A glaciated area in North Wales

Nant Ffrancon, Eryri

[This is the final post in the series on the glaciation of the British Isles. The previous posts can be found in the archive on the Substack app.]

Nant Ffrancon

The Nant Ffrancon region of North Wales exhibits features typical of a glacial eroded landscape, together with aspects of a periglacial landscape. Geomorphologists believe that the ice in this area could once have been up to an altitude of between 820m and 860m, though the last time ice was that extensive in the area may have been 40,000 BP with slightly less ice 20,000 BP.

Nant Ffrancon itself is a glaciated valley, or trough, that was produced by a north-westerly moving glacier flowing away from the central part of Snowdonia. Nant Ffrancon is a wide, flat-bottomed and straight valley, with typical glaciated features of truncated spurs and vertical crags caused by the former interlocking spurs having been clipped by the moving glacier. At the head of the valley there is a rock step, marked by a waterfall, Rhaedr Ogwen. This waterfall is 100m high due to the resistant hard igneous (granitic) rock of the area. At the lower end of the valley is an end moraine. This is where material was deposited by the moving glacier as it came to a standstill as the glacial budget was in equilibrium (accumulation equalling ablation). To the east lies the Ogwen Valley which also has several glacial features such as truncated spurs, a ribbon lake and diffluent cols and troughs. [Figure 1]

Figure 1. Main features of Nant Ffrancon and the Ogwen Valley

Development of the valleys and their landscape

The Nant Ffrancon and Ogwen valleys originally drained in opposite directions, with streams going north-west and east respectively. The watershed would have roughly been where Pen y Ole Wen is today. The reasons for the erosion of the watershed are subject to debate. One theory is that a large glacier emerged from Migneint Moor to the south, flowing north, and as the valley of the Ogwen Valley filled with ice it forced its way over the watershed into the Nant Ffrancon valley, producing what is known as a diffluent col and trough. An alternative theory is that the small glaciers emanating from the corries (cwms) immediately to the south and southwest of the valleys (Figure 1) eroded the watershed. It is difficult to identify precise evidence for either theory but as no erratics from the Migneint Moor area have been found here, this would support the latter theory.

The valleys exhibit common features of a glaciated landscape. At the head of the valleys are several corries, or cwms. However, whereas most textbook glacial corries are circular in shape, with round corrie lakes or tarns within them, here several corries do not fit in with this usual pattern. For example, Cwm Idwal is 1.5km in length and only 1km wide from arête to arête. It is also both deeper and lower in altitude than most other corries. Its floor is at an altitude of 375m, compared to the surrounding corries, Cwm Bochlwyd (550m), Cwm Cneifion (600m) and Cwm Clyd (660m). It is thought that the reason for these characteristics is that the col above it was low enough to allow ice to spill over from the Llanberis Pass. This meant that Cwm Idwal had greater volumes of ice in it and therefore would have been subjected to greater amounts of erosion. Furthermore, it lies along a line of geological weakness - the bedrock here is part of a syncline and so overall erosion would have been greater. Unusually though, it is surprisingly shallow with a 10m depth at its maximum but with an average depth of 3m. Its mouth is dammed by a small end moraine, and much of the land around it is hummocky. Hummocky moraine is a common feature in many areas of the two valleys.

Above the tarn in Cwm Idwal are steep, jagged and slanting rocks called the Idwal Slabs. These rocks are now heavily weathered creating screes at their base and they also create challenging rock-climbing potential. Above these rocks lie a series of knife-shaped edges, or arêtes.

Cwm Idwal

Annotated sketch of Cwm Idwal

Y Gribin is an example of an arête in another part of the area. It separates Cwm Cneifion and Cwm Bochlwyd. Both these corries were eroded back into the mountains and over time the glaciers in these cwms would have increased in size and begun to erode backwards into the mountain plateau, leaving the sharp ridge between them.

Truncated spurs are also a widespread feature of both valleys – an example is the end of Gribin Ridge near Cwm Idwal. As stated above, these mark the residuals of previously existing interlocking spurs.

Examples of glacial landforms in the Nant Ffrancon/Ogwen area

1. Glacial trough Nant Ffrancon

2. Truncated spurs Several on the sides of Nant Ffrancon

3. Arête Y Gribin (separates Cwm Bochlwyd and Cwm Cneifion)

4. Ribbon lake Llyn Ogwen

5. Corrie/Cwm/cirque Cwm Idwal

6. Hanging valley From Cwm Cywion (west of Nant Ffrancon)

7. Rock step Rhaedr Ogwen (western end of Llyn Ogwen ribbon lake)

8. Misfit stream Afon Ogwen

Post glacial modification

Post glacially, meltwater filled the valley to produce a ribbon lake in the over-deepened Nant Ffrancon valley. This ribbon lake had a natural dam to the north-west caused by a rock barrier. This rock barrier is close to the present-day town of Bethesda. As the lake filled with sediments from the streams flowing into the sides of the lake, it produced a flat floor. Today an unexpectedly small stream, called a misfit stream, meanders across the flat floor. Down the sides of the valley there are typical features of triangular shaped alluvial fans deposited by streams flowing off the mountains on either side. These streams slow down significantly and hence shed their load to create the fans. Another common feature is the number of deep, steep sided gorges cut back into the valley sides as waterfalls marking the location of hanging valleys retreat.

Since the end of the Ice Ages, there have been other modifications caused by the periglacial and subsequent processes affecting the area:

· Periglacial – extreme cold temperatures that continued to occur after the ice retreat have created features like those of classic periglacial areas, such as stone stripes and circles (also known as patterned ground). For a period there would have been permafrost in the area.

· Weathering – the usual frost shattering processes that result from alternate freeze-thaw cycles during the long winters that this area experiences have created extensive areas of scree on the slopes below ridges and arêtes. Higher up the slopes and on the mountain summits, collections of larger rock fragments have created blockfields. These have occurred due to the solifluction processes that took place in periglacial times allowing the transport of large rocks by the slow-moving saturated ground during the warmer periods. It is thought that the large blockfields on the Glyder and Carneddau summits would have been above the glacial ice and were therefore shaped by extreme cold and periglacial processes but were not scoured by ice. The clear break between the slightly lower ice scoured and rounded ground and these higher up rocky blockfields provides clear evidence of the depth of the ice described earlier.