If you have reached the GZ area, you will have walked through a unique geological feature to reach a quite unusual beach. The feature is 'machair', and this location is famous as being one the finest examples of machairs. Phonetically, the word ‘machair’ has its origins in Gaelic and, as the wise book says, 'machair' means 'an extensive, low-lying fertile plain'".
But what is a machair, and why it is geologically significant? Under the carpet of grass before you you have windblown sand, and beneath the sand is gneiss. Gneiss is a metamorphic rock, what means that it has been formed deep beneath the surface of the Earth, under the influence of high temperatures and high pressure. These conditions transform older rocks, forming the gneiss. For example, granites, or other metamorphic and sedimentary rocks can be totally transformed when they are changed into a gneis. Gneiss' cross-section often has stripes of various colours (this is called foliation), these often being bent and deformed during the processes where tectonic plates collide, taking millions of years to do so.
The sand has a very high shell content, sometimes up to 90%. This astonishingly high content of shell fragments has its origin during the most recent ice age, when lower sea temperatures killed off many of the carbonate-shelled marine animals, when the meltwaters at the end of the Ice Age transported debris across the land (the continental shelf as a pro-EarthCacher would say). Later on, the combination of onshore sea currents and winds have subsequently reworked these materials, depositing them to form the beaches we see today, all backed by the dunes. The Outer Hebrides are an excellent place to learn about machairs, as this is one of Europe’s rarest landscapes and is unique to the place you’re just visiting.
Walking down towards beach and the sea you’ll pass marram - virtually bare, mobile sand that is highly exposed to the action of both the wind and waves. In this place you can find only grass that has a remarkable ability to withstand dry conditions, and its development quite often opens a way for other plants to settle and grow. This way the machair can expand.
After visiting this EarthCache you should be familiar with following:
- what machair is;
- what makes the Outer Hebrides' beaches special.
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The Tasks were developed with the help of a 12 year old, so they should not be difficult :)
To count the EarthCache, please answer following 4 Tasks. Replies should be sent via email or through the Message Centre. Please do not publish your answers in your log. You do not have to wait for my permission to log your visit; if your answers are wrong, I'll contact you.
1. At GZ take a big pinch of sand from the beach, dry it in your hands, examine it, and then estimate the proportion of sand-shell fragments in it. The shell 'grains' are the lighter-weight fragments, so you may try to blow them away carefully.
2. While examining the sample of sand you will notice shells and sand. Sand is formed mostly from the mineral called quartz. These make white-ish and yellow-ish-coloured grains. Do you notice any other grains of different colours? What do you think these are, and what do you think their source may be?
3. Look towards the sea – the beach is surrounded by gneissose rocks and cliffs. Why do you think the sand has gathered in the middle of the beach?
4. At the additional waypoint you’ll see the head of the sand dune. Estimate its height and describe what the impact of the sea has had on the dune.