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Kilda Rocks

17 May 2011

Recently we have had a team of geologists visiting St Kilda, on a mission to understand the  phases of glaciation that have affected this unique place. It seems that previous work had proposed various ideas regarding the impact of the Ice Age on the archipelago but these have not been proven. New dating techniques could resolve the story.

In simplified form, large glacial erratics (boulders from elsewhere) deposited on the surface of Hirta indicate ice sheet coverage of the island at some stage during the Ice Age. This is backed up by the discovery of moraine material on the sea bed beyond St Kilda. However, the sharp jagged peaks of Dun, the tor of Ruaival and other 'peri-glacial' (cold but not Ice Age cold) deposits, indicate that a long period has elapsed since the last full glaciation over the whole island. The geologists are therefore hoping to resolve the dating of the waxing and waning of the ice sheets across Hirta. In order to do this, further evidence is needed and this came from three main sources.

Firstly, rock samples were collected for dating. Cosmic radiation creates isotopes of certain elements within the rocks, meaning that geologists can measure how long a rock has been exposed to the sky. Samples from Hirta are therefore being used to assess how long they have been visible. For example, if a resulting date is less than 10,000 years the rock must have been covered by the most recent ice sheet. More than 10,000 years and the island was not affected by the most recent ice sheet.

Secondly, the relationships between different types of drift deposits were studied. Drift deposits are the clays and stones which rest upon the bedrock but below the soil. These originate from various different processes, so their order of deposition gives evidence of the order of events.

Thirdly, in two separate areas sands were found containing organic soils below these drift deposits – these had to have been formed in between periods of glaciation. Samples of these were taken for further dating – including radiocarbon dating of the organic components.

sands with organic material underneath glacial deposits..

Sand with organic material within it, underlying the glacial deposits

The results of these studies will be published in due course and an update posted on this blog.

Speaking to the geologists it struck me that the story of the geological formation of St Kilda is not just relevant to the island itself – this story will soon become an important piece of evidence in the much larger story of the geological formation of Britain and the north west of Europe. 

It is inspiring to think of all that evidence just sitting around in the rocks, or among the rocks, waiting to be noticed, and it reminds me of the Kildans' ancient view of their island, according to Martin Martin: that there were spirits all around, in the very rocks and springs..

Ian



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