Cwm Cau

Cwm Cau

The classic view of Cwm Cau on Cadair Idris in southern Snowdonia. Available to buy at my Photobox Gallery.

Cadair Idris is somewhere I know very well. Just up the road from Aberystwyth where I went to University, this is a classic glacial geomorphology trip, and often the first walk for the many outdoors clubs -I really have lost count of the number of times I’ve been up there! Part of that is because I came to study this whole mountain range for my Masters’ degree, so I was up and down like a yo-yo at one point. I guess that’s how there come to be so many photos of the mountain in my gallery. So many, in fact, that my Photobox site has a collection devoted to it, so there are plenty of images for you to choose from if you should choose to buy one.

About this image then: this is from one of the most-walked routes on the mountain, and this lofty vantage point is about two-thirds of the way round, when you’re on the descent from the summit ridge. A great walk, full of interest, you essentially follow the rim of the glacial cwm you see in the picture, having approached the lake from below the rock lip (a great spot for a picnic). I won’t go into details, I’d like to save them for another post to go with this image. The reason the corrie is there comes down to geology (most things do eventually!).

When ice covered the land during the last age, mountain ranges like this often acted as spreading centre, with glaciers radiating out in all directions. Those glaciers were never more effective in modifying the landscape than when there was some geological weakness to exploit (glaciers start out small and weedy, and need a bit of help to get going), such as a fault-line (as at Tal-yllyn, just round the corner), or in this case a soft layer of rock that the ice could work away at. The rock in this case was a mudstone, sandwiched between layers of volcanic rock. The sequence was created during the Ordovician Period, 505-440 Million Years Before Present, when the British Isles weren’t even isles (or Britain), and when tectonic activity caused widespread vulcanism, with ‘brief’ episodes of erosion in between (if you call a few thousand years brief). The mudstones come from eroded material washing down to the sea and being deposited layer on layer until the next volcanic episode. The result is a rock which is especially weak along the bedding plane (i.e. the depositional layers), and very friable (crumbly) -it’s also blue, which comes down to the chemical content of the rock. Anyway, that layer of soft rock lies at the heart of this corrie, running from top right to bottom left  of my picture (more or less), covered by vegetation in the headwall (the cliffs around the lake), continuing along that diagonal line, past the lake and onto the valley floor (see the diagram below).


The approximate line of the mudstone (in blue).

The ice was able to work away at those layers, expanding the cracks and stripping them away, allowing the glacier to grind its way down into mountainside, eventually taking the other rock with it. Hence the scale of this corrie is greater than you might expect for one which is generally east-facing (you’d normally expect the north-facing slopes to have the more significant glaciers as they are more sheltered, and receive less sun) -it also helps that there’s an area down-wind for snow to accumulate in (wind-blown snow is very important in nourishing cirque glaciers). In time, the glacier grew, creating the impressively steep cliffs and classic amphitheatre shape, until it exceeded the confines of the corrie, escaping down into the valley below and beyond, such that ice from here will have joined the massive ice sheet that in the end made it all the way down to the Brecon Beacons (or into the Irish Sea, depending) during the Last Glacial Maximum, around 20,000 years ago.

Incidentally, that same layer of rock that allowed the glacier to create such a stunning landscape, also created a really interesting route up the mountain -we used to call it the chute, because it funnels down into a gully near the top, and is strewn with rock debris. It’s the main scrambling route on this face of the mountain, and therefore not the easiest way to go, but definitely worth the challenge if you like to get your hands on rock.


Click to order prints (opens a new window)

Another image at standard dimensions -I reckon it would look lovely at 18″x12″. On that note, look at the post eBay Sale!!! for details of 10 pictures on special offer at the moment.

All images © Stephen Tyrrell, Mountains, Wild Places & Water, 2013.

2 thoughts on “Cwm Cau

  1. Pingback: Misty Cwm Cau | Mountains, Wild Places & Water

  2. Pingback: Llyn Cau, Cadair Idris | Mountains, Wild Places & Water

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