BARNS OF BYNACH EarthCache
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Please take care in the mountains. This can be a very pleasant few hours walk in the summer, but, if there is snow, or the weather is poor and you are not experienced in the mountains, then this cache is not for you.
Route - Glenmore Lodge - Ryvoan - Summit - Continue downhill to the Barns. Retrace steps to return. A distance of about 23 km and takes about 8 1/2 hours
What is a Tor and how are they formed?
Definition: a small rock hill produced by differential weathering and erosion
In the UK the term Tor is generally used to describe a metre to 10m+ angular residual, upstanding masses of rock; they are particularly a feature of granite bedrock.
The Cairngorm tors are the finest in Scotland, and some of the most spectacular are on Bynack More. The Granite tors are a striking element of the Cairngorm scenery, with the largest summit tors rising over 15 m above their surroundings. There is an intriguing variety of form that reflects structural controls and long-term weathering and erosion. Sheet jointing is a characteristic of the Cairngorms granite. These joints are found in the top 10-15 m of cliffs and tors, and are also called pseudo-bedding. They are thought to be caused by the release of pressure during the removal of the weight of overlying granite by erosion. Modern examples of this can be seen in granite quarries where the granite splits off into slabs parallel to the quarry floor. Many Cairngorm tors have also been modified or destroyed by the movement and erosion of glaciers. The traditional local (Strath Spey) name for a tor in the Cairngorms is ‘barn’, reflected in the names ‘Barns of Bynack’ for tors around the summit of Bynack More.
There are two types of tor:
1. The large monolithic type, which comprise a series of upstanding masses of granite blocks, reaching up to 30 m in height. A few have been disturbed and are weakly moulded by the passage of ice.
2. A smaller type, with encircling block fields.
It is thought that a two-stage process is responsible for the monolithic tors.
1. During the warm, humid conditions of Tertiary times, the granite plateau was deeply weathered, but to an uneven depth. Areas of hard bedrock were left between areas of more deeply weathered and crumbling granite. It may be that the watershed areas were better drained and therefore less rotting occurred. The principal affecting the rate of weathering is the jointing density of the granite.
2. During the Ice Age, the rotted granite was stripped away by solifluction ( the slow creeping of saturated fragmental material such as soil down a slope caused due to recurrent freezing and thawing of the ground) and related nival processes,( objects, organisms or places related to snow) leaving the sound rock as tors.
The survival of the tors was used in the past to suggest that parts of the Cairngorm Mountains were not covered by ice during the last glaciation. However more recent field evidence suggests that the whole area was covered by ice at some time – schist erratics have been found in surrounding scree.
What is granite and how is it formed?
Granite is a common intrusive plutonic igneous rock. Intrusive means that it has moved into other rocks by force coming up from the mantle. Plutonic means that it is magma that does not reach the surface of the earth and so cools very slowly underground. Igneous (derived from the Latin word ignis meaning fire) is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. Because it cools slowly crystals have time to form. Its name comes from the Latin word granum which means “a grain” for the coarse grain crystalline structure of the rock. Individual crystals are visible to the naked eye in all types of granite. It is made up of quartz, mica, and feldspar. It comes in a wide variety of colours including reds, browns, and many shades of grey from almost black to nearly all white.
There are different types of granite; the type of granite a particular specimen is depends upon the percentages of minerals that make up the rock, especially quartz (Greyish colour), K-feldspar (Reddish colour), Na-feldspar (White) and biotite or hornblende (Black). Quartz is a chemically inert mineral under most geological conditions, and in the weathering environment it is only really susceptible to mechanical wear, that is why veins of quartz stand slightly proud of the surrounding
granite.
To claim this earthcache, please send the answers to the questions below to us by email or message centre - do not post in your online log. Your log may be deleted if these criteria are not met. Educational guidelines for Earthcaches are set by Geocaching.com and GeoSociety.org (Earthcache) and have to be adhered to.
1 As you pass the Little Barns, (ref. LB N 57° 08.009 W 003° 34.965 ) you will notice a large vein of quartz on the west side. By how much does it stand proud of the rock encasing it and why is this?
2 Where else in the UK is famous for its tors?
3 Which of the two types of tor, as described above are the Barns of Bynach?
4 How would you describe the overall colour of the granite here at the Barns, and what is it which gives it this colour?
5 What percentage of the rock do you think is made up of biotite and what is the average size of the ‘black spots’?
6 Why not compulsory it is always good to see a photo from your visit
Enjoy the mountains but please be sensible and stay safe!
With thanks to Cairngorms National Park Authority for allowing this earthcache.
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