Misty Cwm Cau
Autumnal moodiness for this post, an image of the beautiful Cwm Cau, taken a little while ago while I was studying at the University of Wales Aberystwyth. Actually looking back, not such a little while ago…but then none of us are getting younger! I used be a member of the walking club there (now called the Uni Hiking Club) and as our closest mountain this was often one of the first of the term -with a range of possible routes, it’s ideal for getting to know a fairly new group of people.
I’ve already blogged a bit about the landscape evolution of Cadair Idris and Cwm Cau; suffice to say that geology had a strong influence on the glacier that once filled the corrie, with the glacier flowing down a band of relatively soft mudstone. In fact, the rock slightly left of the middle of this image is particularly interesting, in that it’s a roche moutonnée -a rock streamlined by the glacier flowing over the top of it. It formed because this is a slightly tougher rock than the surrounding mudstone, resulting from a period of explosive vulcanicity around 450 million years ago (you see the Cadair Idris Massif is the result of the subduction of an area of sea floor and the subsequent undersea eruptions). This area of volcanic tuff, made from volcanic ash flows, eroded more slowly than the surrounding rock, and as the glacier flowed over it, it became more streamlined. On the up-glacier or stoss-side, the glacier and rocks held in its base abraded the surface, grinding it down, polishing away any sharp edges and leaving scratches called striations that align with the direction of flow. Down-glacier, or on the lee-side, the rock became increasingly weakened by the action of freezing and thawing water, loosening great chunks of rock from the developing hummock. The glacier was able to take those loose rocks away wholesale, in a process called plucking, leaving the lee-side much steeper, and forming a cavity (a gap under the ice). There’s a great diagram on the BBC Bitesize website. This only helped further, as the change in pressure under the ice would have allowed greater daily fluctuations and more freezing and thawing.
I know it’s hard to believe water existing under a great body of ice several tens of metres thick, but it has to do with pressure melting point -there’s so much pressure here the the ice flows even more (called regelation) and actually melts to form a film of water that is able to get into joints in the rock where it refreezes and expands, with the loosening effect mentioned before. Actually pressure melting is what goes on when you go ice skating -the force from your weight, pressing down on the runner melts the ice under it, creating a layer of lubrication which allows you to slide. I wouldn’t have been surprised if there wasn’t a film of ice steadily being extruded as the glacier moved over the rock actually -I’ve seen similar things under alpine glaciers before.
Anyway, the roche moutonée provides an ideal focal point and an even better lunch stop. Not only that, it’s common for people to wild camp near here -there’s a source of readily available water (boil it first!), and it’s possible to find a decent level of shelter if the conditions aren’t ideal. It’s also the place where the footpaths split. Go south from here to go round the headwall of the corrie or go north to ascend the chute at the head of the corrie itself. Both excellent walks with plenty to recommend them, although the chute is a bit more challenging with a number of scrambly bits and areas where you’re crossing loose screes. It does keep you sheltered for a long time though, many’s the time we’ve felt next to no wind until we emerged from the top of the chute.
To buy this image please visit: www.photoboxgallery.com/mountainimages
All images © Stephen Tyrrell, Mountains, Wild Places & Water, 2013. All rights reserved.