Mull: Treshnish Raised Beach
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Mull: Treshnish Raised Beach
The sea cliffs of Mull, like all of the Scottish islands, are witness to unremitting grinding down by the sea over thousands of years. Among the many coastal features, there appear what are known as ‘raised beaches’, or ‘marine terraces'. These wave-cut platforms, take the form of an elevated area of ground, sitting prominently 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, although this is not always the case.
Raised beaches are clear evidence that sea level has changed.
There are many reasons for changes in sea level. During glacial periods, areas of the Earth’s crust that were covered by ice-sheets slowly sagged under the weight of the ice, so that sea levels rose. When the ice melted, global sea levels initially rose further, but once the weight of the ice had been removed the depressed areas slowly began to ‘rebound’, consequently, over the next few thousands of years, sea levels fell.
These processes all interact with each other as ice-sheets wax and wane, so the detailed history of sea-level changes is complicated. However, most of the raised shorelines that we see today are the result of the upward rebound of the crust following its depression during the last major ice-sheet glaciation. In effect the islands are rising so that, in general, the lower shorelines are younger than the higher ones.
Two distinct sets of raised beaches occur. 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, 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 beaches occur much closer to present sea level, and many are associated with long-abandoned caves, geos and sea stacks. Raised beaches formed by the actions of isostatic uplift tilt gently towards the west owing to varying uplift since deglaciation.
The highest rock platforms on the coastlines around Mull were probably formed before the last widespread glaciation, 30,000 years ago. The raised beach at Treshnish is an especially fine example.
To claim this cache, answer the following questions:
(a) Without putting yourself at risk, what is your estimate of the height of the raised beach, above sea level?
(b) Did you find any evidence of shingle at the inland (cliff-base) edge of the raised beach?
(c) See if you can detect a westward-sloping angle of tilt of the terrace platform? At what angle do you estimate that it tilts? - ignore the steep cliffs for the purpose of this calculation.
(d) From all the above information, and a visit to the site, can you express a view of whether the raised beach at Treshnish 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.
To reach this EarthCache: It is recommended that you include it as part of a circular walk from the small quarry parking area south of Ensay, and follow the road to the cluster of cottages at Treshnish, before heading for Haunn and the northern end of the raised beach near Dun Haunn. Walk along the raised beach - there is a clear path all the way, although many diversions are tempting - until you reach the GZ. From there, press on a little further, before turning inland and climbing steeply to the ruined village of Crackaig. Beyond lies another ruined village, and from this a path leads across moorland back to the coastal road.
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