COIR A' GHRUNNDA EARTHCACHE EarthCache

Although with most earthcaches the geology come first, in this case I am going to give priority to the route information so geocachers can decide whether they think their mountaineering skills are adequate to be able to attempt this earthcache safely.

THE ROUTE:
According to one Skye walking site on the internet "the walk from Glen Brittle up to Coir' a' Ghrunnda is not really too big an undertaking" but personally I feel that it is a tougher walk than reaching some of the easier tops on the Main Ridge.
Having given that warning the following is the route description that best agrees with my experience:-
It's about 9 km return, with an ascent of 700m, but it rates as 'stretching' for two reasons.
Firstly, the route-finding in the corrie itself can be tricky, especially in poor visibility.
Secondly, a section of simple scrambling is unavoidable on the final approach to the upper corrie.
But in good weather the large sandy shored lochan, surrounded by Cuillin peaks and held by a massive barrier of boiler-plate slabs, is easily reached by any competent hillwalker. For the non-climber, this is an excellent trip to a very worthwhile destination.
From the campsite at Glen Brittle, follow the main (Coire Lagan) path that climbs straight up the hill behind the toilet block. At N57 12.037 W006 16.393 (NG420203), take the branch to the right across the burn. From here, the line of the path is a little different from the one marked on the OS map, but it is well made and very easy to follow, all the way into the mouth of the lower part of the corrie at N57 11.586 W006 13.756 (NG446193).
There are good views of Loch Brittle, Soay and Rum on the way, and a crossing of the Allt Coire Lagan to add some interest - perhaps lots of interest after heavy rain!!
From the steeply rising left turn into the lower corrie onwards, life gets more exciting. The surroundings are suddenly awesome. Massive walls of gabbro on either side funnel towards the lip of the upper corrie.
The burn slides down the middle over flat, smooth slabs and the only real route up keeps to the left of this, above the main stream, close to a cliff towering above on the left.
After passing through an area of large boulders, keep another cliff on the left. Eventually a convex slope of smooth rock bars the way but it can be climbed using a clear narrow crack for footholds.
The path becomes broken, snaking up scree and crossing boulder fields high on the left side. Watch for cairns and smoothed rock marking the way and, if ever in doubt, err on the high (left) side. If you get too low the terrain becomes impassible for the non climber.
Ahead all the way is a skyline that might be mistaken for the Cuillin Ridge itself, were it not for the waterfall coming over it. This is the edge of the upper corrie, and it can be overcome easily by scrambling up to the left from the base of the waterfall.
One site advises “ Pick a safely clear day for this one, and take a picnic to enjoy on the edge of the lochan” while another says “This mystical place has to be visited on a misty day to appreciate our own insignificance.”
I can see that both have their appeal but, whatever the conditions, there are few things that give me a greater thrill in the mountains of Scotland than walking into the magnificent Coir a' Ghrunnda in the Skye Cuillin.

THE GEOLOGY:
The main formation of the rocks of the Black Cuillin occurred in geologically recent times. Volcanic activity poured huge amounts of molten rock over an area far larger than the present island. This is well seen in the basalt moorland in the north of Skye.
Later there were more volcanic eruptions but these lavas cooled more slowly below the surface so that, although they were similar chemical composition to that which formed the basalt, these solidified into a much coarser grained rock with larger crystals. Most of this is dark black gabbro but to the NE side of Coir a’Ghrunnda some of the lava has solidified as ginger brown peridotite.
Later still more molten rock, basalt and dolerite was intruded into the gabbro as vertical dykes and horizontal sills.
The dyke rock is hard and brittle and tends to fracture and therefore erodes more quickly than the surrounding gabbro, frequently leaving gullies or chimneys. Being fine grained and splintery, basalt is much less adhesive underfoot than gabbro, particularly if it is wet.
During the last ice age most of slopes of Skye were covered by glaciers with just the highest peaks, sticking out about these rivers of ice as nunataks and suffering erosion from rain, wind and freeze / thaw processes. This is one of the reasons why the crest of the Cuillin Ridge is so jagged. However, the lower slopes did not escape unscathed.
Glaciers don’t stand still but move under gravity like huge frozen rivers.
And they are so huge and so heavy that they grind away at their base and sides as they inch forward. Hundreds of thousands of tons of ice & snow push down from the mountain tops with tremendous weight and force and the undersides of glaciers are like very coarse sandpaper. They constantly carry stones, rocks and grit down from the upper mountain and, like sandpaper, this has a smoothing effect.
Over centuries, like a huge sanding machine, the glacier works on the rock buried deep beneath it. The last glaciers on Skye melted away as recently as 11,500 years ago but here in Coir a' Ghrunnda you can vividly see signs of glacier activity all around you:-
‘U’ shaped valleys,
hanging valleys,
smoothed rock slabs or boulders (roche moutonees) with parallel grooves or scratch marks and
piles of debris (moraine and drumlins).
Often when glaciers eventually melt away, a lake is left behind in the hollow at the foot of the mountain, held in place with a natural dam of glacial debris or solid rock.
The glacier has pulled away at the downhill side of this lip so the exit streams from such lakes go crashing over the edge in a gorge or waterfall from their hanging valley.
This is particularly well seen in this corrie.
Massive isolated boulders called ‘erratics’ are frequently left behind on the lower sides of the valleys.
All of these glacial features can be seen in Coir a’Ghrunnda.

THE TASKS:
(1) At N57 11.764' W006 13.615 (NG 44767 19612) you should be standing on one of the many "boiler plate" slabs of glacier-soothed rocks. You have to look up into the corrie and estimate the amount of ground that the glaciers have scraped clear off soil and vegetation (ie approximately what percentage of your view is bare rock?)
(2) Look down at the rock you are standing on and tell me what obvious feature you can see near your feet, how wide is it and what rock type is it?
(3) At the listing co-ordinates note the altitude on your gps. Then go to the highest point behind you - approx. N57 11.981 W006 13.408 (NG 45000 20000) - and tell me the difference in height between these two points. This is the maximum height of the lip of the hanging valley above Loch Coir a'Ghrunnda.
(4) Peridotite is the dominant rock of the Earth's mantle above a depth of about 400 km. Oceanic plates consist of up to about 100 km of peridotite covered by a thin crust, commonly about 6 km thick, which consists of basalt, gabbro & minor sediments. All 3 rock types contain pyroxene, plagioclase, amphibole & olivine but peridotite differs from the other two in having less of a particular chemical compound. Do some internet searching to tell me which substance this is. (The answer is NOT one of the 4 just mentioned!)
(5) OPTIONAL FOR THOSE SMITTEN BY THE GRIPPY GABBRO SLABS
(N.B. ONLY IF YOU FEEL CONDITIONS ARE GOOD ENOUGH AND AND YOUR BOOTS ARE UP TO IT.
I WOULD NOT ATTEMPT THIS IN HIGH WINDS OR HEAVY RAIN AS A SLIP COULD HAVE SERIOUS CONSEQUENCES.)
Make your way out to the isolated boulder on the slabs near N 57 11.928 W 006 13.493.
Get a friend to take a photo of you sitting on the block waving your hands in the air and post it with your log. If you are walking alone maybe you could use a self-timer function on a camera.


Be especially careful if your footwear does not have grippy soles. If they don't, however, then you shouldn't really be walking on Skye!!