The origin of the word "gneiss" is thought to be German (verb gneist -to spark) because the rock glitters. It has been in use in English since at least 1757.
This commemorative rock is a fine example of Lewisian Gneiss which is so named as it occurs on the Isle of Lewis in The Outer Hebrides. Lewisian Gneiss is metamorphic, in that volcanic heat and pressure has altered its structure somewhat. Originally the rocks were like granite which changed as the Earth's crust became molten and they solidified. This is the reason you can see great variations in the way the layers are displayed, ranging from the white, to pale grey and even then the really dark grey.
For hundreds of millions of years, these rocks were buried beneath the surface which meant that the intense heat and pressures helped to form the growth of the new minerals in them. It is thought that the original rock was probably sandstone and limestone or igneous rocks like basalt and granite. Gneisses are one of a group of rocks (including slate) exhibiting foliation. In Gneiss, the foliation is more typically represented by compositional banding due to segregation of mineral (mafic and felsic) phases developed under high temperature and pressure conditions.
Gneissic rocks are usually medium- to coarse-foliated and largely recrystallised but do not carry large quantities of micas, chlorite or other platy minerals.
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a combination of the words "magnesium" and "ferric". Most mafic minerals are dark in colour .
Felsic ( a combination of feldspar and silica) is an adjective used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. They are usually light in colour. The most common felsic rock is granite but includes quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars. In terms of chemistry, felsic rocks are on the other side of the rock spectrum from the mafic rocks.
Most of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneiss. These are amongst the oldest rocks in Europe and some of the oldest in the world, having been formed in the Precambrian "super-eon", up to 3 billion years ago. In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree. These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist and intruded by later basaltic dykes and granite magma. The gneiss's delicate pink colourings (due to the feldspar) are exposed throughout the islands.
PLEASE ANSWER THE FOLLOWING QUESTIONS IN ORDER TO CLAIM A FIND HERE. Thank you. Failure to submit answers will result in logs being deleted without notice.
1. Describe the appearance of the rock. What does the foliation look like?
2. What is it's colour? Which mineral is chiefly responsible for this coloration?
3. Name the minerals in this Gneissen example.
4. Count the pebbles around the base of the Stone... only those which are in the ring nearest the Stone and not all the pebbles in the base!!
5. It is no longer a requirement to post a photo of your caching group at the location but we would greatly appreciate one of you and/or your GPS at the GZ. Many thanks for taking part in this Earthcache. We hope you have enjoyed doing it. We certainly had never noticed this Millenium Stone here previously and have enjoyed preparing this cache.
Please do not attempt to park adjacent to the cache. There is a large Pay as you go car park to the rear of the Library and on street parking in adjacent streets although restrictions do apply during daytime hours.