Isle of Skye: The Bile raised beach
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The Bile Raised Beach
A visit to a spectacular raised beach in close proximity to Portree.
The sea cliffs of Skye bear witness to relentless attrition by the sea over many thousands of years. Among the many coastal features there appear so-called ‘raised beaches’, also known as ‘marine terraces; these wave-cut platforms, take the form of an elevated area of sloping ground, sitting above the present tide line. In the past, the area of the platform would have been at sea level, and evidence of this can often be found in the form of beach shingle at the inland edge of the platform.
Raised beaches are therefore clear evidence that sea level has changed.
There are many examples of this feature throughout Britain, particularly along the West coast, because it was the western edge that experienced the greatest weight of ice during the last Ice Age (about 10,000 years ago).
The platforms were created at various times over the past 500,000 years, during glacial periods when the earth's crust sagged under the enormous weight of ice sheets. During these periods the sea level rose in Scotland, even though the world's oceans were partly locked up in great continental ice sheets and sea level at the equator was actually 100 metres or more lower than today.
Two distinct sets of raised beaches occur on Skye. Those lying just a few metres above sea level formed as Scotland's last widespread ice sheet decayed, about 15,000 years ago. This is known as glacio-isostatic rebound, isostasy, or isostatic uplift. This ‘bounce back' motion equates with the localised change in sea level, relative to the land.
The other set of raised beaches formed between 8,000 and 6,500 years ago during a period of rapid rise in world sea level caused by the melting of ice sheets in North America and Scandinavia and before Scotland had completely recovered from the unloading of its former ice cover. These raised beached occur much closer to present sea level; some are associated with long-abandoned caves, geos and sea stacks. The raised beaches all tilt gently towards the west owing to varying uplift since deglaciation.
For visitors to Portree, a spectacular raised beach lies only a short walk away, along the popular path to Scorrybreac. This is known as The Bile, and there was once a small chapel here.
The raised beach at The Bile appears as a distinctive wide, grassy green sward set against a backdrop of lava flows, and displays a well-defined and characteristic tilting along its length, rising from just above sea level to a deep, incised gully below the crags at the southern edge of Sithean Bhealaich Chumhaing, and not far from the small settlement of Torvaig, which is on the headland above.
To view the EarthCache it is not necessary to walk out onto the raised beach; the finest view is from the Scorrybreac walk which starts from the end of the road at Brumhor, and follows the shoreline on a good path, and then reaches a gate at the cache co-ordinates, from which the view of the beach is spectacular. On the way you pass Black Rock, the point from which Bonnie Prince Charlie left Skye for nearby Raasay.
To claim this cache, answer the following questions:
(a) What is you estimate of the height of the raised beach, at its lowest point as you look at it from the gate, and at its highest point, just below the gully below the crags?
(b) Given the above information, can you roughly calculate the angle of tilt of the terrace platform, i.e. at what angle does it elevate from west to east?
(c) From all the above information, and a visit to the site, can you express a view of whether the raised beach at The Bile was the product of isostatic uplift or a change in sea levels?
There is no requirement to submit pictures of the raised beach, but if you were to do so, they would add to the value of the site.
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