Isle of Skye: Talisker and Preashal Mor EarthCache
Hidden : 5/10/2011
Difficulty:
4 out of 5
Terrain:
2.5 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

Talisker and Preashal Mor

The Talisker SSSI on the west coast of the Isle of Skye has been so designated for its geological and biological features, and is a place of considerable earth science importance.

The SSSI area comprises a significant area of land in this remote location, but the focus of this EarthCache is on the shapely summit, Preashal Mor. Geocaching visitors to Talisker, however, should seek to embrace all the fascinating features, and a nearby traditional cache (GC1YXDK) has been placed to encourage this. Please note that to complete this EarthCache there is no need to enter the SSSI itself.

The SSSI is fundamentally in two areas; one being the large area of land above and to the south-east of Preashal Mor, and the other being essentially the entire foreshore of the Bay, below high water mark.

A map is viewable here. NOTE: You will need to click on SSSI map, and then download a pdf map.

Much of the northern part of Skye is underlain by lavas, formed by volcanic eruptions during the Tertiary period of geological time. This thick pile of lavas contains a variety of lava flows of different chemical compositions, and many of these different types can be seen in the Talisker SSSI. The lavas exposed in the Talisker area are some of the youngest on Skye.

Of particular note is the way that individual lava flows can clearly be identified by the presence of a reddened top surface, known as a bole, which formed when the lava was weathered after it had been erupted.

The Talisker SSSI comprises a number of distinct sections, all of which have geological interest. At Talisker Bay, the cliffs provide excellent exposures of the series of lava flows, with each separate flow having a reddened top (bole), which formed when the lava was weathered after it had been erupted.

The two hills of Preashal Mor and Preashal Beag (only the former is visible from the cache co-ordinates) provide exposures of a thick flow of a particular type of lava called olivine tholeiite, which shows excellent columnar jointing (a similar structure to that seen on Staffa, off the coast of Mull, and at the Giant’s Causeway, in Northern Ireland).

In geology, the term 'joint' refers to a fracture in rock where the displacement associated with the opening of the fracture is greater than the displacement due to lateral movement in the plane of the fracture (up, down or sideways) of one side relative to the other. Typically, there is little to no lateral movement across joints. This makes 'joints' different from a 'fault', which is defined as a fracture in rock in which one side slides laterally past the other with a displacement that is greater than the separation between the blocks on either side of the fracture. Columnar jointing is a jointing consisting of parallel columns with polygonal cross-sections, formed by cooling and contraction, with column size generally decreasing as cooling rates increase.

The lava at Talisker has an unusual chemistry compared with other lavas on Skye. It has been interpreted as having in-filled a valley eroded into the underlying lavas, and there must therefore have been a significant time gap before the olivine tholeiite was erupted.

Overall, the Talisker SSSI is important for two reasons: firstly, because of the clear sections through the upper part of the Skye lava succession; and secondly because of the evidence for a long time gap in volcanic activity followed by the eruption of a lava type of unusual composition. This site provides important information about many features of volcanic activity on Skye and is crucial for research and education.


To claim this EarthCache, please respond to the following questions (email your answers, do not place them in your log entry), which can all be answered by careful observation from the given co-ordinates, which are at a gate on the track from Talisker House to the bay:

(a) Below the bulk of Preashal Mor are slopes of scree; what dominant colour do they display, and why do you think this occurs?
(b) As you look at Preashal Mor, you can make out a number of distinctive gullies – how many? How do you think they were formed?
(c) Can you estimate the height of the vertical column section of Preashal Mor, i.e. below the overlying lava flows? - and the length of (c), the olivine tholeiite, as visible from the co-ordinates?

Please do not log your visit until you have sent me answers to the above question and I have replied. I always reply to EarthCaches answers, and usually do so promptly.

There is no requirement to upload images of the area, but if you were to do so, they would demonstrate the attractiveness of Talisker to geocaching colleagues across the UK and wider Europe. There is limited parking (free) at the road end at Talisker, but do take care not to obstruct the road unduly.

Additional Hints (No hints available.)



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