britain. Scot. geogr. Mag., 76, 131, 146; 77, 15-36. E. DERBYSHIRE
Department of Geography Newcastle University College Tighe's Hill, 2N New South Wales