Monday, 29 August 2016
Kingdom of the wind - granite versus gale
The summit plateau of Ben Avon is a windswept sweep of cropped alpine vegetation studded with granite boulders and tors. The summit of the hill is itself a highly featured tor and is visible from a long distances.
Since first acquaintance over 30 years ago I've loved the open nature and the huge skyscapes offered by the Cairngorm giants; in fact the whole area is a great upland plateau cut by steep glacial trenches to form individual hills and ranges. The gravel flats of the higher ground look lifeless, but there are arctic specialists which prefer the conditions found up here....
...like the Ptarmigan (Lagopus mutus). A member of the Grouse family, Ptarmigan are superbly adapted to sub-arctic and arctic conditions. The Linnaean name Lagopus translates as "hare foot" and refers to the birds feathered feet which insulate against ice and snow. The birds are superbly camouflaged, moulting to white in winter and then to mottled grey during the summer, and have feathers adapted to maximise body insulation. The nostrils are also protected by small feathers to warm the air before they breathe it in.
Living high up the Scottish hills (in other regions the bird lives on arctic tundra at lower levels) gives Ptarmigan an advantage as there are few competitor species eating their food plants of bilberry, saxifrages and heather shoots supplemented by insects in summer. The tough plants which the birds eat are difficult to digest, so in common with Red Grouse the birds ingest small quantities of grit which they use to help grind up plant shoots - the gut of a Ptarmigan is the largest of any bird as a proportion of body weight - they are in effect a flying gut! The mottled grey summer plumage is superbly effective amongst boulders and grasses, often the birds are only noticed when they move; a good thing if you are a favoured prey of Eagles. To preserve energy Ptarmigan prefer to walk around rather than fly any distance and are endearingly approachable - this bird moved around me quite unconcernedly, occasionally giving the unusual croaking call which is reflected in its name; an anglicised version of the Gaelic Tarmachan.
Animals and birds can either adapt to the harsh conditions of the Cairngorm plateau, or move downhill when things get really tough. For plants that's not so easy, but there are subtle ways in which vegetation can adapt. These lines of gravel demarcate "wind stripes" of prostrate heather growing tight to the ground aligned to the wind; in this image the prevailing wind is from behind the camera. On the slope across the corrie the bare patches are deflation surfaces where the aspect is so exposed to the wind that little vegetation can survive; the surface itself is being lifted by the wind and deposited elsewhere on lee slopes.
Even the granite bedrock isn't immune to the effects of the wind. The numerous tors which stud the summit plateau of Ben Avon have holes worn into them which are partly formed by the wind. Grit and tiny pebbles which were deposited by the wind are whirled around in rainwater, slowly deepening the depression. Gradually the process forms these deep pots as larger stones and more water are able to accumulate.
Some become quite deep; this one was about half a metre into the rock and the process of the wind whirling the water and disturbing the grit at the bottom of the pool could actually be seen. Near the top of this image, another hole is beginning to form. In the face of Cairngorm gales, even granite gives way....
The summit of Ben Avon is a granite tor at 1171m/3842ft, named Leabaidh an Daimh Bhuidhe (couch (bed) of the yellow stag). A short scramble gains the narrow crest of the tor, normally as on this day quite easy but not if it's plated in ice or lashed by the wind - or both!
From the summit there's a view across to Ben Avon's massive neighbour, Beinn a'Bhuird (table mountain) which has an even larger high plateau.
The two Munros are linked by the narrow connecting bealach (col) of The Sneck which can be seen near the left of the image. The two hills can be combined into a really big day's walking starting (for example) from Invercauld on Deeside, rewarding but shattering! My own day on Ben Avon wasn't quite as long, but the "long walk in" inevitably means a long walk back out too...it was time for me to get moving again.