Pen y Fan & Cwm Llwch
My second post about the highest mountain in southern Britain focusses on one of the corries that nestle below its northern escarpment. Recognised by the Nature Conservancy Council during the Geological Conservation Review in the 1980s as the best example of a corrie in the Beacons, Cwm Llwch provides a great deal of geographical, as well as photographic interest. (The NCC was a QUANGO which later devolved into three statutory bodies representing conservation for each of the nations of the United Kingdom, and which were recently ‘rationalised’ as part of the government’s spending cuts.) This is one of the quieter approaches to the mountain that draws in something in excess of 160,000 visitors each year, as result of the relative remoteness and inaccessibility of the Car Park plus the steepness of the climb. Indeed from here, you face a steady incline of around 35% for more than a kilometre and a half until you reach the corrie rim when it eases to a measly 20%, so you can see it often puts people off!
But for those that do venture up here, the reward is stunning views, a mystical lake and a real sense of the scale of the last ice age (for those that are willing/able to wander off the path a little more, there are also some spectacular waterfalls and a former rifle range to explore). This whole valley is the classic palimpsest landscape, with each geomorphological agent modifying the product of the one before.
Originally this would probably have been a relatively flat plateau, the result of the regular layering in sub-tropical deltas that became the old red sandstone. This plateau would have slowly developed a drainage network, starting with gullies, which became streams, and then rivers dissecting the landscape. When the climate cooled, these valleys protected fallen and wind-blown snow, allowing glaciers to form, which then eroded the valleys further. Ice ages waxed and waned, with fluvial action (i.e. by rivers) in between, so that by the end of the last ice age, there were well established glacial troughs radiating out from the former ice cap that had formed here. When the climate cooled briefly again (the Loch Lomond Stadial/readvance), snow turned to ice in the valley once more, and for 1,000 years a small glacier occupied the north east facing slope of the valley. Because this was a comparatively short-lived cold snap, this glacier didn’t get very far, compounded by the fact that it was probably frozen to its bed for most of the time so didn’t slide very much (yes, glaciers normally do slide a bit -cms a day during the summer, when the sole is lubricated by meltwater), and therefore didn’t escape the trough. This leaves a moraine ridge (bits of rock deposited at the edge of the glacier) some 20-30m high in the upper reaches of the valley at the foot of a cliff made steeper and more unstable by the glacier. This moraine traps water draining off the upper slopes, and a lake forms, which in time breaks through the dam, and a stream forms, cutting down through the glacial deposits and, in time, depositing them lower down. The cliff is gradually broken down by landslides, so that it isn’t as steep as it once was, leaving rusty red scree slopes. The other side of the valley is also typified by mass movement deposits, which formed from rockfalls made more frequent by the freezing and thawing of ice in the cliff-face at the time when the glacier was working away, creating a dome (once more of a cone) below Pen y Fan itself . That too has been worked on since the big thaw, by further mass movement and flowing water.
Since that time, we’ve probably had the biggest effect on the landscape, bringing in livestock, gradually clearing the woodland and creating the grassy moorlands you see today, it’s only in letter days we’ve come to use the landscape for recreation, bringing footpath erosion and associated issues. I find it fascinating that the landscape can show you all that, and that the ice was once so extensive, filling, and even overtopping the current valley sides (some 350m deep) so that the mountain tops only just emerged from the top of the ice cap.
This photo is taken from beside the stream that drains that lake, which is slowly eroding its way through the glacial deposits that line the valley floor -the car park is basically made from them. Like many places in the Beacons I’ve come to know this place for work -this is a great teaching site, showing how stones become aligned with the direction of motion in the process of deposition at the base of glacier. And further, since there aren’t many better corries in the National Park than Cwm Llwch, this combines beautifully to create a very useful day’s fieldwork for any group looking to study a bit of UK glacial ge0morphology. I will say I haven’t always benefited from the weather captured on this particular day -I have been known to be stood at the river cliff with water seeping through my clothes from above and below, and dripping down my arm as we removed rocks from the exposure to measure their orientation.
This is another one to print at standard dimensions, should you choose to buy.
All images © Stephen Tyrrell, Mountains, Wild Places & Water, 2013. All rights reserved.