Fiftytwo

Average glacial conditions and the landscape of Snowdonia

Danny McCarroll

Department of Geography, University of Wales Swansea, Swansea,

The Snowdonian mountains of North Wales hold a special place in the history of glacial geomorphology. It was here that Darwin (1842) was convinced of the former existence of glaciers and of their role in shaping the landscape. In the 19th century, it was the battleground of the 'glacialists' and 'diluvialists', with evidence such as the high-level shelly drift of Moel Tryfan, in the Snow-donian foothills, playing a key role in the debate (Campbell & Bowen, 1989). The glacial cirques (cwms in Wales) and spectacular U-shaped valleys of the region have become classic examples of glacial erosion and generations of geographers, at all levels, have used the area as a classroom. Recent work, however, calls into question the established interpretation of the landscape.

The evidence for glaciations of Snowdonia was reviewed by Whittow & Ball (1970). They argued that at the maximum extent of the last glaciation, ice from a major ice centre located to the southeast of Snowdonia was sufficiently powerful to push straight through the mountains, carving the spectacular U-shaped valleys of the Pass of Llanberis (Fig. 52.1) and the Ogwen Valley. This interpretation has been promulgated by Addison (e.g. 1990), whose works have become standard fare to the many school and university parties visiting the region. The evidence was first questioned by Gemmell et al. (1986), who showed that geomorpho-logical evidence around the triple junction at the southern end of the Pass of Llanberis indicated radial drainage away from the Snowdon (Yr Wyddfa) massif during the last glaciation. Addison (1990, p. 12) countered that although the basal ice might radiate, 'transfluent ice high up in the outflow is believed to have breached the mountains at a number of sites, excavating the major outlet troughs of Nant Ffrancon and Llanberis pass'. This argument requires ice from the southeast to override the mountains and predicts that evidence of ice directions high on the mountains should indicate flow dominantly towards the northwest.

McCarroll & Ballantyne (2000) investigated the evidence for ice thickness in Snowdonia, and for directions of ice movement. A periglacial trimline was mapped, separating areas showing evidence of glacial erosion from palaeonunatak summits with evidence of prolonged weathering. The evidence for former nunataks in Snowdonia proved more spectacular than perhaps anywhere else in Britain. The high peaks of the Glyder range, separating the Pass of Llanberis and the Ogwen Valley, host spectacular blockfields, including the great cantilevered slabs of Glyder Fach, but the lower slopes and cols, including that above Cwm Idwal, are clearly glacially eroded. The lower slopes of the Carned-dau, to the east, retain beautifully striated vertical faces to at least 700ma.s.l., but the high plateau above 850ma.s.l. is swathed in largely peat-covered blockfield punctured by relict tors, the finest example of which rises 8-12 m above the blockfield on Yr Aryg (865ma.s.l.). The interpretation of the trimline as the surface of the last ice sheet at its maximum is supported by the presence of gibbsite in relict soils on the weathered summits. Gibbsite is an end product of the weathering of silicates and in mountainous environments is regarded as evidence of a very prolonged period of weathering (Ballantyne et al., 1998a). The trimline cannot, therefore, represent the surface of a later readvance.

The altitude of the ice surface reconstructed by McCarroll & Ballantyne (1990), and the evidence for ice movement high on the mountains, provides no support for the argument that ice from the southeast has ever overridden the mountains of Snowdonia, or that the major valleys represent 'outlet troughs'. On the contrary, all of the evidence points to radial drainage of ice away from Snowdonia, even when the ice was at its thickest (Fig. 52.2). The periglacial trimline reaches 850ma.s.l. in the main mountain ranges of Snowdonia, including the Snowdon massif, the Glyders and the Carneddau, but only about 820ma.s.l. to the southeast on Moel Siabod. The ridge of Moel Siabod (Fig. 52.2) lies directly in the path of the ice purported to have overridden Snowdonia, but striae demonstrate that ice crossed it not from the south but from the west.

The geomorphological evidence in Snowdonia clearly demonstrates that the mountains remained an area of ice dispersal throughout the last glaciation, with no evidence of ice from the southeast ever having overridden them. If the Pass of Llanberis and Ogwen Valley were cut by ice flowing northwest through Snowdonia, they would have to have been cut in some previous

Figure 52.1 The Pass of Llanberis, looking to the northwest. Breaching of former ice-sheds, to produce the major valleys cutting through Snowdonia, can be explained by the action of local ice. There is no need to invoke ice from the southeast overriding the mountains. (See www.blackwellpublishing.com/knight for colour version.)

Figure 52.1 The Pass of Llanberis, looking to the northwest. Breaching of former ice-sheds, to produce the major valleys cutting through Snowdonia, can be explained by the action of local ice. There is no need to invoke ice from the southeast overriding the mountains. (See www.blackwellpublishing.com/knight for colour version.)

Figure 52.2 Digital elevation model (DEM) of Snowdonia, viewed from the southeast, showing the dominantly radial drainage away from the mountains of Snowdonia during the last glaciation. (DEM courtesy of Rob Davies.)

glaciation, when the ice from the southeast was very much more powerful. However, there is no direct evidence for such a major event and it is possible to explain these valleys as produced by the action of local ice.

It is a mistake to interpret a mountainous landscape like Snowdonia as the product only of major glaciations, with huge ice sheets, and of minor events such as the Younger Dryas (Loch Lomond Stadial) when only the cirques were occupied by ice. For most of Pleistocene time the climate of northern Europe has been much colder than today, but not cold enough to generate major ice sheets. Porter (1989) has stressed that these 'average' glacial conditions may be much more important in shaping landscapes than the infrequent and short-lived extremes. Under these 'average' conditions, Snowdonia's glaciers would radiate out of the cirques and into the valleys, varying in size as the climate fluctuated. In early glaciations the drainage would have been radial, with ice moving away from the mountains in all directions.

However, during 'major' glaciations, when ice was able to build up over the hills and plateaux to the southeast of Snowdonia, the southerly drainage of the Snowdonian ice would be constrained. As ice flow to the south was restricted, southerly flowing glaciers would thicken, allowing them to overflow their own ice-sheds, forming the major glacial breaches.

Most of the Snowdonian landscape, therefore, is tuned to 'average' glacial conditions, with ice flowing out of cirques and being evacuated radially via valley glaciers of varying size. The often-spectacular erosional landscapes of the cirques are not the product of minor events such as the Younger Dryas, when the cirques were occupied by small glaciers and glacial erosion was minimal, but have been occupied by ice for much of the Pleistocene. The major through valleys, which are clearly the product of breached ice-sheds, are the product of much thicker local ice constrained by the advance of ice from high ground in the southeast and also southwards down the basin of the Irish Sea.

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