Skye: Elgol honeycomb EarthCache
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A visit to one of the most geologically evocative locations on the Isle of Skye.
Permission to create an EarthCache at this location is given by the John Muir Trust,
which owns much of the land in the Strathaird peninsula.
Outcrops on the southern part of the Strathaird peninsula are chiefly of Jurassic-age sedimentary rocks, together with small areas of rocks formed during the Cretaceous, capped by more resistant basalt lavas of Palaeogene age on the hills of Slat Bheinn, Beinn Leacach, Ben Cleat and Ben Meabost. Both the sedimentary rocks and the lavas dip gently westwards, so that the hills have steep eastern cliffs but gently dipping western slopes.
The sedimentary rocks can be studied along the coast north of Elgol, which provides an excellent section through Middle to Upper Jurassic sandstones, mudstones, and limestones, cut by later dykes.
A well-known feature is the line of cliffs of pale-coloured sandstone with conspicuous honeycomb weathering, immediately to the north of Elgol village, caused by, among other things, sand grains from the sea in rough weather pitting the sandstone: over thousands of years this pattern is formed. In these sandstones, hard quartz grains are cemented together by calcium carbonate, which has been dissolved and eroded away to give the honeycomb appearance. There are two distinct types of coastal honeycomb weathering intertidal and supratidal.
For honeycomb weathering to occur, a source of salt is needed because the basic mechanism for this kind of weathering is salt heaving. Salt is deposited on the surface of the rock by saltwater spray or by wind. Moisture must be present to allow for the salt to settle on the rocks so that as the salt solution evaporates the salt begins to crystallize within the pore-spaces of the rock. Porous rock is also needed so that there are pore-spaces for the salt to crystallize within. These salt crystals prise apart the mineral grains, leaving them vulnerable to other forms of weathering. It takes prolonged periods for this weathering to become visible, as the rock goes through cycles of wetting and drying. Intertidal honeycomb weathering such as at Elgol, is found on horizontal planes in rock within the tidal zone. This type of honeycomb weathering is limited in its growth by the rate of evaporation from the sun. Once the depressions have grown large enough that the sun can not evaporate all of the water left in the gap by the retreating wave, the holes are as large as they will get, because the salt can not dry out and wedge grains apart any longer.
The rocks on the beach below these sandstones are mudstones, which contain fossil fish scales and other fossils such as bivalves and gastropods. Many of the sedimentary rocks in this area contain abundant fossils, including bivalves, brachiopods and belemnites. Of particular interest to palaeontologists are the mudstones of the Kilmaluag Formation, which contain fossils of small amphibians, reptiles and mammals that lived around the lagoons in which the mud was deposited.
To claim this cache, please answer the following questions:
(a) When you look at the cliff with the honeycomb weathering, there appear to be three distinct layers of rocks, starting with the honeycomb at the bottom, and a more solid-looking top section. How many layers of sediment do your estimate/count in the layer between these top and bottom layers? No one is expecting you to be precise, but it will give you a good idea of the everlasting processes of deposition.
(b) How high do you estimate the honeycomb section to be above the boulder beach below?
(c) If you touch one of the honeycombs, very lightly, can you describe what it feels like on your fingers.
(d) How long ago, in geological terms, was the Jurassic period? [You'll need to look it up.]
There is no obligation to upload images, but it would be good if the cache page contained a series of images taken over time.
PLEASE DO NOT ENTER YOUR ANSWERS TO THESE QUESTIONS IN YOUR LOG; EMAIL THEM TO ME FIRST, AND AWAIT A REPLY BEFORE LOGGING YOUR VISIT. I AM USUALLY VERY PROMPT IN DEALING WITH EARTHCACHES.
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