WRITTEN CONTRIBUTION TO THE DISCUSSION OF A PAPERPREVIOUSLY TAKEN AS READ

The EditorDear Sir,

In a recent paper (Derbyshire, 1962) the late-Glacial drainage in north-east Waleswas discussed, with particular reference to the Wheeler Valley and the adjacentpart of the Lower Alyn Valley . Whilst welcoming further study of a districthitherto somewhat neglected I wish to reply to the criticisms of my recent paper(Peake, 1961), and to point out some of the weaknesses in this new hypothesis.

In an area where direct ice deposits and resorted material are frequently sointermixed as to be practically indistinguishable, the absence of any reference tothe Boulder Clay and its important erratic content is a serious omission in thispaper. Moreover, the opinion that the gravels are of local origin (Derbyshire,1962,327) is at variance with published surveys dating back to the earliest workof Mackintosh (1873). Reports on the content of both the Boulder Clay and thesands and gravels have invariably shown that among the local detritus is aproportionately small but very significant amount of far-travelled material(Wedd & King, 1924, 144). The contrast between Boulder Clay containingerratics from Welsh sources far to the west and that with erratics from distantnorthern sources permits a rough division between the areas dominated by Welshice moving eastward in general and Irish Sea ice moving south-eastward. Never­theless, as stated in my paper (Peake, 1961,340), resorted material of both typesis to be found east and west of the line at varying heights above sea-level. In viewof this , in the description of the sands and gra vels of the Pant Gwyn Valley, thereference to my paper (Derbyshire, 1962, 328) is incomprehensible, the more sobecause nowhere in my paper did I touch on the composition of the sands andgravels of the Pant Gwyn Valley. I have found, however, that these deposits, likeall the sands and gravels occurring in scattered patches across the high groundfrom Moel-y-crio (199696) to Gop Hill (087802), contain northern erratics andmarine shells, the erratics also occurring throughout the gravel train that followsthe Afon Pant Gwyn down into the Wheeler Valley.

Research in glaciology abroad as well as in Britain has produced irrefutableevidence of a number of glacial advances and retreats within the Pleistocene; thussome reference to the effects of earlier glaciations seems indispensable in a studyof what are mainly the effects of the final retreat-yet with the exception of asingle reference to inter-Glacial valleys (Derbyshire, 1962,331) there is no intro­duction of this important aspect. Indeed, the statement (Derbyshire, 1962,333)that the dra inage returned to the pre-Glacial valleys implies a monoglacia!approach. No mention is made of the three glacial gorges prominent in FigureI, i.e. the two limestone gorges west of Rhydymwyn (205669) and the BellanGorge to the south, all too deep to have been excavated in a single retreat.Substantial evidence for ice retreats pre-dating the last retreat is to be found inthe area. (Peake, 1961).lt has been shown (Embleton, 1957b, 258) that during thePleistocene the Upper Alyn was permanently diverted through the Alyn Gorgeat Rhydyrnwyn. In the Hope Mountain area (Peake, 1961, 338) the Cegidog wassimilarly diverted southward through a 2oo-foot gorge in limestone, and theLower Alyn was diverted to cross two prominent rock spurs of Hope Mountain.These rivers did not return to their pre-Glacial valleys, and their striking gorgescould scarcely have been cut by subglacial stream action; thus the likelihoodemerges that sub-aerial marginal drainage involving ponded diversions was arecurring feature of the glacial retreats in north-east Wales .

367

368

It cannot be denied that a proportion of the meltwater, increasing withstagnation, repeatedly escaped through or beneath the ice-sheets. North-eastof Mold the peculiarities of the drift topography around Northop may well haveoriginated in subglacial channels. In the Lower Alyn Valley, however, I supportthe view of Embleton (l957a) that a series of pro-glacial lakes and overflowchannels was in operation in the final major retreat of an Irish Sea ice-sheet.Comparison of his map (Embleton, 1957a, fig. 5) with mine (Peake, 1961, pl.XIII) shows that our views on the positions of ice-fronts and lakes in the Moldarea differed, my conclusion being that the drainage direction was in generalsouth-eastward, as shown by the dip of the beds and the form and placing of thedeltaic terraces and spillways. Contrary to the statement that the sands and gravelsare in general a pitted deposit (Derbyshire, 1962, 330), in the area of the MoldLake no kettle-holes occur. It would be especially difficult to imagine how thebeautiful little series of deltaic terraces at the mouth of the Bellan Gorge couldhave originated either subglacially or round stagnant ice. Its progressive forma­tion during the excavation of the gorge was clearly linked with the uncovering ofspillways to the south.

In the Wheeler Valley, not included in my detailed survey (Peake, 1961), Iagree that north-westward from the Nannerch locality the sand and graveldeposits are more in the nature of kame terraces built out on either side of ashrinking ice-tongue. The same is true of the Upper Alyn Valley, where theWelsh ice melted to a valley glacier (cf. Peake, 1961,340). Between Nannerch andStar Crossing, however, the sands and gravels assume a definite terrace aspect atroughly 600 feet a.D., continuing on either side of the narrowing valley acrossthe watershed, where in Pen-bedw Park they have been trenched down to thelimestone by a flat-floored channel. Evidence for the strong south-eastward flowof water from the Wheeler Valley over into the Alyn system by way ofthe HendreGorge is seen in the presence of the bright red sand carried through from theTriassic outcrops of the Vale of Clwyd, the only possible source (Wedd & King,1924, 148). The ice did not bring it, for the Boulder Clay lining the WheelerValley beneath the sand and gravel kames is almost invariably a tough yellow­grey till, packed with local Silurian and Lower Carboniferous detritus and theoccasional Welsh erratics; very seldom are red day bands seen. In the main themasses of red sand were certainly waterborne, and could scarcely have beenforced uphill through the Pass of Bodfari underneath the ice, to be ejected oneither side of the glacier as pitted hummocks on the steep hill-slopes. Red sandyclay derived from the low-lying Triassic outcrops in the Vale of Clwyd has beencarried far up on the western flanks of the Clwydian Range by Welsh ice movingeastward; in my viewthe bulk of the red sands of the Wheeler Valley were washedout of these deposits or from crevasses, to be transported by marginal drainagealong the edge of the shrinking Clwydian ice-mass, a tongue of which extended upthe Wheeler Valley. The flow of meltwater was continued through into the Alynsystem, first by a series of lakes (Peake, 1961, pI. XXIII), where the red sandaccumulated in the deltaic terraces, and finally as a torrent which temporarilyswept the valley floor from Star Crossing to Mold.

With regard to the sequence of marginal channels which carried the drainagefrom East Halkyn southwards into the Lower Alyn Valley (Peake, 1961, 345),no other system could account satisfactorily for the great channelled accumulationof sand and gravel on the south-eastern flanks of the mountain ridge. The some­what spectacular gulch at Sarn Galed, suggested to have initially carried all thedrainage of the vicinity subglacially (Derbyshire, 1962, fig. I, col gully 1), simplyresulted from a deep land-slip into the main trench valley. In crossing the

369

anticlinal saddle of high ground linking Halkyn Mountain and Warren Mountainthe Flint-Alyn trench valley follows the base of the Middle Coal Measures(Peake , 1961, 347). Thus its high western wall, drift-plastered for much of itslength and locally very steep, is formed by the more resistant Holywell Shales.The outcrop is reinforced by a broken rampart of the fault-smashed GwespyrSandstone, which occurs at the top of the Holywell Shales, though here muchreduced in thickness from its massive character farther north. In the last ice­retreat from East Halkyn there was intermittent collapse of the eastern wall ofthe high Sarn Galed channel where there were gaps in the sandstone rim. In somecases the fallen drift was carried southward in the scouring of the main trenchvalley a hundred feet below, a meltwater excavation which admittedly may havecommenced under shelving ice when the direct route southward into the MoldLake was opened. Elsewhere the slipped masses of more recent date are stilldiscernible at the foot of the slopes (cf. Wedd & King, 1924, 113).

The description of the Sam Adda col (Derbyshire, 1962, fig. I, col gully 2, 332)is yet another example of the monoglacial approach. Previously in operation atleast once, this channel lies at the head of a deep valley which continues throughto the Cegidog Gorge (Wedd & King, 1924, 153). There is little doubt of theglacial origin of this valley (Peake, 1961 ,338), but its gorge-like nature and greatdepth farther south preclude a subglacial origin. What is meant by a 'natural' col(Derbyshire, 1962,331) is not made clear, but when the last Irish Sea ice-retreatcommenced the Sarn Adda channel was already a glacial trench little shallowerthan it is today, as proved in the colliery workings. Its relatively gentle downstreamgradient cannot be compared with the spectacular drop of the Sarn Galed landslip,with which the channel is grouped.

Some of the channels of the second group, the sub-glacial chutes (Derbyshire ,1962, fig. I), are post-glacial ravines ; others link sections of demonstrably differ­ing stages, e.g. no. 4. The head section trending southward from Halkyn Moun­tain can be seen in the field to form a hanging valley above the newer sectioncurving round from Sarn Galed (cf. Peake, 1963, pI. XIII). Indeed , the smallrejuvenated stream issuing from the higher channel has notched the lip in astriking little ravine a quarter of a mile long. Further, it is difficult to visualise theexcavation by meltwater of the very deep and now abandoned Gwysaney Halldepress ion (Derbyshire, 1962, 333), if, as depicted in Fig. I, S, a subglacial chutewas operating at right angles at its head .

From the evidence of northern drift occurring on the crest of the Halkynridge it may be deduced that, late in the Pleistocene, Irish Sea ice in the Deeestuary had a surface at least a thousand feet above sea level. Similar depositsat comparable heights can be found on the uplands of the north Welsh border asfar south as the Longmynd . Along the foothills of north-east Wales the southerlydiversion of the main drainage suggests that Irish Sea ice was repeatedly themain barrier to the escape of meltwater.

REFERENCESDERBYSHIRE, E. 1962. Late-GlacialDrainage in Part of North-East Wales: An Alterna­

tive Hypothesis. Proc. Geol. Ass ., Lond., 73, 327-34.EMBLETON, C. 1957a. Late-Glacial Drainage in Part of North-East Wales. Proc. Geol.

Ass., Lond., 67, 393-404.---. 1957b. SomeStagesin the Drainage Evolutionof Part of North-East Wales.

Trans. Inst, Brit. Geogr ., 23, 19-35.MACKINTOSH, D. 1873. Observationson the More Remarkable Bouldersof the North­

West of England and the Welsh Borders. Quart. J. geol . Soc., Lond., 29,351-60.

370

PEAKE, D. S. 1961. Glacial Changes in the Alyn River System and their Significancein 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.

D. S. PEAKE

104 Old Farleigh RoadSelsdonSurrey

REPLY BY THE AUTHOR

In reply to Dr. Peake's discussion of my paper on the deglaciation of the Alynand Wheeler Valleys, it is necessary to stress that it attempted to review the groundevidence left by the last ice to occupy this small area. Evidence for the diversionof drainage in an earlier ice-retreat, summarised by Embleton (1957), is notreviewed, although I remain critical of the assumption that it was necessarilymarginal. It is not enough merely to state that subglacial meltwaters could scarcelyhave been the major factor in the formation of some diversion gorges. Moreover,the unqualified statement that the two limestone gorges west of Rhydymwyn andthe Bellan gorge are 'too deep to have been excavated in a single retreat' begs amultitude of questions, among them the problem of the rate and periodicity ofthe retreat. Severe modification of pre-Glacial (or inter-Glacial) cols and streamvalleys by subglacial torrents has been accepted in Europe for many years andexamples 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 StarCrossing, the kame deposits clearly indicate abundant meltwater in associationwith immobile ice. Ice gradients, at least until the ice surface was reduced to600 feet O.D., appear to have been from west to east, a situation consistent withenglacial 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 forthe view that the drainage was marginal, the waters lacustrine, and the beddingdeltaic. A demonstration of the reality or otherwise of the supposed terracingat 600 feet O.D. is still lacking.

Two small misreadings of my paper should be mentioned. My reference toDr. Peake's paper (Derbyshire, 1963,328) concerns the morphology of the gravels,not their content. Secondly, subglacial meltwaters cut the surface of the gravelsin the Gwysaney Hall col, which is neither mapped nor referred to as a glacialdrainage channel (Derbyshire, 1963,333).

Dr. Peake's contribution on the character of the unsorted drift fills a gap inmy paper. I agree that while the f1uvioglacial deposits in the area in question areoverwhelmingly 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 erraticcontent.

The paucity of channelling, the lack of correlation between channel heightsand proposed lake levels, the lack of channelling at the outlets of some proposed