Difference between revisions of "Pleistocene and Recent deposits - Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75"

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From: Howells, M. F., Francis, E. H., Leveridge, B. E. and Evans, C. D. R. 1978 Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75 Classical areas of British geology, Institute of Geological Sciences. (London: Her Majesty's Stationery Office.)

Map: Sheet SH 75 Capel Curig and Betws-y-Coed. 1:25 000 series - Classical areas of British geology]

Under construction

Figure 27 Llyn Elsi south-west of Betws-y-Coed.
Figure 28 Swallow Falls near Betws-y-Coed.

Chapter 9 Pleistocene and Recent deposits[edit]

During the Pleistocene, North Wales suffered severe glaciation, the complex history of which IS s still not fully elucidated (Table 1; Whittow and Ball, 1970). Its main effect in the Capel Curig–Betws-y-Coed district was of intense erosion. Glacial drift deposits are relatively thin, of restricted extent and are entirely the products of local ice during the last glaciation (Devensian, Wiirm or Weischel).

The movement of the ice and its drainage was in a general easterly to north-easterly direction, towards the Conway Valley. A planation between 274 and 305 m on the eastern side of the district is also recognised east of the Conway Valley and indicates that the main ice overrode the valley from the west. For this reason, Warren and others (in preparation) maintain that the valley exerted only local control over an integral North Welsh ice-sheet, contrary to Embleton's (1961) postulate of a Conway Glacier. However, the profiles of the Llugwy and Lledr valleys suggest that in part they were overdeepened by subglacial drainage rather than glacial scouring. Moreover, where the valleys are clearly glacially scoured, as immediately west of Betws-y-Coed and in Glyn Lledr, good hanging-valley features from the south-west are preserved at Rhiwddolion and Gwibernant respectively. The proximity of these features to the Conway Valley would support the existence of a Conway Glacier, albeit at a late stage. The only high-level cwm in the district is that on Moel Siâbod (872 m OD), facing east and occupied by a lake – Llyn-y-Foel (535 m OD). Most of the exposed crags show clear evidence of glacial scouring. The broken ridges of volcanic rocks, on the northern limb of the Dolwyddelan Syncline and the prominent feature of the Garth Tuff at Plas-y-Brenin show good examples of roches moutonnees.

Boulder clay is widespread although it forms moderately extensive tracts only near Dol-Llech in the Llugwy Valley, on the smooth slopes between 700 and 900 ft OD N of Dolwyddelan and in the valley of the Afon Machno. Elsewhere it locally fills hollows, as in the valley about Rhiwddolion. It consists typically of grey to greyish brown tenacious clay with a high proportion of pebbles and boulders, all of local origin.

Fluvioglacial sand and gravel deposits are preserved only as small remnants of high terraces, near Plasglasgwm, near Penmachno, and in the Lledr Valley, near Gethin's Bridge.

During the period of deglaciation meltwaters were periodically dammed in shallow lakes, many of which (such as Llynnau Mymbyr and Llyn Goddionduon) have survived naturally to the present day. Others were re-established by man during the 19th century and utilised as sources of power, as settling pools in the mining area about Sarnau, and for water supply, as at Llyn Elsi. All that now remains of some of these lakes, however, are marshy peat tracts or mires, as at Cors Geuallt, north of Bryn-tyrch-uchaf. Diatomaceous muds have been recorded at Llynnau Mymbyr, Cors Geuallt, Llyn Sarnau and Llyn Elsi (Thomas, 1972) (Figure 27). At Llyn Sarnau, peat up to 2.5 m with reed and wood remains forms a layer over most of the lake bed, covering a saucer-shaped deposit of peaty diatomaceous mud, generally about 1 m thick, which in turn overlies fine grey glacial clay with slate fragments. Crabtree (1966) examined the deposits at Cors Geuallt and found evidence for both the Allerød Phase and an earlier warm phase which may possibly represent the Bölling Oscillation.

Exposed slopes were subject to solifluction in the Late-Glacial and Post-Glacial stages. Most have a mantle of ochreous weathered clay with angular to subangular rock fragments and isolated well-rounded boulders. In Cwm Penamnen, it is possible to limit the deposits below the rock scarps on the steep valley slopes and above the alluvium.

The screes of the district were mainly formed in periglacial conditions during the Late-Glacial Period. It is only on the higher ground on Moel Siâbod, above the cwm of Llyn-y-Foel, that they have continued to accrue to the present day. On lower ground, they are stabilised and so mixed with hillwash and overgrown that they cannot be delineated; a scree below the east-facing scarp of Grinllwm Slates in Coed-yr-Allt Goch, north of Betws-y-Coed (Ball, 1966) is an example which was dense forest at the time of mapping.

The main alluvial tract extends along the River Conway, about Betws-y-Coed. It includes well-defined terraces 3 and 6 m above the river, the higher probably having been deposited by the Mon Llugwy which joins the Conway, 1 km NNE of the station. The alluvium, commonly exposed in the eroded banks of the river, consists of clayey sand overlying coarse gravels. Smaller terraced alluvial tracts occur near Penmachno and around Mon Lledr near Dolwyddelan village. At Blaenau Dolwyddelan the low-angled profile of a cone, encroaching on the flat, suggests that the flat was lacustrine at some time. Waterfalls or rapids occur in both the Llugwy and Lledr valleys. In the Llugwy Valley these nick points are encountered at Pont-y-Pair (30.5 m OD), near Miners' Bridge (91 m OD), Swallow Falls (122 m OD) (Figure 28) and Cyfyng Falls (183 m OD). Similarly four nick points can be determined in Mon Lledr between Pont-y-Pair and Glan-y-Wern though at slightly different levels. Before they were breached these obstructions would have held the meltwater in small lakes and it is possible that the alluvial flats now seen on their upstream side were, in part, lacustrine.